Merge git://git.kernel.org/pub/scm/linux/kernel/git/pablo/nf

[platform/kernel/linux-starfive.git] / kernel / futex.c

// SPDX-License-Identifier: GPL-2.0-or-later
/*
 *  Fast Userspace Mutexes (which I call "Futexes!").
 *  (C) Rusty Russell, IBM 2002
 *
 *  Generalized futexes, futex requeueing, misc fixes by Ingo Molnar
 *  (C) Copyright 2003 Red Hat Inc, All Rights Reserved
 *
 *  Removed page pinning, fix privately mapped COW pages and other cleanups
 *  (C) Copyright 2003, 2004 Jamie Lokier
 *
 *  Robust futex support started by Ingo Molnar
 *  (C) Copyright 2006 Red Hat Inc, All Rights Reserved
 *  Thanks to Thomas Gleixner for suggestions, analysis and fixes.
 *
 *  PI-futex support started by Ingo Molnar and Thomas Gleixner
 *  Copyright (C) 2006 Red Hat, Inc., Ingo Molnar <mingo@redhat.com>
 *  Copyright (C) 2006 Timesys Corp., Thomas Gleixner <tglx@timesys.com>
 *
 *  PRIVATE futexes by Eric Dumazet
 *  Copyright (C) 2007 Eric Dumazet <dada1@cosmosbay.com>
 *
 *  Requeue-PI support by Darren Hart <dvhltc@us.ibm.com>
 *  Copyright (C) IBM Corporation, 2009
 *  Thanks to Thomas Gleixner for conceptual design and careful reviews.
 *
 *  Thanks to Ben LaHaise for yelling "hashed waitqueues" loudly
 *  enough at me, Linus for the original (flawed) idea, Matthew
 *  Kirkwood for proof-of-concept implementation.
 *
 *  "The futexes are also cursed."
 *  "But they come in a choice of three flavours!"
 */
#include <linux/compat.h>
#include <linux/jhash.h>
#include <linux/pagemap.h>
#include <linux/syscalls.h>
#include <linux/freezer.h>
#include <linux/memblock.h>
#include <linux/fault-inject.h>
#include <linux/time_namespace.h>

#include <asm/futex.h>

#include "locking/rtmutex_common.h"

/*
 * READ this before attempting to hack on futexes!
 *
 * Basic futex operation and ordering guarantees
 * =============================================
 *
 * The waiter reads the futex value in user space and calls
 * futex_wait(). This function computes the hash bucket and acquires
 * the hash bucket lock. After that it reads the futex user space value
 * again and verifies that the data has not changed. If it has not changed
 * it enqueues itself into the hash bucket, releases the hash bucket lock
 * and schedules.
 *
 * The waker side modifies the user space value of the futex and calls
 * futex_wake(). This function computes the hash bucket and acquires the
 * hash bucket lock. Then it looks for waiters on that futex in the hash
 * bucket and wakes them.
 *
 * In futex wake up scenarios where no tasks are blocked on a futex, taking
 * the hb spinlock can be avoided and simply return. In order for this
 * optimization to work, ordering guarantees must exist so that the waiter
 * being added to the list is acknowledged when the list is concurrently being
 * checked by the waker, avoiding scenarios like the following:
 *
 * CPU 0                               CPU 1
 * val = *futex;
 * sys_futex(WAIT, futex, val);
 *   futex_wait(futex, val);
 *   uval = *futex;
 *                                     *futex = newval;
 *                                     sys_futex(WAKE, futex);
 *                                       futex_wake(futex);
 *                                       if (queue_empty())
 *                                         return;
 *   if (uval == val)
 *      lock(hash_bucket(futex));
 *      queue();
 *     unlock(hash_bucket(futex));
 *     schedule();
 *
 * This would cause the waiter on CPU 0 to wait forever because it
 * missed the transition of the user space value from val to newval
 * and the waker did not find the waiter in the hash bucket queue.
 *
 * The correct serialization ensures that a waiter either observes
 * the changed user space value before blocking or is woken by a
 * concurrent waker:
 *
 * CPU 0                                 CPU 1
 * val = *futex;
 * sys_futex(WAIT, futex, val);
 *   futex_wait(futex, val);
 *
 *   waiters++; (a)
 *   smp_mb(); (A) <-- paired with -.
 *                                  |
 *   lock(hash_bucket(futex));      |
 *                                  |
 *   uval = *futex;                 |
 *                                  |        *futex = newval;
 *                                  |        sys_futex(WAKE, futex);
 *                                  |          futex_wake(futex);
 *                                  |
 *                                  `--------> smp_mb(); (B)
 *   if (uval == val)
 *     queue();
 *     unlock(hash_bucket(futex));
 *     schedule();                         if (waiters)
 *                                           lock(hash_bucket(futex));
 *   else                                    wake_waiters(futex);
 *     waiters--; (b)                        unlock(hash_bucket(futex));
 *
 * Where (A) orders the waiters increment and the futex value read through
 * atomic operations (see hb_waiters_inc) and where (B) orders the write
 * to futex and the waiters read (see hb_waiters_pending()).
 *
 * This yields the following case (where X:=waiters, Y:=futex):
 *
 *      X = Y = 0
 *
 *      w[X]=1          w[Y]=1
 *      MB              MB
 *      r[Y]=y          r[X]=x
 *
 * Which guarantees that x==0 && y==0 is impossible; which translates back into
 * the guarantee that we cannot both miss the futex variable change and the
 * enqueue.
 *
 * Note that a new waiter is accounted for in (a) even when it is possible that
 * the wait call can return error, in which case we backtrack from it in (b).
 * Refer to the comment in queue_lock().
 *
 * Similarly, in order to account for waiters being requeued on another
 * address we always increment the waiters for the destination bucket before
 * acquiring the lock. It then decrements them again  after releasing it -
 * the code that actually moves the futex(es) between hash buckets (requeue_futex)
 * will do the additional required waiter count housekeeping. This is done for
 * double_lock_hb() and double_unlock_hb(), respectively.
 */

#ifdef CONFIG_HAVE_FUTEX_CMPXCHG
#define futex_cmpxchg_enabled 1
#else
static int  __read_mostly futex_cmpxchg_enabled;
#endif

/*
 * Futex flags used to encode options to functions and preserve them across
 * restarts.
 */
#ifdef CONFIG_MMU
# define FLAGS_SHARED           0x01
#else
/*
 * NOMMU does not have per process address space. Let the compiler optimize
 * code away.
 */
# define FLAGS_SHARED           0x00
#endif
#define FLAGS_CLOCKRT           0x02
#define FLAGS_HAS_TIMEOUT       0x04

/*
 * Priority Inheritance state:
 */
struct futex_pi_state {
        /*
         * list of 'owned' pi_state instances - these have to be
         * cleaned up in do_exit() if the task exits prematurely:
         */
        struct list_head list;

        /*
         * The PI object:
         */
        struct rt_mutex_base pi_mutex;

        struct task_struct *owner;
        refcount_t refcount;

        union futex_key key;
} __randomize_layout;

/**
 * struct futex_q - The hashed futex queue entry, one per waiting task
 * @list:               priority-sorted list of tasks waiting on this futex
 * @task:               the task waiting on the futex
 * @lock_ptr:           the hash bucket lock
 * @key:                the key the futex is hashed on
 * @pi_state:           optional priority inheritance state
 * @rt_waiter:          rt_waiter storage for use with requeue_pi
 * @requeue_pi_key:     the requeue_pi target futex key
 * @bitset:             bitset for the optional bitmasked wakeup
 * @requeue_state:      State field for futex_requeue_pi()
 * @requeue_wait:       RCU wait for futex_requeue_pi() (RT only)
 *
 * We use this hashed waitqueue, instead of a normal wait_queue_entry_t, so
 * we can wake only the relevant ones (hashed queues may be shared).
 *
 * A futex_q has a woken state, just like tasks have TASK_RUNNING.
 * It is considered woken when plist_node_empty(&q->list) || q->lock_ptr == 0.
 * The order of wakeup is always to make the first condition true, then
 * the second.
 *
 * PI futexes are typically woken before they are removed from the hash list via
 * the rt_mutex code. See unqueue_me_pi().
 */
struct futex_q {
        struct plist_node list;

        struct task_struct *task;
        spinlock_t *lock_ptr;
        union futex_key key;
        struct futex_pi_state *pi_state;
        struct rt_mutex_waiter *rt_waiter;
        union futex_key *requeue_pi_key;
        u32 bitset;
        atomic_t requeue_state;
#ifdef CONFIG_PREEMPT_RT
        struct rcuwait requeue_wait;
#endif
} __randomize_layout;

/*
 * On PREEMPT_RT, the hash bucket lock is a 'sleeping' spinlock with an
 * underlying rtmutex. The task which is about to be requeued could have
 * just woken up (timeout, signal). After the wake up the task has to
 * acquire hash bucket lock, which is held by the requeue code.  As a task
 * can only be blocked on _ONE_ rtmutex at a time, the proxy lock blocking
 * and the hash bucket lock blocking would collide and corrupt state.
 *
 * On !PREEMPT_RT this is not a problem and everything could be serialized
 * on hash bucket lock, but aside of having the benefit of common code,
 * this allows to avoid doing the requeue when the task is already on the
 * way out and taking the hash bucket lock of the original uaddr1 when the
 * requeue has been completed.
 *
 * The following state transitions are valid:
 *
 * On the waiter side:
 *   Q_REQUEUE_PI_NONE          -> Q_REQUEUE_PI_IGNORE
 *   Q_REQUEUE_PI_IN_PROGRESS   -> Q_REQUEUE_PI_WAIT
 *
 * On the requeue side:
 *   Q_REQUEUE_PI_NONE          -> Q_REQUEUE_PI_INPROGRESS
 *   Q_REQUEUE_PI_IN_PROGRESS   -> Q_REQUEUE_PI_DONE/LOCKED
 *   Q_REQUEUE_PI_IN_PROGRESS   -> Q_REQUEUE_PI_NONE (requeue failed)
 *   Q_REQUEUE_PI_WAIT          -> Q_REQUEUE_PI_DONE/LOCKED
 *   Q_REQUEUE_PI_WAIT          -> Q_REQUEUE_PI_IGNORE (requeue failed)
 *
 * The requeue side ignores a waiter with state Q_REQUEUE_PI_IGNORE as this
 * signals that the waiter is already on the way out. It also means that
 * the waiter is still on the 'wait' futex, i.e. uaddr1.
 *
 * The waiter side signals early wakeup to the requeue side either through
 * setting state to Q_REQUEUE_PI_IGNORE or to Q_REQUEUE_PI_WAIT depending
 * on the current state. In case of Q_REQUEUE_PI_IGNORE it can immediately
 * proceed to take the hash bucket lock of uaddr1. If it set state to WAIT,
 * which means the wakeup is interleaving with a requeue in progress it has
 * to wait for the requeue side to change the state. Either to DONE/LOCKED
 * or to IGNORE. DONE/LOCKED means the waiter q is now on the uaddr2 futex
 * and either blocked (DONE) or has acquired it (LOCKED). IGNORE is set by
 * the requeue side when the requeue attempt failed via deadlock detection
 * and therefore the waiter q is still on the uaddr1 futex.
 */
enum {
        Q_REQUEUE_PI_NONE               =  0,
        Q_REQUEUE_PI_IGNORE,
        Q_REQUEUE_PI_IN_PROGRESS,
        Q_REQUEUE_PI_WAIT,
        Q_REQUEUE_PI_DONE,
        Q_REQUEUE_PI_LOCKED,
};

static const struct futex_q futex_q_init = {
        /* list gets initialized in queue_me()*/
        .key            = FUTEX_KEY_INIT,
        .bitset         = FUTEX_BITSET_MATCH_ANY,
        .requeue_state  = ATOMIC_INIT(Q_REQUEUE_PI_NONE),
};

/*
 * Hash buckets are shared by all the futex_keys that hash to the same
 * location.  Each key may have multiple futex_q structures, one for each task
 * waiting on a futex.
 */
struct futex_hash_bucket {
        atomic_t waiters;
        spinlock_t lock;
        struct plist_head chain;
} ____cacheline_aligned_in_smp;

/*
 * The base of the bucket array and its size are always used together
 * (after initialization only in hash_futex()), so ensure that they
 * reside in the same cacheline.
 */
static struct {
        struct futex_hash_bucket *queues;
        unsigned long            hashsize;
} __futex_data __read_mostly __aligned(2*sizeof(long));
#define futex_queues   (__futex_data.queues)
#define futex_hashsize (__futex_data.hashsize)


/*
 * Fault injections for futexes.
 */
#ifdef CONFIG_FAIL_FUTEX

static struct {
        struct fault_attr attr;

        bool ignore_private;
} fail_futex = {
        .attr = FAULT_ATTR_INITIALIZER,
        .ignore_private = false,
};

static int __init setup_fail_futex(char *str)
{
        return setup_fault_attr(&fail_futex.attr, str);
}
__setup("fail_futex=", setup_fail_futex);

static bool should_fail_futex(bool fshared)
{
        if (fail_futex.ignore_private && !fshared)
                return false;

        return should_fail(&fail_futex.attr, 1);
}

#ifdef CONFIG_FAULT_INJECTION_DEBUG_FS

static int __init fail_futex_debugfs(void)
{
        umode_t mode = S_IFREG | S_IRUSR | S_IWUSR;
        struct dentry *dir;

        dir = fault_create_debugfs_attr("fail_futex", NULL,
                                        &fail_futex.attr);
        if (IS_ERR(dir))
                return PTR_ERR(dir);

        debugfs_create_bool("ignore-private", mode, dir,
                            &fail_futex.ignore_private);
        return 0;
}

late_initcall(fail_futex_debugfs);

#endif /* CONFIG_FAULT_INJECTION_DEBUG_FS */

#else
static inline bool should_fail_futex(bool fshared)
{
        return false;
}
#endif /* CONFIG_FAIL_FUTEX */

#ifdef CONFIG_COMPAT
static void compat_exit_robust_list(struct task_struct *curr);
#endif

/*
 * Reflects a new waiter being added to the waitqueue.
 */
static inline void hb_waiters_inc(struct futex_hash_bucket *hb)
{
#ifdef CONFIG_SMP
        atomic_inc(&hb->waiters);
        /*
         * Full barrier (A), see the ordering comment above.
         */
        smp_mb__after_atomic();
#endif
}

/*
 * Reflects a waiter being removed from the waitqueue by wakeup
 * paths.
 */
static inline void hb_waiters_dec(struct futex_hash_bucket *hb)
{
#ifdef CONFIG_SMP
        atomic_dec(&hb->waiters);
#endif
}

static inline int hb_waiters_pending(struct futex_hash_bucket *hb)
{
#ifdef CONFIG_SMP
        /*
         * Full barrier (B), see the ordering comment above.
         */
        smp_mb();
        return atomic_read(&hb->waiters);
#else
        return 1;
#endif
}

/**
 * hash_futex - Return the hash bucket in the global hash
 * @key:        Pointer to the futex key for which the hash is calculated
 *
 * We hash on the keys returned from get_futex_key (see below) and return the
 * corresponding hash bucket in the global hash.
 */
static struct futex_hash_bucket *hash_futex(union futex_key *key)
{
        u32 hash = jhash2((u32 *)key, offsetof(typeof(*key), both.offset) / 4,
                          key->both.offset);

        return &futex_queues[hash & (futex_hashsize - 1)];
}


/**
 * match_futex - Check whether two futex keys are equal
 * @key1:       Pointer to key1
 * @key2:       Pointer to key2
 *
 * Return 1 if two futex_keys are equal, 0 otherwise.
 */
static inline int match_futex(union futex_key *key1, union futex_key *key2)
{
        return (key1 && key2
                && key1->both.word == key2->both.word
                && key1->both.ptr == key2->both.ptr
                && key1->both.offset == key2->both.offset);
}

enum futex_access {
        FUTEX_READ,
        FUTEX_WRITE
};

/**
 * futex_setup_timer - set up the sleeping hrtimer.
 * @time:       ptr to the given timeout value
 * @timeout:    the hrtimer_sleeper structure to be set up
 * @flags:      futex flags
 * @range_ns:   optional range in ns
 *
 * Return: Initialized hrtimer_sleeper structure or NULL if no timeout
 *         value given
 */
static inline struct hrtimer_sleeper *
futex_setup_timer(ktime_t *time, struct hrtimer_sleeper *timeout,
                  int flags, u64 range_ns)
{
        if (!time)
                return NULL;

        hrtimer_init_sleeper_on_stack(timeout, (flags & FLAGS_CLOCKRT) ?
                                      CLOCK_REALTIME : CLOCK_MONOTONIC,
                                      HRTIMER_MODE_ABS);
        /*
         * If range_ns is 0, calling hrtimer_set_expires_range_ns() is
         * effectively the same as calling hrtimer_set_expires().
         */
        hrtimer_set_expires_range_ns(&timeout->timer, *time, range_ns);

        return timeout;
}

/*
 * Generate a machine wide unique identifier for this inode.
 *
 * This relies on u64 not wrapping in the life-time of the machine; which with
 * 1ns resolution means almost 585 years.
 *
 * This further relies on the fact that a well formed program will not unmap
 * the file while it has a (shared) futex waiting on it. This mapping will have
 * a file reference which pins the mount and inode.
 *
 * If for some reason an inode gets evicted and read back in again, it will get
 * a new sequence number and will _NOT_ match, even though it is the exact same
 * file.
 *
 * It is important that match_futex() will never have a false-positive, esp.
 * for PI futexes that can mess up the state. The above argues that false-negatives
 * are only possible for malformed programs.
 */
static u64 get_inode_sequence_number(struct inode *inode)
{
        static atomic64_t i_seq;
        u64 old;

        /* Does the inode already have a sequence number? */
        old = atomic64_read(&inode->i_sequence);
        if (likely(old))
                return old;

        for (;;) {
                u64 new = atomic64_add_return(1, &i_seq);
                if (WARN_ON_ONCE(!new))
                        continue;

                old = atomic64_cmpxchg_relaxed(&inode->i_sequence, 0, new);
                if (old)
                        return old;
                return new;
        }
}

/**
 * get_futex_key() - Get parameters which are the keys for a futex
 * @uaddr:      virtual address of the futex
 * @fshared:    false for a PROCESS_PRIVATE futex, true for PROCESS_SHARED
 * @key:        address where result is stored.
 * @rw:         mapping needs to be read/write (values: FUTEX_READ,
 *              FUTEX_WRITE)
 *
 * Return: a negative error code or 0
 *
 * The key words are stored in @key on success.
 *
 * For shared mappings (when @fshared), the key is:
 *
 *   ( inode->i_sequence, page->index, offset_within_page )
 *
 * [ also see get_inode_sequence_number() ]
 *
 * For private mappings (or when !@fshared), the key is:
 *
 *   ( current->mm, address, 0 )
 *
 * This allows (cross process, where applicable) identification of the futex
 * without keeping the page pinned for the duration of the FUTEX_WAIT.
 *
 * lock_page() might sleep, the caller should not hold a spinlock.
 */
static int get_futex_key(u32 __user *uaddr, bool fshared, union futex_key *key,
                         enum futex_access rw)
{
        unsigned long address = (unsigned long)uaddr;
        struct mm_struct *mm = current->mm;
        struct page *page, *tail;
        struct address_space *mapping;
        int err, ro = 0;

        /*
         * The futex address must be "naturally" aligned.
         */
        key->both.offset = address % PAGE_SIZE;
        if (unlikely((address % sizeof(u32)) != 0))
                return -EINVAL;
        address -= key->both.offset;

        if (unlikely(!access_ok(uaddr, sizeof(u32))))
                return -EFAULT;

        if (unlikely(should_fail_futex(fshared)))
                return -EFAULT;

        /*
         * PROCESS_PRIVATE futexes are fast.
         * As the mm cannot disappear under us and the 'key' only needs
         * virtual address, we dont even have to find the underlying vma.
         * Note : We do have to check 'uaddr' is a valid user address,
         *        but access_ok() should be faster than find_vma()
         */
        if (!fshared) {
                key->private.mm = mm;
                key->private.address = address;
                return 0;
        }

again:
        /* Ignore any VERIFY_READ mapping (futex common case) */
        if (unlikely(should_fail_futex(true)))
                return -EFAULT;

        err = get_user_pages_fast(address, 1, FOLL_WRITE, &page);
        /*
         * If write access is not required (eg. FUTEX_WAIT), try
         * and get read-only access.
         */
        if (err == -EFAULT && rw == FUTEX_READ) {
                err = get_user_pages_fast(address, 1, 0, &page);
                ro = 1;
        }
        if (err < 0)
                return err;
        else
                err = 0;

        /*
         * The treatment of mapping from this point on is critical. The page
         * lock protects many things but in this context the page lock
         * stabilizes mapping, prevents inode freeing in the shared
         * file-backed region case and guards against movement to swap cache.
         *
         * Strictly speaking the page lock is not needed in all cases being
         * considered here and page lock forces unnecessarily serialization
         * From this point on, mapping will be re-verified if necessary and
         * page lock will be acquired only if it is unavoidable
         *
         * Mapping checks require the head page for any compound page so the
         * head page and mapping is looked up now. For anonymous pages, it
         * does not matter if the page splits in the future as the key is
         * based on the address. For filesystem-backed pages, the tail is
         * required as the index of the page determines the key. For
         * base pages, there is no tail page and tail == page.
         */
        tail = page;
        page = compound_head(page);
        mapping = READ_ONCE(page->mapping);

        /*
         * If page->mapping is NULL, then it cannot be a PageAnon
         * page; but it might be the ZERO_PAGE or in the gate area or
         * in a special mapping (all cases which we are happy to fail);
         * or it may have been a good file page when get_user_pages_fast
         * found it, but truncated or holepunched or subjected to
         * invalidate_complete_page2 before we got the page lock (also
         * cases which we are happy to fail).  And we hold a reference,
         * so refcount care in invalidate_complete_page's remove_mapping
         * prevents drop_caches from setting mapping to NULL beneath us.
         *
         * The case we do have to guard against is when memory pressure made
         * shmem_writepage move it from filecache to swapcache beneath us:
         * an unlikely race, but we do need to retry for page->mapping.
         */
        if (unlikely(!mapping)) {
                int shmem_swizzled;

                /*
                 * Page lock is required to identify which special case above
                 * applies. If this is really a shmem page then the page lock
                 * will prevent unexpected transitions.
                 */
                lock_page(page);
                shmem_swizzled = PageSwapCache(page) || page->mapping;
                unlock_page(page);
                put_page(page);

                if (shmem_swizzled)
                        goto again;

                return -EFAULT;
        }

        /*
         * Private mappings are handled in a simple way.
         *
         * If the futex key is stored on an anonymous page, then the associated
         * object is the mm which is implicitly pinned by the calling process.
         *
         * NOTE: When userspace waits on a MAP_SHARED mapping, even if
         * it's a read-only handle, it's expected that futexes attach to
         * the object not the particular process.
         */
        if (PageAnon(page)) {
                /*
                 * A RO anonymous page will never change and thus doesn't make
                 * sense for futex operations.
                 */
                if (unlikely(should_fail_futex(true)) || ro) {
                        err = -EFAULT;
                        goto out;
                }

                key->both.offset |= FUT_OFF_MMSHARED; /* ref taken on mm */
                key->private.mm = mm;
                key->private.address = address;

        } else {
                struct inode *inode;

                /*
                 * The associated futex object in this case is the inode and
                 * the page->mapping must be traversed. Ordinarily this should
                 * be stabilised under page lock but it's not strictly
                 * necessary in this case as we just want to pin the inode, not
                 * update the radix tree or anything like that.
                 *
                 * The RCU read lock is taken as the inode is finally freed
                 * under RCU. If the mapping still matches expectations then the
                 * mapping->host can be safely accessed as being a valid inode.
                 */
                rcu_read_lock();

                if (READ_ONCE(page->mapping) != mapping) {
                        rcu_read_unlock();
                        put_page(page);

                        goto again;
                }

                inode = READ_ONCE(mapping->host);
                if (!inode) {
                        rcu_read_unlock();
                        put_page(page);

                        goto again;
                }

                key->both.offset |= FUT_OFF_INODE; /* inode-based key */
                key->shared.i_seq = get_inode_sequence_number(inode);
                key->shared.pgoff = page_to_pgoff(tail);
                rcu_read_unlock();
        }

out:
        put_page(page);
        return err;
}

/**
 * fault_in_user_writeable() - Fault in user address and verify RW access
 * @uaddr:      pointer to faulting user space address
 *
 * Slow path to fixup the fault we just took in the atomic write
 * access to @uaddr.
 *
 * We have no generic implementation of a non-destructive write to the
 * user address. We know that we faulted in the atomic pagefault
 * disabled section so we can as well avoid the #PF overhead by
 * calling get_user_pages() right away.
 */
static int fault_in_user_writeable(u32 __user *uaddr)
{
        struct mm_struct *mm = current->mm;
        int ret;

        mmap_read_lock(mm);
        ret = fixup_user_fault(mm, (unsigned long)uaddr,
                               FAULT_FLAG_WRITE, NULL);
        mmap_read_unlock(mm);

        return ret < 0 ? ret : 0;
}

/**
 * futex_top_waiter() - Return the highest priority waiter on a futex
 * @hb:         the hash bucket the futex_q's reside in
 * @key:        the futex key (to distinguish it from other futex futex_q's)
 *
 * Must be called with the hb lock held.
 */
static struct futex_q *futex_top_waiter(struct futex_hash_bucket *hb,
                                        union futex_key *key)
{
        struct futex_q *this;

        plist_for_each_entry(this, &hb->chain, list) {
                if (match_futex(&this->key, key))
                        return this;
        }
        return NULL;
}

static int cmpxchg_futex_value_locked(u32 *curval, u32 __user *uaddr,
                                      u32 uval, u32 newval)
{
        int ret;

        pagefault_disable();
        ret = futex_atomic_cmpxchg_inatomic(curval, uaddr, uval, newval);
        pagefault_enable();

        return ret;
}

static int get_futex_value_locked(u32 *dest, u32 __user *from)
{
        int ret;

        pagefault_disable();
        ret = __get_user(*dest, from);
        pagefault_enable();

        return ret ? -EFAULT : 0;
}


/*
 * PI code:
 */
static int refill_pi_state_cache(void)
{
        struct futex_pi_state *pi_state;

        if (likely(current->pi_state_cache))
                return 0;

        pi_state = kzalloc(sizeof(*pi_state), GFP_KERNEL);

        if (!pi_state)
                return -ENOMEM;

        INIT_LIST_HEAD(&pi_state->list);
        /* pi_mutex gets initialized later */
        pi_state->owner = NULL;
        refcount_set(&pi_state->refcount, 1);
        pi_state->key = FUTEX_KEY_INIT;

        current->pi_state_cache = pi_state;

        return 0;
}

static struct futex_pi_state *alloc_pi_state(void)
{
        struct futex_pi_state *pi_state = current->pi_state_cache;

        WARN_ON(!pi_state);
        current->pi_state_cache = NULL;

        return pi_state;
}

static void pi_state_update_owner(struct futex_pi_state *pi_state,
                                  struct task_struct *new_owner)
{
        struct task_struct *old_owner = pi_state->owner;

        lockdep_assert_held(&pi_state->pi_mutex.wait_lock);

        if (old_owner) {
                raw_spin_lock(&old_owner->pi_lock);
                WARN_ON(list_empty(&pi_state->list));
                list_del_init(&pi_state->list);
                raw_spin_unlock(&old_owner->pi_lock);
        }

        if (new_owner) {
                raw_spin_lock(&new_owner->pi_lock);
                WARN_ON(!list_empty(&pi_state->list));
                list_add(&pi_state->list, &new_owner->pi_state_list);
                pi_state->owner = new_owner;
                raw_spin_unlock(&new_owner->pi_lock);
        }
}

static void get_pi_state(struct futex_pi_state *pi_state)
{
        WARN_ON_ONCE(!refcount_inc_not_zero(&pi_state->refcount));
}

/*
 * Drops a reference to the pi_state object and frees or caches it
 * when the last reference is gone.
 */
static void put_pi_state(struct futex_pi_state *pi_state)
{
        if (!pi_state)
                return;

        if (!refcount_dec_and_test(&pi_state->refcount))
                return;

        /*
         * If pi_state->owner is NULL, the owner is most probably dying
         * and has cleaned up the pi_state already
         */
        if (pi_state->owner) {
                unsigned long flags;

                raw_spin_lock_irqsave(&pi_state->pi_mutex.wait_lock, flags);
                pi_state_update_owner(pi_state, NULL);
                rt_mutex_proxy_unlock(&pi_state->pi_mutex);
                raw_spin_unlock_irqrestore(&pi_state->pi_mutex.wait_lock, flags);
        }

        if (current->pi_state_cache) {
                kfree(pi_state);
        } else {
                /*
                 * pi_state->list is already empty.
                 * clear pi_state->owner.
                 * refcount is at 0 - put it back to 1.
                 */
                pi_state->owner = NULL;
                refcount_set(&pi_state->refcount, 1);
                current->pi_state_cache = pi_state;
        }
}

#ifdef CONFIG_FUTEX_PI

/*
 * This task is holding PI mutexes at exit time => bad.
 * Kernel cleans up PI-state, but userspace is likely hosed.
 * (Robust-futex cleanup is separate and might save the day for userspace.)
 */
static void exit_pi_state_list(struct task_struct *curr)
{
        struct list_head *next, *head = &curr->pi_state_list;
        struct futex_pi_state *pi_state;
        struct futex_hash_bucket *hb;
        union futex_key key = FUTEX_KEY_INIT;

        if (!futex_cmpxchg_enabled)
                return;
        /*
         * We are a ZOMBIE and nobody can enqueue itself on
         * pi_state_list anymore, but we have to be careful
         * versus waiters unqueueing themselves:
         */
        raw_spin_lock_irq(&curr->pi_lock);
        while (!list_empty(head)) {
                next = head->next;
                pi_state = list_entry(next, struct futex_pi_state, list);
                key = pi_state->key;
                hb = hash_futex(&key);

                /*
                 * We can race against put_pi_state() removing itself from the
                 * list (a waiter going away). put_pi_state() will first
                 * decrement the reference count and then modify the list, so
                 * its possible to see the list entry but fail this reference
                 * acquire.
                 *
                 * In that case; drop the locks to let put_pi_state() make
                 * progress and retry the loop.
                 */
                if (!refcount_inc_not_zero(&pi_state->refcount)) {
                        raw_spin_unlock_irq(&curr->pi_lock);
                        cpu_relax();
                        raw_spin_lock_irq(&curr->pi_lock);
                        continue;
                }
                raw_spin_unlock_irq(&curr->pi_lock);

                spin_lock(&hb->lock);
                raw_spin_lock_irq(&pi_state->pi_mutex.wait_lock);
                raw_spin_lock(&curr->pi_lock);
                /*
                 * We dropped the pi-lock, so re-check whether this
                 * task still owns the PI-state:
                 */
                if (head->next != next) {
                        /* retain curr->pi_lock for the loop invariant */
                        raw_spin_unlock(&pi_state->pi_mutex.wait_lock);
                        spin_unlock(&hb->lock);
                        put_pi_state(pi_state);
                        continue;
                }

                WARN_ON(pi_state->owner != curr);
                WARN_ON(list_empty(&pi_state->list));
                list_del_init(&pi_state->list);
                pi_state->owner = NULL;

                raw_spin_unlock(&curr->pi_lock);
                raw_spin_unlock_irq(&pi_state->pi_mutex.wait_lock);
                spin_unlock(&hb->lock);

                rt_mutex_futex_unlock(&pi_state->pi_mutex);
                put_pi_state(pi_state);

                raw_spin_lock_irq(&curr->pi_lock);
        }
        raw_spin_unlock_irq(&curr->pi_lock);
}
#else
static inline void exit_pi_state_list(struct task_struct *curr) { }
#endif

/*
 * We need to check the following states:
 *
 *      Waiter | pi_state | pi->owner | uTID      | uODIED | ?
 *
 * [1]  NULL   | ---      | ---       | 0         | 0/1    | Valid
 * [2]  NULL   | ---      | ---       | >0        | 0/1    | Valid
 *
 * [3]  Found  | NULL     | --        | Any       | 0/1    | Invalid
 *
 * [4]  Found  | Found    | NULL      | 0         | 1      | Valid
 * [5]  Found  | Found    | NULL      | >0        | 1      | Invalid
 *
 * [6]  Found  | Found    | task      | 0         | 1      | Valid
 *
 * [7]  Found  | Found    | NULL      | Any       | 0      | Invalid
 *
 * [8]  Found  | Found    | task      | ==taskTID | 0/1    | Valid
 * [9]  Found  | Found    | task      | 0         | 0      | Invalid
 * [10] Found  | Found    | task      | !=taskTID | 0/1    | Invalid
 *
 * [1]  Indicates that the kernel can acquire the futex atomically. We
 *      came here due to a stale FUTEX_WAITERS/FUTEX_OWNER_DIED bit.
 *
 * [2]  Valid, if TID does not belong to a kernel thread. If no matching
 *      thread is found then it indicates that the owner TID has died.
 *
 * [3]  Invalid. The waiter is queued on a non PI futex
 *
 * [4]  Valid state after exit_robust_list(), which sets the user space
 *      value to FUTEX_WAITERS | FUTEX_OWNER_DIED.
 *
 * [5]  The user space value got manipulated between exit_robust_list()
 *      and exit_pi_state_list()
 *
 * [6]  Valid state after exit_pi_state_list() which sets the new owner in
 *      the pi_state but cannot access the user space value.
 *
 * [7]  pi_state->owner can only be NULL when the OWNER_DIED bit is set.
 *
 * [8]  Owner and user space value match
 *
 * [9]  There is no transient state which sets the user space TID to 0
 *      except exit_robust_list(), but this is indicated by the
 *      FUTEX_OWNER_DIED bit. See [4]
 *
 * [10] There is no transient state which leaves owner and user space
 *      TID out of sync. Except one error case where the kernel is denied
 *      write access to the user address, see fixup_pi_state_owner().
 *
 *
 * Serialization and lifetime rules:
 *
 * hb->lock:
 *
 *      hb -> futex_q, relation
 *      futex_q -> pi_state, relation
 *
 *      (cannot be raw because hb can contain arbitrary amount
 *       of futex_q's)
 *
 * pi_mutex->wait_lock:
 *
 *      {uval, pi_state}
 *
 *      (and pi_mutex 'obviously')
 *
 * p->pi_lock:
 *
 *      p->pi_state_list -> pi_state->list, relation
 *      pi_mutex->owner -> pi_state->owner, relation
 *
 * pi_state->refcount:
 *
 *      pi_state lifetime
 *
 *
 * Lock order:
 *
 *   hb->lock
 *     pi_mutex->wait_lock
 *       p->pi_lock
 *
 */

/*
 * Validate that the existing waiter has a pi_state and sanity check
 * the pi_state against the user space value. If correct, attach to
 * it.
 */
static int attach_to_pi_state(u32 __user *uaddr, u32 uval,
                              struct futex_pi_state *pi_state,
                              struct futex_pi_state **ps)
{
        pid_t pid = uval & FUTEX_TID_MASK;
        u32 uval2;
        int ret;

        /*
         * Userspace might have messed up non-PI and PI futexes [3]
         */
        if (unlikely(!pi_state))
                return -EINVAL;

        /*
         * We get here with hb->lock held, and having found a
         * futex_top_waiter(). This means that futex_lock_pi() of said futex_q
         * has dropped the hb->lock in between queue_me() and unqueue_me_pi(),
         * which in turn means that futex_lock_pi() still has a reference on
         * our pi_state.
         *
         * The waiter holding a reference on @pi_state also protects against
         * the unlocked put_pi_state() in futex_unlock_pi(), futex_lock_pi()
         * and futex_wait_requeue_pi() as it cannot go to 0 and consequently
         * free pi_state before we can take a reference ourselves.
         */
        WARN_ON(!refcount_read(&pi_state->refcount));

        /*
         * Now that we have a pi_state, we can acquire wait_lock
         * and do the state validation.
         */
        raw_spin_lock_irq(&pi_state->pi_mutex.wait_lock);

        /*
         * Since {uval, pi_state} is serialized by wait_lock, and our current
         * uval was read without holding it, it can have changed. Verify it
         * still is what we expect it to be, otherwise retry the entire
         * operation.
         */
        if (get_futex_value_locked(&uval2, uaddr))
                goto out_efault;

        if (uval != uval2)
                goto out_eagain;

        /*
         * Handle the owner died case:
         */
        if (uval & FUTEX_OWNER_DIED) {
                /*
                 * exit_pi_state_list sets owner to NULL and wakes the
                 * topmost waiter. The task which acquires the
                 * pi_state->rt_mutex will fixup owner.
                 */
                if (!pi_state->owner) {
                        /*
                         * No pi state owner, but the user space TID
                         * is not 0. Inconsistent state. [5]
                         */
                        if (pid)
                                goto out_einval;
                        /*
                         * Take a ref on the state and return success. [4]
                         */
                        goto out_attach;
                }

                /*
                 * If TID is 0, then either the dying owner has not
                 * yet executed exit_pi_state_list() or some waiter
                 * acquired the rtmutex in the pi state, but did not
                 * yet fixup the TID in user space.
                 *
                 * Take a ref on the state and return success. [6]
                 */
                if (!pid)
                        goto out_attach;
        } else {
                /*
                 * If the owner died bit is not set, then the pi_state
                 * must have an owner. [7]
                 */
                if (!pi_state->owner)
                        goto out_einval;
        }

        /*
         * Bail out if user space manipulated the futex value. If pi
         * state exists then the owner TID must be the same as the
         * user space TID. [9/10]
         */
        if (pid != task_pid_vnr(pi_state->owner))
                goto out_einval;

out_attach:
        get_pi_state(pi_state);
        raw_spin_unlock_irq(&pi_state->pi_mutex.wait_lock);
        *ps = pi_state;
        return 0;

out_einval:
        ret = -EINVAL;
        goto out_error;

out_eagain:
        ret = -EAGAIN;
        goto out_error;

out_efault:
        ret = -EFAULT;
        goto out_error;

out_error:
        raw_spin_unlock_irq(&pi_state->pi_mutex.wait_lock);
        return ret;
}

/**
 * wait_for_owner_exiting - Block until the owner has exited
 * @ret: owner's current futex lock status
 * @exiting:    Pointer to the exiting task
 *
 * Caller must hold a refcount on @exiting.
 */
static void wait_for_owner_exiting(int ret, struct task_struct *exiting)
{
        if (ret != -EBUSY) {
                WARN_ON_ONCE(exiting);
                return;
        }

        if (WARN_ON_ONCE(ret == -EBUSY && !exiting))
                return;

        mutex_lock(&exiting->futex_exit_mutex);
        /*
         * No point in doing state checking here. If the waiter got here
         * while the task was in exec()->exec_futex_release() then it can
         * have any FUTEX_STATE_* value when the waiter has acquired the
         * mutex. OK, if running, EXITING or DEAD if it reached exit()
         * already. Highly unlikely and not a problem. Just one more round
         * through the futex maze.
         */
        mutex_unlock(&exiting->futex_exit_mutex);

        put_task_struct(exiting);
}

static int handle_exit_race(u32 __user *uaddr, u32 uval,
                            struct task_struct *tsk)
{
        u32 uval2;

        /*
         * If the futex exit state is not yet FUTEX_STATE_DEAD, tell the
         * caller that the alleged owner is busy.
         */
        if (tsk && tsk->futex_state != FUTEX_STATE_DEAD)
                return -EBUSY;

        /*
         * Reread the user space value to handle the following situation:
         *
         * CPU0                         CPU1
         *
         * sys_exit()                   sys_futex()
         *  do_exit()                    futex_lock_pi()
         *                                futex_lock_pi_atomic()
         *   exit_signals(tsk)              No waiters:
         *    tsk->flags |= PF_EXITING;     *uaddr == 0x00000PID
         *  mm_release(tsk)                 Set waiter bit
         *   exit_robust_list(tsk) {        *uaddr = 0x80000PID;
         *      Set owner died              attach_to_pi_owner() {
         *    *uaddr = 0xC0000000;           tsk = get_task(PID);
         *   }                               if (!tsk->flags & PF_EXITING) {
         *  ...                                attach();
         *  tsk->futex_state =               } else {
         *      FUTEX_STATE_DEAD;              if (tsk->futex_state !=
         *                                        FUTEX_STATE_DEAD)
         *                                       return -EAGAIN;
         *                                     return -ESRCH; <--- FAIL
         *                                   }
         *
         * Returning ESRCH unconditionally is wrong here because the
         * user space value has been changed by the exiting task.
         *
         * The same logic applies to the case where the exiting task is
         * already gone.
         */
        if (get_futex_value_locked(&uval2, uaddr))
                return -EFAULT;

        /* If the user space value has changed, try again. */
        if (uval2 != uval)
                return -EAGAIN;

        /*
         * The exiting task did not have a robust list, the robust list was
         * corrupted or the user space value in *uaddr is simply bogus.
         * Give up and tell user space.
         */
        return -ESRCH;
}

static void __attach_to_pi_owner(struct task_struct *p, union futex_key *key,
                                 struct futex_pi_state **ps)
{
        /*
         * No existing pi state. First waiter. [2]
         *
         * This creates pi_state, we have hb->lock held, this means nothing can
         * observe this state, wait_lock is irrelevant.
         */
        struct futex_pi_state *pi_state = alloc_pi_state();

        /*
         * Initialize the pi_mutex in locked state and make @p
         * the owner of it:
         */
        rt_mutex_init_proxy_locked(&pi_state->pi_mutex, p);

        /* Store the key for possible exit cleanups: */
        pi_state->key = *key;

        WARN_ON(!list_empty(&pi_state->list));
        list_add(&pi_state->list, &p->pi_state_list);
        /*
         * Assignment without holding pi_state->pi_mutex.wait_lock is safe
         * because there is no concurrency as the object is not published yet.
         */
        pi_state->owner = p;

        *ps = pi_state;
}
/*
 * Lookup the task for the TID provided from user space and attach to
 * it after doing proper sanity checks.
 */
static int attach_to_pi_owner(u32 __user *uaddr, u32 uval, union futex_key *key,
                              struct futex_pi_state **ps,
                              struct task_struct **exiting)
{
        pid_t pid = uval & FUTEX_TID_MASK;
        struct task_struct *p;

        /*
         * We are the first waiter - try to look up the real owner and attach
         * the new pi_state to it, but bail out when TID = 0 [1]
         *
         * The !pid check is paranoid. None of the call sites should end up
         * with pid == 0, but better safe than sorry. Let the caller retry
         */
        if (!pid)
                return -EAGAIN;
        p = find_get_task_by_vpid(pid);
        if (!p)
                return handle_exit_race(uaddr, uval, NULL);

        if (unlikely(p->flags & PF_KTHREAD)) {
                put_task_struct(p);
                return -EPERM;
        }

        /*
         * We need to look at the task state to figure out, whether the
         * task is exiting. To protect against the change of the task state
         * in futex_exit_release(), we do this protected by p->pi_lock:
         */
        raw_spin_lock_irq(&p->pi_lock);
        if (unlikely(p->futex_state != FUTEX_STATE_OK)) {
                /*
                 * The task is on the way out. When the futex state is
                 * FUTEX_STATE_DEAD, we know that the task has finished
                 * the cleanup:
                 */
                int ret = handle_exit_race(uaddr, uval, p);

                raw_spin_unlock_irq(&p->pi_lock);
                /*
                 * If the owner task is between FUTEX_STATE_EXITING and
                 * FUTEX_STATE_DEAD then store the task pointer and keep
                 * the reference on the task struct. The calling code will
                 * drop all locks, wait for the task to reach
                 * FUTEX_STATE_DEAD and then drop the refcount. This is
                 * required to prevent a live lock when the current task
                 * preempted the exiting task between the two states.
                 */
                if (ret == -EBUSY)
                        *exiting = p;
                else
                        put_task_struct(p);
                return ret;
        }

        __attach_to_pi_owner(p, key, ps);
        raw_spin_unlock_irq(&p->pi_lock);

        put_task_struct(p);

        return 0;
}

static int lock_pi_update_atomic(u32 __user *uaddr, u32 uval, u32 newval)
{
        int err;
        u32 curval;

        if (unlikely(should_fail_futex(true)))
                return -EFAULT;

        err = cmpxchg_futex_value_locked(&curval, uaddr, uval, newval);
        if (unlikely(err))
                return err;

        /* If user space value changed, let the caller retry */
        return curval != uval ? -EAGAIN : 0;
}

/**
 * futex_lock_pi_atomic() - Atomic work required to acquire a pi aware futex
 * @uaddr:              the pi futex user address
 * @hb:                 the pi futex hash bucket
 * @key:                the futex key associated with uaddr and hb
 * @ps:                 the pi_state pointer where we store the result of the
 *                      lookup
 * @task:               the task to perform the atomic lock work for.  This will
 *                      be "current" except in the case of requeue pi.
 * @exiting:            Pointer to store the task pointer of the owner task
 *                      which is in the middle of exiting
 * @set_waiters:        force setting the FUTEX_WAITERS bit (1) or not (0)
 *
 * Return:
 *  -  0 - ready to wait;
 *  -  1 - acquired the lock;
 *  - <0 - error
 *
 * The hb->lock must be held by the caller.
 *
 * @exiting is only set when the return value is -EBUSY. If so, this holds
 * a refcount on the exiting task on return and the caller needs to drop it
 * after waiting for the exit to complete.
 */
static int futex_lock_pi_atomic(u32 __user *uaddr, struct futex_hash_bucket *hb,
                                union futex_key *key,
                                struct futex_pi_state **ps,
                                struct task_struct *task,
                                struct task_struct **exiting,
                                int set_waiters)
{
        u32 uval, newval, vpid = task_pid_vnr(task);
        struct futex_q *top_waiter;
        int ret;

        /*
         * Read the user space value first so we can validate a few
         * things before proceeding further.
         */
        if (get_futex_value_locked(&uval, uaddr))
                return -EFAULT;

        if (unlikely(should_fail_futex(true)))
                return -EFAULT;

        /*
         * Detect deadlocks.
         */
        if ((unlikely((uval & FUTEX_TID_MASK) == vpid)))
                return -EDEADLK;

        if ((unlikely(should_fail_futex(true))))
                return -EDEADLK;

        /*
         * Lookup existing state first. If it exists, try to attach to
         * its pi_state.
         */
        top_waiter = futex_top_waiter(hb, key);
        if (top_waiter)
                return attach_to_pi_state(uaddr, uval, top_waiter->pi_state, ps);

        /*
         * No waiter and user TID is 0. We are here because the
         * waiters or the owner died bit is set or called from
         * requeue_cmp_pi or for whatever reason something took the
         * syscall.
         */
        if (!(uval & FUTEX_TID_MASK)) {
                /*
                 * We take over the futex. No other waiters and the user space
                 * TID is 0. We preserve the owner died bit.
                 */
                newval = uval & FUTEX_OWNER_DIED;
                newval |= vpid;

                /* The futex requeue_pi code can enforce the waiters bit */
                if (set_waiters)
                        newval |= FUTEX_WAITERS;

                ret = lock_pi_update_atomic(uaddr, uval, newval);
                if (ret)
                        return ret;

                /*
                 * If the waiter bit was requested the caller also needs PI
                 * state attached to the new owner of the user space futex.
                 *
                 * @task is guaranteed to be alive and it cannot be exiting
                 * because it is either sleeping or waiting in
                 * futex_requeue_pi_wakeup_sync().
                 *
                 * No need to do the full attach_to_pi_owner() exercise
                 * because @task is known and valid.
                 */
                if (set_waiters) {
                        raw_spin_lock_irq(&task->pi_lock);
                        __attach_to_pi_owner(task, key, ps);
                        raw_spin_unlock_irq(&task->pi_lock);
                }
                return 1;
        }

        /*
         * First waiter. Set the waiters bit before attaching ourself to
         * the owner. If owner tries to unlock, it will be forced into
         * the kernel and blocked on hb->lock.
         */
        newval = uval | FUTEX_WAITERS;
        ret = lock_pi_update_atomic(uaddr, uval, newval);
        if (ret)
                return ret;
        /*
         * If the update of the user space value succeeded, we try to
         * attach to the owner. If that fails, no harm done, we only
         * set the FUTEX_WAITERS bit in the user space variable.
         */
        return attach_to_pi_owner(uaddr, newval, key, ps, exiting);
}

/**
 * __unqueue_futex() - Remove the futex_q from its futex_hash_bucket
 * @q:  The futex_q to unqueue
 *
 * The q->lock_ptr must not be NULL and must be held by the caller.
 */
static void __unqueue_futex(struct futex_q *q)
{
        struct futex_hash_bucket *hb;

        if (WARN_ON_SMP(!q->lock_ptr) || WARN_ON(plist_node_empty(&q->list)))
                return;
        lockdep_assert_held(q->lock_ptr);

        hb = container_of(q->lock_ptr, struct futex_hash_bucket, lock);
        plist_del(&q->list, &hb->chain);
        hb_waiters_dec(hb);
}

/*
 * The hash bucket lock must be held when this is called.
 * Afterwards, the futex_q must not be accessed. Callers
 * must ensure to later call wake_up_q() for the actual
 * wakeups to occur.
 */
static void mark_wake_futex(struct wake_q_head *wake_q, struct futex_q *q)
{
        struct task_struct *p = q->task;

        if (WARN(q->pi_state || q->rt_waiter, "refusing to wake PI futex\n"))
                return;

        get_task_struct(p);
        __unqueue_futex(q);
        /*
         * The waiting task can free the futex_q as soon as q->lock_ptr = NULL
         * is written, without taking any locks. This is possible in the event
         * of a spurious wakeup, for example. A memory barrier is required here
         * to prevent the following store to lock_ptr from getting ahead of the
         * plist_del in __unqueue_futex().
         */
        smp_store_release(&q->lock_ptr, NULL);

        /*
         * Queue the task for later wakeup for after we've released
         * the hb->lock.
         */
        wake_q_add_safe(wake_q, p);
}

/*
 * Caller must hold a reference on @pi_state.
 */
static int wake_futex_pi(u32 __user *uaddr, u32 uval, struct futex_pi_state *pi_state)
{
        struct rt_mutex_waiter *top_waiter;
        struct task_struct *new_owner;
        bool postunlock = false;
        DEFINE_RT_WAKE_Q(wqh);
        u32 curval, newval;
        int ret = 0;

        top_waiter = rt_mutex_top_waiter(&pi_state->pi_mutex);
        if (WARN_ON_ONCE(!top_waiter)) {
                /*
                 * As per the comment in futex_unlock_pi() this should not happen.
                 *
                 * When this happens, give up our locks and try again, giving
                 * the futex_lock_pi() instance time to complete, either by
                 * waiting on the rtmutex or removing itself from the futex
                 * queue.
                 */
                ret = -EAGAIN;
                goto out_unlock;
        }

        new_owner = top_waiter->task;

        /*
         * We pass it to the next owner. The WAITERS bit is always kept
         * enabled while there is PI state around. We cleanup the owner
         * died bit, because we are the owner.
         */
        newval = FUTEX_WAITERS | task_pid_vnr(new_owner);

        if (unlikely(should_fail_futex(true))) {
                ret = -EFAULT;
                goto out_unlock;
        }

        ret = cmpxchg_futex_value_locked(&curval, uaddr, uval, newval);
        if (!ret && (curval != uval)) {
                /*
                 * If a unconditional UNLOCK_PI operation (user space did not
                 * try the TID->0 transition) raced with a waiter setting the
                 * FUTEX_WAITERS flag between get_user() and locking the hash
                 * bucket lock, retry the operation.
                 */
                if ((FUTEX_TID_MASK & curval) == uval)
                        ret = -EAGAIN;
                else
                        ret = -EINVAL;
        }

        if (!ret) {
                /*
                 * This is a point of no return; once we modified the uval
                 * there is no going back and subsequent operations must
                 * not fail.
                 */
                pi_state_update_owner(pi_state, new_owner);
                postunlock = __rt_mutex_futex_unlock(&pi_state->pi_mutex, &wqh);
        }

out_unlock:
        raw_spin_unlock_irq(&pi_state->pi_mutex.wait_lock);

        if (postunlock)
                rt_mutex_postunlock(&wqh);

        return ret;
}

/*
 * Express the locking dependencies for lockdep:
 */
static inline void
double_lock_hb(struct futex_hash_bucket *hb1, struct futex_hash_bucket *hb2)
{
        if (hb1 <= hb2) {
                spin_lock(&hb1->lock);
                if (hb1 < hb2)
                        spin_lock_nested(&hb2->lock, SINGLE_DEPTH_NESTING);
        } else { /* hb1 > hb2 */
                spin_lock(&hb2->lock);
                spin_lock_nested(&hb1->lock, SINGLE_DEPTH_NESTING);
        }
}

static inline void
double_unlock_hb(struct futex_hash_bucket *hb1, struct futex_hash_bucket *hb2)
{
        spin_unlock(&hb1->lock);
        if (hb1 != hb2)
                spin_unlock(&hb2->lock);
}

/*
 * Wake up waiters matching bitset queued on this futex (uaddr).
 */
static int
futex_wake(u32 __user *uaddr, unsigned int flags, int nr_wake, u32 bitset)
{
        struct futex_hash_bucket *hb;
        struct futex_q *this, *next;
        union futex_key key = FUTEX_KEY_INIT;
        int ret;
        DEFINE_WAKE_Q(wake_q);

        if (!bitset)
                return -EINVAL;

        ret = get_futex_key(uaddr, flags & FLAGS_SHARED, &key, FUTEX_READ);
        if (unlikely(ret != 0))
                return ret;

        hb = hash_futex(&key);

        /* Make sure we really have tasks to wakeup */
        if (!hb_waiters_pending(hb))
                return ret;

        spin_lock(&hb->lock);

        plist_for_each_entry_safe(this, next, &hb->chain, list) {
                if (match_futex (&this->key, &key)) {
                        if (this->pi_state || this->rt_waiter) {
                                ret = -EINVAL;
                                break;
                        }

                        /* Check if one of the bits is set in both bitsets */
                        if (!(this->bitset & bitset))
                                continue;

                        mark_wake_futex(&wake_q, this);
                        if (++ret >= nr_wake)
                                break;
                }
        }

        spin_unlock(&hb->lock);
        wake_up_q(&wake_q);
        return ret;
}

static int futex_atomic_op_inuser(unsigned int encoded_op, u32 __user *uaddr)
{
        unsigned int op =         (encoded_op & 0x70000000) >> 28;
        unsigned int cmp =        (encoded_op & 0x0f000000) >> 24;
        int oparg = sign_extend32((encoded_op & 0x00fff000) >> 12, 11);
        int cmparg = sign_extend32(encoded_op & 0x00000fff, 11);
        int oldval, ret;

        if (encoded_op & (FUTEX_OP_OPARG_SHIFT << 28)) {
                if (oparg < 0 || oparg > 31) {
                        char comm[sizeof(current->comm)];
                        /*
                         * kill this print and return -EINVAL when userspace
                         * is sane again
                         */
                        pr_info_ratelimited("futex_wake_op: %s tries to shift op by %d; fix this program\n",
                                        get_task_comm(comm, current), oparg);
                        oparg &= 31;
                }
                oparg = 1 << oparg;
        }

        pagefault_disable();
        ret = arch_futex_atomic_op_inuser(op, oparg, &oldval, uaddr);
        pagefault_enable();
        if (ret)
                return ret;

        switch (cmp) {
        case FUTEX_OP_CMP_EQ:
                return oldval == cmparg;
        case FUTEX_OP_CMP_NE:
                return oldval != cmparg;
        case FUTEX_OP_CMP_LT:
                return oldval < cmparg;
        case FUTEX_OP_CMP_GE:
                return oldval >= cmparg;
        case FUTEX_OP_CMP_LE:
                return oldval <= cmparg;
        case FUTEX_OP_CMP_GT:
                return oldval > cmparg;
        default:
                return -ENOSYS;
        }
}

/*
 * Wake up all waiters hashed on the physical page that is mapped
 * to this virtual address:
 */
static int
futex_wake_op(u32 __user *uaddr1, unsigned int flags, u32 __user *uaddr2,
              int nr_wake, int nr_wake2, int op)
{
        union futex_key key1 = FUTEX_KEY_INIT, key2 = FUTEX_KEY_INIT;
        struct futex_hash_bucket *hb1, *hb2;
        struct futex_q *this, *next;
        int ret, op_ret;
        DEFINE_WAKE_Q(wake_q);

retry:
        ret = get_futex_key(uaddr1, flags & FLAGS_SHARED, &key1, FUTEX_READ);
        if (unlikely(ret != 0))
                return ret;
        ret = get_futex_key(uaddr2, flags & FLAGS_SHARED, &key2, FUTEX_WRITE);
        if (unlikely(ret != 0))
                return ret;

        hb1 = hash_futex(&key1);
        hb2 = hash_futex(&key2);

retry_private:
        double_lock_hb(hb1, hb2);
        op_ret = futex_atomic_op_inuser(op, uaddr2);
        if (unlikely(op_ret < 0)) {
                double_unlock_hb(hb1, hb2);

                if (!IS_ENABLED(CONFIG_MMU) ||
                    unlikely(op_ret != -EFAULT && op_ret != -EAGAIN)) {
                        /*
                         * we don't get EFAULT from MMU faults if we don't have
                         * an MMU, but we might get them from range checking
                         */
                        ret = op_ret;
                        return ret;
                }

                if (op_ret == -EFAULT) {
                        ret = fault_in_user_writeable(uaddr2);
                        if (ret)
                                return ret;
                }

                cond_resched();
                if (!(flags & FLAGS_SHARED))
                        goto retry_private;
                goto retry;
        }

        plist_for_each_entry_safe(this, next, &hb1->chain, list) {
                if (match_futex (&this->key, &key1)) {
                        if (this->pi_state || this->rt_waiter) {
                                ret = -EINVAL;
                                goto out_unlock;
                        }
                        mark_wake_futex(&wake_q, this);
                        if (++ret >= nr_wake)
                                break;
                }
        }

        if (op_ret > 0) {
                op_ret = 0;
                plist_for_each_entry_safe(this, next, &hb2->chain, list) {
                        if (match_futex (&this->key, &key2)) {
                                if (this->pi_state || this->rt_waiter) {
                                        ret = -EINVAL;
                                        goto out_unlock;
                                }
                                mark_wake_futex(&wake_q, this);
                                if (++op_ret >= nr_wake2)
                                        break;
                        }
                }
                ret += op_ret;
        }

out_unlock:
        double_unlock_hb(hb1, hb2);
        wake_up_q(&wake_q);
        return ret;
}

/**
 * requeue_futex() - Requeue a futex_q from one hb to another
 * @q:          the futex_q to requeue
 * @hb1:        the source hash_bucket
 * @hb2:        the target hash_bucket
 * @key2:       the new key for the requeued futex_q
 */
static inline
void requeue_futex(struct futex_q *q, struct futex_hash_bucket *hb1,
                   struct futex_hash_bucket *hb2, union futex_key *key2)
{

        /*
         * If key1 and key2 hash to the same bucket, no need to
         * requeue.
         */
        if (likely(&hb1->chain != &hb2->chain)) {
                plist_del(&q->list, &hb1->chain);
                hb_waiters_dec(hb1);
                hb_waiters_inc(hb2);
                plist_add(&q->list, &hb2->chain);
                q->lock_ptr = &hb2->lock;
        }
        q->key = *key2;
}

static inline bool futex_requeue_pi_prepare(struct futex_q *q,
                                            struct futex_pi_state *pi_state)
{
        int old, new;

        /*
         * Set state to Q_REQUEUE_PI_IN_PROGRESS unless an early wakeup has
         * already set Q_REQUEUE_PI_IGNORE to signal that requeue should
         * ignore the waiter.
         */
        old = atomic_read_acquire(&q->requeue_state);
        do {
                if (old == Q_REQUEUE_PI_IGNORE)
                        return false;

                /*
                 * futex_proxy_trylock_atomic() might have set it to
                 * IN_PROGRESS and a interleaved early wake to WAIT.
                 *
                 * It was considered to have an extra state for that
                 * trylock, but that would just add more conditionals
                 * all over the place for a dubious value.
                 */
                if (old != Q_REQUEUE_PI_NONE)
                        break;

                new = Q_REQUEUE_PI_IN_PROGRESS;
        } while (!atomic_try_cmpxchg(&q->requeue_state, &old, new));

        q->pi_state = pi_state;
        return true;
}

static inline void futex_requeue_pi_complete(struct futex_q *q, int locked)
{
        int old, new;

        old = atomic_read_acquire(&q->requeue_state);
        do {
                if (old == Q_REQUEUE_PI_IGNORE)
                        return;

                if (locked >= 0) {
                        /* Requeue succeeded. Set DONE or LOCKED */
                        WARN_ON_ONCE(old != Q_REQUEUE_PI_IN_PROGRESS &&
                                     old != Q_REQUEUE_PI_WAIT);
                        new = Q_REQUEUE_PI_DONE + locked;
                } else if (old == Q_REQUEUE_PI_IN_PROGRESS) {
                        /* Deadlock, no early wakeup interleave */
                        new = Q_REQUEUE_PI_NONE;
                } else {
                        /* Deadlock, early wakeup interleave. */
                        WARN_ON_ONCE(old != Q_REQUEUE_PI_WAIT);
                        new = Q_REQUEUE_PI_IGNORE;
                }
        } while (!atomic_try_cmpxchg(&q->requeue_state, &old, new));

#ifdef CONFIG_PREEMPT_RT
        /* If the waiter interleaved with the requeue let it know */
        if (unlikely(old == Q_REQUEUE_PI_WAIT))
                rcuwait_wake_up(&q->requeue_wait);
#endif
}

static inline int futex_requeue_pi_wakeup_sync(struct futex_q *q)
{
        int old, new;

        old = atomic_read_acquire(&q->requeue_state);
        do {
                /* Is requeue done already? */
                if (old >= Q_REQUEUE_PI_DONE)
                        return old;

                /*
                 * If not done, then tell the requeue code to either ignore
                 * the waiter or to wake it up once the requeue is done.
                 */
                new = Q_REQUEUE_PI_WAIT;
                if (old == Q_REQUEUE_PI_NONE)
                        new = Q_REQUEUE_PI_IGNORE;
        } while (!atomic_try_cmpxchg(&q->requeue_state, &old, new));

        /* If the requeue was in progress, wait for it to complete */
        if (old == Q_REQUEUE_PI_IN_PROGRESS) {
#ifdef CONFIG_PREEMPT_RT
                rcuwait_wait_event(&q->requeue_wait,
                                   atomic_read(&q->requeue_state) != Q_REQUEUE_PI_WAIT,
                                   TASK_UNINTERRUPTIBLE);
#else
                (void)atomic_cond_read_relaxed(&q->requeue_state, VAL != Q_REQUEUE_PI_WAIT);
#endif
        }

        /*
         * Requeue is now either prohibited or complete. Reread state
         * because during the wait above it might have changed. Nothing
         * will modify q->requeue_state after this point.
         */
        return atomic_read(&q->requeue_state);
}

/**
 * requeue_pi_wake_futex() - Wake a task that acquired the lock during requeue
 * @q:          the futex_q
 * @key:        the key of the requeue target futex
 * @hb:         the hash_bucket of the requeue target futex
 *
 * During futex_requeue, with requeue_pi=1, it is possible to acquire the
 * target futex if it is uncontended or via a lock steal.
 *
 * 1) Set @q::key to the requeue target futex key so the waiter can detect
 *    the wakeup on the right futex.
 *
 * 2) Dequeue @q from the hash bucket.
 *
 * 3) Set @q::rt_waiter to NULL so the woken up task can detect atomic lock
 *    acquisition.
 *
 * 4) Set the q->lock_ptr to the requeue target hb->lock for the case that
 *    the waiter has to fixup the pi state.
 *
 * 5) Complete the requeue state so the waiter can make progress. After
 *    this point the waiter task can return from the syscall immediately in
 *    case that the pi state does not have to be fixed up.
 *
 * 6) Wake the waiter task.
 *
 * Must be called with both q->lock_ptr and hb->lock held.
 */
static inline
void requeue_pi_wake_futex(struct futex_q *q, union futex_key *key,
                           struct futex_hash_bucket *hb)
{
        q->key = *key;

        __unqueue_futex(q);

        WARN_ON(!q->rt_waiter);
        q->rt_waiter = NULL;

        q->lock_ptr = &hb->lock;

        /* Signal locked state to the waiter */
        futex_requeue_pi_complete(q, 1);
        wake_up_state(q->task, TASK_NORMAL);
}

/**
 * futex_proxy_trylock_atomic() - Attempt an atomic lock for the top waiter
 * @pifutex:            the user address of the to futex
 * @hb1:                the from futex hash bucket, must be locked by the caller
 * @hb2:                the to futex hash bucket, must be locked by the caller
 * @key1:               the from futex key
 * @key2:               the to futex key
 * @ps:                 address to store the pi_state pointer
 * @exiting:            Pointer to store the task pointer of the owner task
 *                      which is in the middle of exiting
 * @set_waiters:        force setting the FUTEX_WAITERS bit (1) or not (0)
 *
 * Try and get the lock on behalf of the top waiter if we can do it atomically.
 * Wake the top waiter if we succeed.  If the caller specified set_waiters,
 * then direct futex_lock_pi_atomic() to force setting the FUTEX_WAITERS bit.
 * hb1 and hb2 must be held by the caller.
 *
 * @exiting is only set when the return value is -EBUSY. If so, this holds
 * a refcount on the exiting task on return and the caller needs to drop it
 * after waiting for the exit to complete.
 *
 * Return:
 *  -  0 - failed to acquire the lock atomically;
 *  - >0 - acquired the lock, return value is vpid of the top_waiter
 *  - <0 - error
 */
static int
futex_proxy_trylock_atomic(u32 __user *pifutex, struct futex_hash_bucket *hb1,
                           struct futex_hash_bucket *hb2, union futex_key *key1,
                           union futex_key *key2, struct futex_pi_state **ps,
                           struct task_struct **exiting, int set_waiters)
{
        struct futex_q *top_waiter = NULL;
        u32 curval;
        int ret;

        if (get_futex_value_locked(&curval, pifutex))
                return -EFAULT;

        if (unlikely(should_fail_futex(true)))
                return -EFAULT;

        /*
         * Find the top_waiter and determine if there are additional waiters.
         * If the caller intends to requeue more than 1 waiter to pifutex,
         * force futex_lock_pi_atomic() to set the FUTEX_WAITERS bit now,
         * as we have means to handle the possible fault.  If not, don't set
         * the bit unnecessarily as it will force the subsequent unlock to enter
         * the kernel.
         */
        top_waiter = futex_top_waiter(hb1, key1);

        /* There are no waiters, nothing for us to do. */
        if (!top_waiter)
                return 0;

        /*
         * Ensure that this is a waiter sitting in futex_wait_requeue_pi()
         * and waiting on the 'waitqueue' futex which is always !PI.
         */
        if (!top_waiter->rt_waiter || top_waiter->pi_state)
                return -EINVAL;

        /* Ensure we requeue to the expected futex. */
        if (!match_futex(top_waiter->requeue_pi_key, key2))
                return -EINVAL;

        /* Ensure that this does not race against an early wakeup */
        if (!futex_requeue_pi_prepare(top_waiter, NULL))
                return -EAGAIN;

        /*
         * Try to take the lock for top_waiter and set the FUTEX_WAITERS bit
         * in the contended case or if @set_waiters is true.
         *
         * In the contended case PI state is attached to the lock owner. If
         * the user space lock can be acquired then PI state is attached to
         * the new owner (@top_waiter->task) when @set_waiters is true.
         */
        ret = futex_lock_pi_atomic(pifutex, hb2, key2, ps, top_waiter->task,
                                   exiting, set_waiters);
        if (ret == 1) {
                /*
                 * Lock was acquired in user space and PI state was
                 * attached to @top_waiter->task. That means state is fully
                 * consistent and the waiter can return to user space
                 * immediately after the wakeup.
                 */
                requeue_pi_wake_futex(top_waiter, key2, hb2);
        } else if (ret < 0) {
                /* Rewind top_waiter::requeue_state */
                futex_requeue_pi_complete(top_waiter, ret);
        } else {
                /*
                 * futex_lock_pi_atomic() did not acquire the user space
                 * futex, but managed to establish the proxy lock and pi
                 * state. top_waiter::requeue_state cannot be fixed up here
                 * because the waiter is not enqueued on the rtmutex
                 * yet. This is handled at the callsite depending on the
                 * result of rt_mutex_start_proxy_lock() which is
                 * guaranteed to be reached with this function returning 0.
                 */
        }
        return ret;
}

/**
 * futex_requeue() - Requeue waiters from uaddr1 to uaddr2
 * @uaddr1:     source futex user address
 * @flags:      futex flags (FLAGS_SHARED, etc.)
 * @uaddr2:     target futex user address
 * @nr_wake:    number of waiters to wake (must be 1 for requeue_pi)
 * @nr_requeue: number of waiters to requeue (0-INT_MAX)
 * @cmpval:     @uaddr1 expected value (or %NULL)
 * @requeue_pi: if we are attempting to requeue from a non-pi futex to a
 *              pi futex (pi to pi requeue is not supported)
 *
 * Requeue waiters on uaddr1 to uaddr2. In the requeue_pi case, try to acquire
 * uaddr2 atomically on behalf of the top waiter.
 *
 * Return:
 *  - >=0 - on success, the number of tasks requeued or woken;
 *  -  <0 - on error
 */
static int futex_requeue(u32 __user *uaddr1, unsigned int flags,
                         u32 __user *uaddr2, int nr_wake, int nr_requeue,
                         u32 *cmpval, int requeue_pi)
{
        union futex_key key1 = FUTEX_KEY_INIT, key2 = FUTEX_KEY_INIT;
        int task_count = 0, ret;
        struct futex_pi_state *pi_state = NULL;
        struct futex_hash_bucket *hb1, *hb2;
        struct futex_q *this, *next;
        DEFINE_WAKE_Q(wake_q);

        if (nr_wake < 0 || nr_requeue < 0)
                return -EINVAL;

        /*
         * When PI not supported: return -ENOSYS if requeue_pi is true,
         * consequently the compiler knows requeue_pi is always false past
         * this point which will optimize away all the conditional code
         * further down.
         */
        if (!IS_ENABLED(CONFIG_FUTEX_PI) && requeue_pi)
                return -ENOSYS;

        if (requeue_pi) {
                /*
                 * Requeue PI only works on two distinct uaddrs. This
                 * check is only valid for private futexes. See below.
                 */
                if (uaddr1 == uaddr2)
                        return -EINVAL;

                /*
                 * futex_requeue() allows the caller to define the number
                 * of waiters to wake up via the @nr_wake argument. With
                 * REQUEUE_PI, waking up more than one waiter is creating
                 * more problems than it solves. Waking up a waiter makes
                 * only sense if the PI futex @uaddr2 is uncontended as
                 * this allows the requeue code to acquire the futex
                 * @uaddr2 before waking the waiter. The waiter can then
                 * return to user space without further action. A secondary
                 * wakeup would just make the futex_wait_requeue_pi()
                 * handling more complex, because that code would have to
                 * look up pi_state and do more or less all the handling
                 * which the requeue code has to do for the to be requeued
                 * waiters. So restrict the number of waiters to wake to
                 * one, and only wake it up when the PI futex is
                 * uncontended. Otherwise requeue it and let the unlock of
                 * the PI futex handle the wakeup.
                 *
                 * All REQUEUE_PI users, e.g. pthread_cond_signal() and
                 * pthread_cond_broadcast() must use nr_wake=1.
                 */
                if (nr_wake != 1)
                        return -EINVAL;

                /*
                 * requeue_pi requires a pi_state, try to allocate it now
                 * without any locks in case it fails.
                 */
                if (refill_pi_state_cache())
                        return -ENOMEM;
        }

retry:
        ret = get_futex_key(uaddr1, flags & FLAGS_SHARED, &key1, FUTEX_READ);
        if (unlikely(ret != 0))
                return ret;
        ret = get_futex_key(uaddr2, flags & FLAGS_SHARED, &key2,
                            requeue_pi ? FUTEX_WRITE : FUTEX_READ);
        if (unlikely(ret != 0))
                return ret;

        /*
         * The check above which compares uaddrs is not sufficient for
         * shared futexes. We need to compare the keys:
         */
        if (requeue_pi && match_futex(&key1, &key2))
                return -EINVAL;

        hb1 = hash_futex(&key1);
        hb2 = hash_futex(&key2);

retry_private:
        hb_waiters_inc(hb2);
        double_lock_hb(hb1, hb2);

        if (likely(cmpval != NULL)) {
                u32 curval;

                ret = get_futex_value_locked(&curval, uaddr1);

                if (unlikely(ret)) {
                        double_unlock_hb(hb1, hb2);
                        hb_waiters_dec(hb2);

                        ret = get_user(curval, uaddr1);
                        if (ret)
                                return ret;

                        if (!(flags & FLAGS_SHARED))
                                goto retry_private;

                        goto retry;
                }
                if (curval != *cmpval) {
                        ret = -EAGAIN;
                        goto out_unlock;
                }
        }

        if (requeue_pi) {
                struct task_struct *exiting = NULL;

                /*
                 * Attempt to acquire uaddr2 and wake the top waiter. If we
                 * intend to requeue waiters, force setting the FUTEX_WAITERS
                 * bit.  We force this here where we are able to easily handle
                 * faults rather in the requeue loop below.
                 *
                 * Updates topwaiter::requeue_state if a top waiter exists.
                 */
                ret = futex_proxy_trylock_atomic(uaddr2, hb1, hb2, &key1,
                                                 &key2, &pi_state,
                                                 &exiting, nr_requeue);

                /*
                 * At this point the top_waiter has either taken uaddr2 or
                 * is waiting on it. In both cases pi_state has been
                 * established and an initial refcount on it. In case of an
                 * error there's nothing.
                 *
                 * The top waiter's requeue_state is up to date:
                 *
                 *  - If the lock was acquired atomically (ret == 1), then
                 *    the state is Q_REQUEUE_PI_LOCKED.
                 *
                 *    The top waiter has been dequeued and woken up and can
                 *    return to user space immediately. The kernel/user
                 *    space state is consistent. In case that there must be
                 *    more waiters requeued the WAITERS bit in the user
                 *    space futex is set so the top waiter task has to go
                 *    into the syscall slowpath to unlock the futex. This
                 *    will block until this requeue operation has been
                 *    completed and the hash bucket locks have been
                 *    dropped.
                 *
                 *  - If the trylock failed with an error (ret < 0) then
                 *    the state is either Q_REQUEUE_PI_NONE, i.e. "nothing
                 *    happened", or Q_REQUEUE_PI_IGNORE when there was an
                 *    interleaved early wakeup.
                 *
                 *  - If the trylock did not succeed (ret == 0) then the
                 *    state is either Q_REQUEUE_PI_IN_PROGRESS or
                 *    Q_REQUEUE_PI_WAIT if an early wakeup interleaved.
                 *    This will be cleaned up in the loop below, which
                 *    cannot fail because futex_proxy_trylock_atomic() did
                 *    the same sanity checks for requeue_pi as the loop
                 *    below does.
                 */
                switch (ret) {
                case 0:
                        /* We hold a reference on the pi state. */
                        break;

                case 1:
                        /*
                         * futex_proxy_trylock_atomic() acquired the user space
                         * futex. Adjust task_count.
                         */
                        task_count++;
                        ret = 0;
                        break;

                /*
                 * If the above failed, then pi_state is NULL and
                 * waiter::requeue_state is correct.
                 */
                case -EFAULT:
                        double_unlock_hb(hb1, hb2);
                        hb_waiters_dec(hb2);
                        ret = fault_in_user_writeable(uaddr2);
                        if (!ret)
                                goto retry;
                        return ret;
                case -EBUSY:
                case -EAGAIN:
                        /*
                         * Two reasons for this:
                         * - EBUSY: Owner is exiting and we just wait for the
                         *   exit to complete.
                         * - EAGAIN: The user space value changed.
                         */
                        double_unlock_hb(hb1, hb2);
                        hb_waiters_dec(hb2);
                        /*
                         * Handle the case where the owner is in the middle of
                         * exiting. Wait for the exit to complete otherwise
                         * this task might loop forever, aka. live lock.
                         */
                        wait_for_owner_exiting(ret, exiting);
                        cond_resched();
                        goto retry;
                default:
                        goto out_unlock;
                }
        }

        plist_for_each_entry_safe(this, next, &hb1->chain, list) {
                if (task_count - nr_wake >= nr_requeue)
                        break;

                if (!match_futex(&this->key, &key1))
                        continue;

                /*
                 * FUTEX_WAIT_REQUEUE_PI and FUTEX_CMP_REQUEUE_PI should always
                 * be paired with each other and no other futex ops.
                 *
                 * We should never be requeueing a futex_q with a pi_state,
                 * which is awaiting a futex_unlock_pi().
                 */
                if ((requeue_pi && !this->rt_waiter) ||
                    (!requeue_pi && this->rt_waiter) ||
                    this->pi_state) {
                        ret = -EINVAL;
                        break;
                }

                /* Plain futexes just wake or requeue and are done */
                if (!requeue_pi) {
                        if (++task_count <= nr_wake)
                                mark_wake_futex(&wake_q, this);
                        else
                                requeue_futex(this, hb1, hb2, &key2);
                        continue;
                }

                /* Ensure we requeue to the expected futex for requeue_pi. */
                if (!match_futex(this->requeue_pi_key, &key2)) {
                        ret = -EINVAL;
                        break;
                }

                /*
                 * Requeue nr_requeue waiters and possibly one more in the case
                 * of requeue_pi if we couldn't acquire the lock atomically.
                 *
                 * Prepare the waiter to take the rt_mutex. Take a refcount
                 * on the pi_state and store the pointer in the futex_q
                 * object of the waiter.
                 */
                get_pi_state(pi_state);

                /* Don't requeue when the waiter is already on the way out. */
                if (!futex_requeue_pi_prepare(this, pi_state)) {
                        /*
                         * Early woken waiter signaled that it is on the
                         * way out. Drop the pi_state reference and try the
                         * next waiter. @this->pi_state is still NULL.
                         */
                        put_pi_state(pi_state);
                        continue;
                }

                ret = rt_mutex_start_proxy_lock(&pi_state->pi_mutex,
                                                this->rt_waiter,
                                                this->task);

                if (ret == 1) {
                        /*
                         * We got the lock. We do neither drop the refcount
                         * on pi_state nor clear this->pi_state because the
                         * waiter needs the pi_state for cleaning up the
                         * user space value. It will drop the refcount
                         * after doing so. this::requeue_state is updated
                         * in the wakeup as well.
                         */
                        requeue_pi_wake_futex(this, &key2, hb2);
                        task_count++;
                } else if (!ret) {
                        /* Waiter is queued, move it to hb2 */
                        requeue_futex(this, hb1, hb2, &key2);
                        futex_requeue_pi_complete(this, 0);
                        task_count++;
                } else {
                        /*
                         * rt_mutex_start_proxy_lock() detected a potential
                         * deadlock when we tried to queue that waiter.
                         * Drop the pi_state reference which we took above
                         * and remove the pointer to the state from the
                         * waiters futex_q object.
                         */
                        this->pi_state = NULL;
                        put_pi_state(pi_state);
                        futex_requeue_pi_complete(this, ret);
                        /*
                         * We stop queueing more waiters and let user space
                         * deal with the mess.
                         */
                        break;
                }
        }

        /*
         * We took an extra initial reference to the pi_state in
         * futex_proxy_trylock_atomic(). We need to drop it here again.
         */
        put_pi_state(pi_state);

out_unlock:
        double_unlock_hb(hb1, hb2);
        wake_up_q(&wake_q);
        hb_waiters_dec(hb2);
        return ret ? ret : task_count;
}

/* The key must be already stored in q->key. */
static inline struct futex_hash_bucket *queue_lock(struct futex_q *q)
        __acquires(&hb->lock)
{
        struct futex_hash_bucket *hb;

        hb = hash_futex(&q->key);

        /*
         * Increment the counter before taking the lock so that
         * a potential waker won't miss a to-be-slept task that is
         * waiting for the spinlock. This is safe as all queue_lock()
         * users end up calling queue_me(). Similarly, for housekeeping,
         * decrement the counter at queue_unlock() when some error has
         * occurred and we don't end up adding the task to the list.
         */
        hb_waiters_inc(hb); /* implies smp_mb(); (A) */

        q->lock_ptr = &hb->lock;

        spin_lock(&hb->lock);
        return hb;
}

static inline void
queue_unlock(struct futex_hash_bucket *hb)
        __releases(&hb->lock)
{
        spin_unlock(&hb->lock);
        hb_waiters_dec(hb);
}

static inline void __queue_me(struct futex_q *q, struct futex_hash_bucket *hb)
{
        int prio;

        /*
         * The priority used to register this element is
         * - either the real thread-priority for the real-time threads
         * (i.e. threads with a priority lower than MAX_RT_PRIO)
         * - or MAX_RT_PRIO for non-RT threads.
         * Thus, all RT-threads are woken first in priority order, and
         * the others are woken last, in FIFO order.
         */
        prio = min(current->normal_prio, MAX_RT_PRIO);

        plist_node_init(&q->list, prio);
        plist_add(&q->list, &hb->chain);
        q->task = current;
}

/**
 * queue_me() - Enqueue the futex_q on the futex_hash_bucket
 * @q:  The futex_q to enqueue
 * @hb: The destination hash bucket
 *
 * The hb->lock must be held by the caller, and is released here. A call to
 * queue_me() is typically paired with exactly one call to unqueue_me().  The
 * exceptions involve the PI related operations, which may use unqueue_me_pi()
 * or nothing if the unqueue is done as part of the wake process and the unqueue
 * state is implicit in the state of woken task (see futex_wait_requeue_pi() for
 * an example).
 */
static inline void queue_me(struct futex_q *q, struct futex_hash_bucket *hb)
        __releases(&hb->lock)
{
        __queue_me(q, hb);
        spin_unlock(&hb->lock);
}

/**
 * unqueue_me() - Remove the futex_q from its futex_hash_bucket
 * @q:  The futex_q to unqueue
 *
 * The q->lock_ptr must not be held by the caller. A call to unqueue_me() must
 * be paired with exactly one earlier call to queue_me().
 *
 * Return:
 *  - 1 - if the futex_q was still queued (and we removed unqueued it);
 *  - 0 - if the futex_q was already removed by the waking thread
 */
static int unqueue_me(struct futex_q *q)
{
        spinlock_t *lock_ptr;
        int ret = 0;

        /* In the common case we don't take the spinlock, which is nice. */
retry:
        /*
         * q->lock_ptr can change between this read and the following spin_lock.
         * Use READ_ONCE to forbid the compiler from reloading q->lock_ptr and
         * optimizing lock_ptr out of the logic below.
         */
        lock_ptr = READ_ONCE(q->lock_ptr);
        if (lock_ptr != NULL) {
                spin_lock(lock_ptr);
                /*
                 * q->lock_ptr can change between reading it and
                 * spin_lock(), causing us to take the wrong lock.  This
                 * corrects the race condition.
                 *
                 * Reasoning goes like this: if we have the wrong lock,
                 * q->lock_ptr must have changed (maybe several times)
                 * between reading it and the spin_lock().  It can
                 * change again after the spin_lock() but only if it was
                 * already changed before the spin_lock().  It cannot,
                 * however, change back to the original value.  Therefore
                 * we can detect whether we acquired the correct lock.
                 */
                if (unlikely(lock_ptr != q->lock_ptr)) {
                        spin_unlock(lock_ptr);
                        goto retry;
                }
                __unqueue_futex(q);

                BUG_ON(q->pi_state);

                spin_unlock(lock_ptr);
                ret = 1;
        }

        return ret;
}

/*
 * PI futexes can not be requeued and must remove themselves from the
 * hash bucket. The hash bucket lock (i.e. lock_ptr) is held.
 */
static void unqueue_me_pi(struct futex_q *q)
{
        __unqueue_futex(q);

        BUG_ON(!q->pi_state);
        put_pi_state(q->pi_state);
        q->pi_state = NULL;
}

static int __fixup_pi_state_owner(u32 __user *uaddr, struct futex_q *q,
                                  struct task_struct *argowner)
{
        struct futex_pi_state *pi_state = q->pi_state;
        struct task_struct *oldowner, *newowner;
        u32 uval, curval, newval, newtid;
        int err = 0;

        oldowner = pi_state->owner;

        /*
         * We are here because either:
         *
         *  - we stole the lock and pi_state->owner needs updating to reflect
         *    that (@argowner == current),
         *
         * or:
         *
         *  - someone stole our lock and we need to fix things to point to the
         *    new owner (@argowner == NULL).
         *
         * Either way, we have to replace the TID in the user space variable.
         * This must be atomic as we have to preserve the owner died bit here.
         *
         * Note: We write the user space value _before_ changing the pi_state
         * because we can fault here. Imagine swapped out pages or a fork
         * that marked all the anonymous memory readonly for cow.
         *
         * Modifying pi_state _before_ the user space value would leave the
         * pi_state in an inconsistent state when we fault here, because we
         * need to drop the locks to handle the fault. This might be observed
         * in the PID checks when attaching to PI state .
         */
retry:
        if (!argowner) {
                if (oldowner != current) {
                        /*
                         * We raced against a concurrent self; things are
                         * already fixed up. Nothing to do.
                         */
                        return 0;
                }

                if (__rt_mutex_futex_trylock(&pi_state->pi_mutex)) {
                        /* We got the lock. pi_state is correct. Tell caller. */
                        return 1;
                }

                /*
                 * The trylock just failed, so either there is an owner or
                 * there is a higher priority waiter than this one.
                 */
                newowner = rt_mutex_owner(&pi_state->pi_mutex);
                /*
                 * If the higher priority waiter has not yet taken over the
                 * rtmutex then newowner is NULL. We can't return here with
                 * that state because it's inconsistent vs. the user space
                 * state. So drop the locks and try again. It's a valid
                 * situation and not any different from the other retry
                 * conditions.
                 */
                if (unlikely(!newowner)) {
                        err = -EAGAIN;
                        goto handle_err;
                }
        } else {
                WARN_ON_ONCE(argowner != current);
                if (oldowner == current) {
                        /*
                         * We raced against a concurrent self; things are
                         * already fixed up. Nothing to do.
                         */
                        return 1;
                }
                newowner = argowner;
        }

        newtid = task_pid_vnr(newowner) | FUTEX_WAITERS;
        /* Owner died? */
        if (!pi_state->owner)
                newtid |= FUTEX_OWNER_DIED;

        err = get_futex_value_locked(&uval, uaddr);
        if (err)
                goto handle_err;

        for (;;) {
                newval = (uval & FUTEX_OWNER_DIED) | newtid;

                err = cmpxchg_futex_value_locked(&curval, uaddr, uval, newval);
                if (err)
                        goto handle_err;

                if (curval == uval)
                        break;
                uval = curval;
        }

        /*
         * We fixed up user space. Now we need to fix the pi_state
         * itself.
         */
        pi_state_update_owner(pi_state, newowner);

        return argowner == current;

        /*
         * In order to reschedule or handle a page fault, we need to drop the
         * locks here. In the case of a fault, this gives the other task
         * (either the highest priority waiter itself or the task which stole
         * the rtmutex) the chance to try the fixup of the pi_state. So once we
         * are back from handling the fault we need to check the pi_state after
         * reacquiring the locks and before trying to do another fixup. When
         * the fixup has been done already we simply return.
         *
         * Note: we hold both hb->lock and pi_mutex->wait_lock. We can safely
         * drop hb->lock since the caller owns the hb -> futex_q relation.
         * Dropping the pi_mutex->wait_lock requires the state revalidate.
         */
handle_err:
        raw_spin_unlock_irq(&pi_state->pi_mutex.wait_lock);
        spin_unlock(q->lock_ptr);

        switch (err) {
        case -EFAULT:
                err = fault_in_user_writeable(uaddr);
                break;

        case -EAGAIN:
                cond_resched();
                err = 0;
                break;

        default:
                WARN_ON_ONCE(1);
                break;
        }

        spin_lock(q->lock_ptr);
        raw_spin_lock_irq(&pi_state->pi_mutex.wait_lock);

        /*
         * Check if someone else fixed it for us:
         */
        if (pi_state->owner != oldowner)
                return argowner == current;

        /* Retry if err was -EAGAIN or the fault in succeeded */
        if (!err)
                goto retry;

        /*
         * fault_in_user_writeable() failed so user state is immutable. At
         * best we can make the kernel state consistent but user state will
         * be most likely hosed and any subsequent unlock operation will be
         * rejected due to PI futex rule [10].
         *
         * Ensure that the rtmutex owner is also the pi_state owner despite
         * the user space value claiming something different. There is no
         * point in unlocking the rtmutex if current is the owner as it
         * would need to wait until the next waiter has taken the rtmutex
         * to guarantee consistent state. Keep it simple. Userspace asked
         * for this wreckaged state.
         *
         * The rtmutex has an owner - either current or some other
         * task. See the EAGAIN loop above.
         */
        pi_state_update_owner(pi_state, rt_mutex_owner(&pi_state->pi_mutex));

        return err;
}

static int fixup_pi_state_owner(u32 __user *uaddr, struct futex_q *q,
                                struct task_struct *argowner)
{
        struct futex_pi_state *pi_state = q->pi_state;
        int ret;

        lockdep_assert_held(q->lock_ptr);

        raw_spin_lock_irq(&pi_state->pi_mutex.wait_lock);
        ret = __fixup_pi_state_owner(uaddr, q, argowner);
        raw_spin_unlock_irq(&pi_state->pi_mutex.wait_lock);
        return ret;
}

static long futex_wait_restart(struct restart_block *restart);

/**
 * fixup_owner() - Post lock pi_state and corner case management
 * @uaddr:      user address of the futex
 * @q:          futex_q (contains pi_state and access to the rt_mutex)
 * @locked:     if the attempt to take the rt_mutex succeeded (1) or not (0)
 *
 * After attempting to lock an rt_mutex, this function is called to cleanup
 * the pi_state owner as well as handle race conditions that may allow us to
 * acquire the lock. Must be called with the hb lock held.
 *
 * Return:
 *  -  1 - success, lock taken;
 *  -  0 - success, lock not taken;
 *  - <0 - on error (-EFAULT)
 */
static int fixup_owner(u32 __user *uaddr, struct futex_q *q, int locked)
{
        if (locked) {
                /*
                 * Got the lock. We might not be the anticipated owner if we
                 * did a lock-steal - fix up the PI-state in that case:
                 *
                 * Speculative pi_state->owner read (we don't hold wait_lock);
                 * since we own the lock pi_state->owner == current is the
                 * stable state, anything else needs more attention.
                 */
                if (q->pi_state->owner != current)
                        return fixup_pi_state_owner(uaddr, q, current);
                return 1;
        }

        /*
         * If we didn't get the lock; check if anybody stole it from us. In
         * that case, we need to fix up the uval to point to them instead of
         * us, otherwise bad things happen. [10]
         *
         * Another speculative read; pi_state->owner == current is unstable
         * but needs our attention.
         */
        if (q->pi_state->owner == current)
                return fixup_pi_state_owner(uaddr, q, NULL);

        /*
         * Paranoia check. If we did not take the lock, then we should not be
         * the owner of the rt_mutex. Warn and establish consistent state.
         */
        if (WARN_ON_ONCE(rt_mutex_owner(&q->pi_state->pi_mutex) == current))
                return fixup_pi_state_owner(uaddr, q, current);

        return 0;
}

/**
 * futex_wait_queue_me() - queue_me() and wait for wakeup, timeout, or signal
 * @hb:         the futex hash bucket, must be locked by the caller
 * @q:          the futex_q to queue up on
 * @timeout:    the prepared hrtimer_sleeper, or null for no timeout
 */
static void futex_wait_queue_me(struct futex_hash_bucket *hb, struct futex_q *q,
                                struct hrtimer_sleeper *timeout)
{
        /*
         * The task state is guaranteed to be set before another task can
         * wake it. set_current_state() is implemented using smp_store_mb() and
         * queue_me() calls spin_unlock() upon completion, both serializing
         * access to the hash list and forcing another memory barrier.
         */
        set_current_state(TASK_INTERRUPTIBLE);
        queue_me(q, hb);

        /* Arm the timer */
        if (timeout)
                hrtimer_sleeper_start_expires(timeout, HRTIMER_MODE_ABS);

        /*
         * If we have been removed from the hash list, then another task
         * has tried to wake us, and we can skip the call to schedule().
         */
        if (likely(!plist_node_empty(&q->list))) {
                /*
                 * If the timer has already expired, current will already be
                 * flagged for rescheduling. Only call schedule if there
                 * is no timeout, or if it has yet to expire.
                 */
                if (!timeout || timeout->task)
                        freezable_schedule();
        }
        __set_current_state(TASK_RUNNING);
}

/**
 * futex_wait_setup() - Prepare to wait on a futex
 * @uaddr:      the futex userspace address
 * @val:        the expected value
 * @flags:      futex flags (FLAGS_SHARED, etc.)
 * @q:          the associated futex_q
 * @hb:         storage for hash_bucket pointer to be returned to caller
 *
 * Setup the futex_q and locate the hash_bucket.  Get the futex value and
 * compare it with the expected value.  Handle atomic faults internally.
 * Return with the hb lock held on success, and unlocked on failure.
 *
 * Return:
 *  -  0 - uaddr contains val and hb has been locked;
 *  - <1 - -EFAULT or -EWOULDBLOCK (uaddr does not contain val) and hb is unlocked
 */
static int futex_wait_setup(u32 __user *uaddr, u32 val, unsigned int flags,
                           struct futex_q *q, struct futex_hash_bucket **hb)
{
        u32 uval;
        int ret;

        /*
         * Access the page AFTER the hash-bucket is locked.
         * Order is important:
         *
         *   Userspace waiter: val = var; if (cond(val)) futex_wait(&var, val);
         *   Userspace waker:  if (cond(var)) { var = new; futex_wake(&var); }
         *
         * The basic logical guarantee of a futex is that it blocks ONLY
         * if cond(var) is known to be true at the time of blocking, for
         * any cond.  If we locked the hash-bucket after testing *uaddr, that
         * would open a race condition where we could block indefinitely with
         * cond(var) false, which would violate the guarantee.
         *
         * On the other hand, we insert q and release the hash-bucket only
         * after testing *uaddr.  This guarantees that futex_wait() will NOT
         * absorb a wakeup if *uaddr does not match the desired values
         * while the syscall executes.
         */
retry:
        ret = get_futex_key(uaddr, flags & FLAGS_SHARED, &q->key, FUTEX_READ);
        if (unlikely(ret != 0))
                return ret;

retry_private:
        *hb = queue_lock(q);

        ret = get_futex_value_locked(&uval, uaddr);

        if (ret) {
                queue_unlock(*hb);

                ret = get_user(uval, uaddr);
                if (ret)
                        return ret;

                if (!(flags & FLAGS_SHARED))
                        goto retry_private;

                goto retry;
        }

        if (uval != val) {
                queue_unlock(*hb);
                ret = -EWOULDBLOCK;
        }

        return ret;
}

static int futex_wait(u32 __user *uaddr, unsigned int flags, u32 val,
                      ktime_t *abs_time, u32 bitset)
{
        struct hrtimer_sleeper timeout, *to;
        struct restart_block *restart;
        struct futex_hash_bucket *hb;
        struct futex_q q = futex_q_init;
        int ret;

        if (!bitset)
                return -EINVAL;
        q.bitset = bitset;

        to = futex_setup_timer(abs_time, &timeout, flags,
                               current->timer_slack_ns);
retry:
        /*
         * Prepare to wait on uaddr. On success, it holds hb->lock and q
         * is initialized.
         */
        ret = futex_wait_setup(uaddr, val, flags, &q, &hb);
        if (ret)
                goto out;

        /* queue_me and wait for wakeup, timeout, or a signal. */
        futex_wait_queue_me(hb, &q, to);

        /* If we were woken (and unqueued), we succeeded, whatever. */
        ret = 0;
        if (!unqueue_me(&q))
                goto out;
        ret = -ETIMEDOUT;
        if (to && !to->task)
                goto out;

        /*
         * We expect signal_pending(current), but we might be the
         * victim of a spurious wakeup as well.
         */
        if (!signal_pending(current))
                goto retry;

        ret = -ERESTARTSYS;
        if (!abs_time)
                goto out;

        restart = &current->restart_block;
        restart->futex.uaddr = uaddr;
        restart->futex.val = val;
        restart->futex.time = *abs_time;
        restart->futex.bitset = bitset;
        restart->futex.flags = flags | FLAGS_HAS_TIMEOUT;

        ret = set_restart_fn(restart, futex_wait_restart);

out:
        if (to) {
                hrtimer_cancel(&to->timer);
                destroy_hrtimer_on_stack(&to->timer);
        }
        return ret;
}


static long futex_wait_restart(struct restart_block *restart)
{
        u32 __user *uaddr = restart->futex.uaddr;
        ktime_t t, *tp = NULL;

        if (restart->futex.flags & FLAGS_HAS_TIMEOUT) {
                t = restart->futex.time;
                tp = &t;
        }
        restart->fn = do_no_restart_syscall;

        return (long)futex_wait(uaddr, restart->futex.flags,
                                restart->futex.val, tp, restart->futex.bitset);
}


/*
 * Userspace tried a 0 -> TID atomic transition of the futex value
 * and failed. The kernel side here does the whole locking operation:
 * if there are waiters then it will block as a consequence of relying
 * on rt-mutexes, it does PI, etc. (Due to races the kernel might see
 * a 0 value of the futex too.).
 *
 * Also serves as futex trylock_pi()'ing, and due semantics.
 */
static int futex_lock_pi(u32 __user *uaddr, unsigned int flags,
                         ktime_t *time, int trylock)
{
        struct hrtimer_sleeper timeout, *to;
        struct task_struct *exiting = NULL;
        struct rt_mutex_waiter rt_waiter;
        struct futex_hash_bucket *hb;
        struct futex_q q = futex_q_init;
        int res, ret;

        if (!IS_ENABLED(CONFIG_FUTEX_PI))
                return -ENOSYS;

        if (refill_pi_state_cache())
                return -ENOMEM;

        to = futex_setup_timer(time, &timeout, flags, 0);

retry:
        ret = get_futex_key(uaddr, flags & FLAGS_SHARED, &q.key, FUTEX_WRITE);
        if (unlikely(ret != 0))
                goto out;

retry_private:
        hb = queue_lock(&q);

        ret = futex_lock_pi_atomic(uaddr, hb, &q.key, &q.pi_state, current,
                                   &exiting, 0);
        if (unlikely(ret)) {
                /*
                 * Atomic work succeeded and we got the lock,
                 * or failed. Either way, we do _not_ block.
                 */
                switch (ret) {
                case 1:
                        /* We got the lock. */
                        ret = 0;
                        goto out_unlock_put_key;
                case -EFAULT:
                        goto uaddr_faulted;
                case -EBUSY:
                case -EAGAIN:
                        /*
                         * Two reasons for this:
                         * - EBUSY: Task is exiting and we just wait for the
                         *   exit to complete.
                         * - EAGAIN: The user space value changed.
                         */
                        queue_unlock(hb);
                        /*
                         * Handle the case where the owner is in the middle of
                         * exiting. Wait for the exit to complete otherwise
                         * this task might loop forever, aka. live lock.
                         */
                        wait_for_owner_exiting(ret, exiting);
                        cond_resched();
                        goto retry;
                default:
                        goto out_unlock_put_key;
                }
        }

        WARN_ON(!q.pi_state);

        /*
         * Only actually queue now that the atomic ops are done:
         */
        __queue_me(&q, hb);

        if (trylock) {
                ret = rt_mutex_futex_trylock(&q.pi_state->pi_mutex);
                /* Fixup the trylock return value: */
                ret = ret ? 0 : -EWOULDBLOCK;
                goto no_block;
        }

        rt_mutex_init_waiter(&rt_waiter);

        /*
         * On PREEMPT_RT_FULL, when hb->lock becomes an rt_mutex, we must not
         * hold it while doing rt_mutex_start_proxy(), because then it will
         * include hb->lock in the blocking chain, even through we'll not in
         * fact hold it while blocking. This will lead it to report -EDEADLK
         * and BUG when futex_unlock_pi() interleaves with this.
         *
         * Therefore acquire wait_lock while holding hb->lock, but drop the
         * latter before calling __rt_mutex_start_proxy_lock(). This
         * interleaves with futex_unlock_pi() -- which does a similar lock
         * handoff -- such that the latter can observe the futex_q::pi_state
         * before __rt_mutex_start_proxy_lock() is done.
         */
        raw_spin_lock_irq(&q.pi_state->pi_mutex.wait_lock);
        spin_unlock(q.lock_ptr);
        /*
         * __rt_mutex_start_proxy_lock() unconditionally enqueues the @rt_waiter
         * such that futex_unlock_pi() is guaranteed to observe the waiter when
         * it sees the futex_q::pi_state.
         */
        ret = __rt_mutex_start_proxy_lock(&q.pi_state->pi_mutex, &rt_waiter, current);
        raw_spin_unlock_irq(&q.pi_state->pi_mutex.wait_lock);

        if (ret) {
                if (ret == 1)
                        ret = 0;
                goto cleanup;
        }

        if (unlikely(to))
                hrtimer_sleeper_start_expires(to, HRTIMER_MODE_ABS);

        ret = rt_mutex_wait_proxy_lock(&q.pi_state->pi_mutex, to, &rt_waiter);

cleanup:
        spin_lock(q.lock_ptr);
        /*
         * If we failed to acquire the lock (deadlock/signal/timeout), we must
         * first acquire the hb->lock before removing the lock from the
         * rt_mutex waitqueue, such that we can keep the hb and rt_mutex wait
         * lists consistent.
         *
         * In particular; it is important that futex_unlock_pi() can not
         * observe this inconsistency.
         */
        if (ret && !rt_mutex_cleanup_proxy_lock(&q.pi_state->pi_mutex, &rt_waiter))
                ret = 0;

no_block:
        /*
         * Fixup the pi_state owner and possibly acquire the lock if we
         * haven't already.
         */
        res = fixup_owner(uaddr, &q, !ret);
        /*
         * If fixup_owner() returned an error, propagate that.  If it acquired
         * the lock, clear our -ETIMEDOUT or -EINTR.
         */
        if (res)
                ret = (res < 0) ? res : 0;

        unqueue_me_pi(&q);
        spin_unlock(q.lock_ptr);
        goto out;

out_unlock_put_key:
        queue_unlock(hb);

out:
        if (to) {
                hrtimer_cancel(&to->timer);
                destroy_hrtimer_on_stack(&to->timer);
        }
        return ret != -EINTR ? ret : -ERESTARTNOINTR;

uaddr_faulted:
        queue_unlock(hb);

        ret = fault_in_user_writeable(uaddr);
        if (ret)
                goto out;

        if (!(flags & FLAGS_SHARED))
                goto retry_private;

        goto retry;
}

/*
 * Userspace attempted a TID -> 0 atomic transition, and failed.
 * This is the in-kernel slowpath: we look up the PI state (if any),
 * and do the rt-mutex unlock.
 */
static int futex_unlock_pi(u32 __user *uaddr, unsigned int flags)
{
        u32 curval, uval, vpid = task_pid_vnr(current);
        union futex_key key = FUTEX_KEY_INIT;
        struct futex_hash_bucket *hb;
        struct futex_q *top_waiter;
        int ret;

        if (!IS_ENABLED(CONFIG_FUTEX_PI))
                return -ENOSYS;

retry:
        if (get_user(uval, uaddr))
                return -EFAULT;
        /*
         * We release only a lock we actually own:
         */
        if ((uval & FUTEX_TID_MASK) != vpid)
                return -EPERM;

        ret = get_futex_key(uaddr, flags & FLAGS_SHARED, &key, FUTEX_WRITE);
        if (ret)
                return ret;

        hb = hash_futex(&key);
        spin_lock(&hb->lock);

        /*
         * Check waiters first. We do not trust user space values at
         * all and we at least want to know if user space fiddled
         * with the futex value instead of blindly unlocking.
         */
        top_waiter = futex_top_waiter(hb, &key);
        if (top_waiter) {
                struct futex_pi_state *pi_state = top_waiter->pi_state;

                ret = -EINVAL;
                if (!pi_state)
                        goto out_unlock;

                /*
                 * If current does not own the pi_state then the futex is
                 * inconsistent and user space fiddled with the futex value.
                 */
                if (pi_state->owner != current)
                        goto out_unlock;

                get_pi_state(pi_state);
                /*
                 * By taking wait_lock while still holding hb->lock, we ensure
                 * there is no point where we hold neither; and therefore
                 * wake_futex_pi() must observe a state consistent with what we
                 * observed.
                 *
                 * In particular; this forces __rt_mutex_start_proxy() to
                 * complete such that we're guaranteed to observe the
                 * rt_waiter. Also see the WARN in wake_futex_pi().
                 */
                raw_spin_lock_irq(&pi_state->pi_mutex.wait_lock);
                spin_unlock(&hb->lock);

                /* drops pi_state->pi_mutex.wait_lock */
                ret = wake_futex_pi(uaddr, uval, pi_state);

                put_pi_state(pi_state);

                /*
                 * Success, we're done! No tricky corner cases.
                 */
                if (!ret)
                        return ret;
                /*
                 * The atomic access to the futex value generated a
                 * pagefault, so retry the user-access and the wakeup:
                 */
                if (ret == -EFAULT)
                        goto pi_faulted;
                /*
                 * A unconditional UNLOCK_PI op raced against a waiter
                 * setting the FUTEX_WAITERS bit. Try again.
                 */
                if (ret == -EAGAIN)
                        goto pi_retry;
                /*
                 * wake_futex_pi has detected invalid state. Tell user
                 * space.
                 */
                return ret;
        }

        /*
         * We have no kernel internal state, i.e. no waiters in the
         * kernel. Waiters which are about to queue themselves are stuck
         * on hb->lock. So we can safely ignore them. We do neither
         * preserve the WAITERS bit not the OWNER_DIED one. We are the
         * owner.
         */
        if ((ret = cmpxchg_futex_value_locked(&curval, uaddr, uval, 0))) {
                spin_unlock(&hb->lock);
                switch (ret) {
                case -EFAULT:
                        goto pi_faulted;

                case -EAGAIN:
                        goto pi_retry;

                default:
                        WARN_ON_ONCE(1);
                        return ret;
                }
        }

        /*
         * If uval has changed, let user space handle it.
         */
        ret = (curval == uval) ? 0 : -EAGAIN;

out_unlock:
        spin_unlock(&hb->lock);
        return ret;

pi_retry:
        cond_resched();
        goto retry;

pi_faulted:

        ret = fault_in_user_writeable(uaddr);
        if (!ret)
                goto retry;

        return ret;
}

/**
 * handle_early_requeue_pi_wakeup() - Handle early wakeup on the initial futex
 * @hb:         the hash_bucket futex_q was original enqueued on
 * @q:          the futex_q woken while waiting to be requeued
 * @timeout:    the timeout associated with the wait (NULL if none)
 *
 * Determine the cause for the early wakeup.
 *
 * Return:
 *  -EWOULDBLOCK or -ETIMEDOUT or -ERESTARTNOINTR
 */
static inline
int handle_early_requeue_pi_wakeup(struct futex_hash_bucket *hb,
                                   struct futex_q *q,
                                   struct hrtimer_sleeper *timeout)
{
        int ret;

        /*
         * With the hb lock held, we avoid races while we process the wakeup.
         * We only need to hold hb (and not hb2) to ensure atomicity as the
         * wakeup code can't change q.key from uaddr to uaddr2 if we hold hb.
         * It can't be requeued from uaddr2 to something else since we don't
         * support a PI aware source futex for requeue.
         */
        WARN_ON_ONCE(&hb->lock != q->lock_ptr);

        /*
         * We were woken prior to requeue by a timeout or a signal.
         * Unqueue the futex_q and determine which it was.
         */
        plist_del(&q->list, &hb->chain);
        hb_waiters_dec(hb);

        /* Handle spurious wakeups gracefully */
        ret = -EWOULDBLOCK;
        if (timeout && !timeout->task)
                ret = -ETIMEDOUT;
        else if (signal_pending(current))
                ret = -ERESTARTNOINTR;
        return ret;
}

/**
 * futex_wait_requeue_pi() - Wait on uaddr and take uaddr2
 * @uaddr:      the futex we initially wait on (non-pi)
 * @flags:      futex flags (FLAGS_SHARED, FLAGS_CLOCKRT, etc.), they must be
 *              the same type, no requeueing from private to shared, etc.
 * @val:        the expected value of uaddr
 * @abs_time:   absolute timeout
 * @bitset:     32 bit wakeup bitset set by userspace, defaults to all
 * @uaddr2:     the pi futex we will take prior to returning to user-space
 *
 * The caller will wait on uaddr and will be requeued by futex_requeue() to
 * uaddr2 which must be PI aware and unique from uaddr.  Normal wakeup will wake
 * on uaddr2 and complete the acquisition of the rt_mutex prior to returning to
 * userspace.  This ensures the rt_mutex maintains an owner when it has waiters;
 * without one, the pi logic would not know which task to boost/deboost, if
 * there was a need to.
 *
 * We call schedule in futex_wait_queue_me() when we enqueue and return there
 * via the following--
 * 1) wakeup on uaddr2 after an atomic lock acquisition by futex_requeue()
 * 2) wakeup on uaddr2 after a requeue
 * 3) signal
 * 4) timeout
 *
 * If 3, cleanup and return -ERESTARTNOINTR.
 *
 * If 2, we may then block on trying to take the rt_mutex and return via:
 * 5) successful lock
 * 6) signal
 * 7) timeout
 * 8) other lock acquisition failure
 *
 * If 6, return -EWOULDBLOCK (restarting the syscall would do the same).
 *
 * If 4 or 7, we cleanup and return with -ETIMEDOUT.
 *
 * Return:
 *  -  0 - On success;
 *  - <0 - On error
 */
static int futex_wait_requeue_pi(u32 __user *uaddr, unsigned int flags,
                                 u32 val, ktime_t *abs_time, u32 bitset,
                                 u32 __user *uaddr2)
{
        struct hrtimer_sleeper timeout, *to;
        struct rt_mutex_waiter rt_waiter;
        struct futex_hash_bucket *hb;
        union futex_key key2 = FUTEX_KEY_INIT;
        struct futex_q q = futex_q_init;
        struct rt_mutex_base *pi_mutex;
        int res, ret;

        if (!IS_ENABLED(CONFIG_FUTEX_PI))
                return -ENOSYS;

        if (uaddr == uaddr2)
                return -EINVAL;

        if (!bitset)
                return -EINVAL;

        to = futex_setup_timer(abs_time, &timeout, flags,
                               current->timer_slack_ns);

        /*
         * The waiter is allocated on our stack, manipulated by the requeue
         * code while we sleep on uaddr.
         */
        rt_mutex_init_waiter(&rt_waiter);

        ret = get_futex_key(uaddr2, flags & FLAGS_SHARED, &key2, FUTEX_WRITE);
        if (unlikely(ret != 0))
                goto out;

        q.bitset = bitset;
        q.rt_waiter = &rt_waiter;
        q.requeue_pi_key = &key2;

        /*
         * Prepare to wait on uaddr. On success, it holds hb->lock and q
         * is initialized.
         */
        ret = futex_wait_setup(uaddr, val, flags, &q, &hb);
        if (ret)
                goto out;

        /*
         * The check above which compares uaddrs is not sufficient for
         * shared futexes. We need to compare the keys:
         */
        if (match_futex(&q.key, &key2)) {
                queue_unlock(hb);
                ret = -EINVAL;
                goto out;
        }

        /* Queue the futex_q, drop the hb lock, wait for wakeup. */
        futex_wait_queue_me(hb, &q, to);

        switch (futex_requeue_pi_wakeup_sync(&q)) {
        case Q_REQUEUE_PI_IGNORE:
                /* The waiter is still on uaddr1 */
                spin_lock(&hb->lock);
                ret = handle_early_requeue_pi_wakeup(hb, &q, to);
                spin_unlock(&hb->lock);
                break;

        case Q_REQUEUE_PI_LOCKED:
                /* The requeue acquired the lock */
                if (q.pi_state && (q.pi_state->owner != current)) {
                        spin_lock(q.lock_ptr);
                        ret = fixup_owner(uaddr2, &q, true);
                        /*
                         * Drop the reference to the pi state which the
                         * requeue_pi() code acquired for us.
                         */
                        put_pi_state(q.pi_state);
                        spin_unlock(q.lock_ptr);
                        /*
                         * Adjust the return value. It's either -EFAULT or
                         * success (1) but the caller expects 0 for success.
                         */
                        ret = ret < 0 ? ret : 0;
                }
                break;

        case Q_REQUEUE_PI_DONE:
                /* Requeue completed. Current is 'pi_blocked_on' the rtmutex */
                pi_mutex = &q.pi_state->pi_mutex;
                ret = rt_mutex_wait_proxy_lock(pi_mutex, to, &rt_waiter);

                /* Current is not longer pi_blocked_on */
                spin_lock(q.lock_ptr);
                if (ret && !rt_mutex_cleanup_proxy_lock(pi_mutex, &rt_waiter))
                        ret = 0;

                debug_rt_mutex_free_waiter(&rt_waiter);
                /*
                 * Fixup the pi_state owner and possibly acquire the lock if we
                 * haven't already.
                 */
                res = fixup_owner(uaddr2, &q, !ret);
                /*
                 * If fixup_owner() returned an error, propagate that.  If it
                 * acquired the lock, clear -ETIMEDOUT or -EINTR.
                 */
                if (res)
                        ret = (res < 0) ? res : 0;

                unqueue_me_pi(&q);
                spin_unlock(q.lock_ptr);

                if (ret == -EINTR) {
                        /*
                         * We've already been requeued, but cannot restart
                         * by calling futex_lock_pi() directly. We could
                         * restart this syscall, but it would detect that
                         * the user space "val" changed and return
                         * -EWOULDBLOCK.  Save the overhead of the restart
                         * and return -EWOULDBLOCK directly.
                         */
                        ret = -EWOULDBLOCK;
                }
                break;
        default:
                BUG();
        }

out:
        if (to) {
                hrtimer_cancel(&to->timer);
                destroy_hrtimer_on_stack(&to->timer);
        }
        return ret;
}

/*
 * Support for robust futexes: the kernel cleans up held futexes at
 * thread exit time.
 *
 * Implementation: user-space maintains a per-thread list of locks it
 * is holding. Upon do_exit(), the kernel carefully walks this list,
 * and marks all locks that are owned by this thread with the
 * FUTEX_OWNER_DIED bit, and wakes up a waiter (if any). The list is
 * always manipulated with the lock held, so the list is private and
 * per-thread. Userspace also maintains a per-thread 'list_op_pending'
 * field, to allow the kernel to clean up if the thread dies after
 * acquiring the lock, but just before it could have added itself to
 * the list. There can only be one such pending lock.
 */

/**
 * sys_set_robust_list() - Set the robust-futex list head of a task
 * @head:       pointer to the list-head
 * @len:        length of the list-head, as userspace expects
 */
SYSCALL_DEFINE2(set_robust_list, struct robust_list_head __user *, head,
                size_t, len)
{
        if (!futex_cmpxchg_enabled)
                return -ENOSYS;
        /*
         * The kernel knows only one size for now:
         */
        if (unlikely(len != sizeof(*head)))
                return -EINVAL;

        current->robust_list = head;

        return 0;
}

/**
 * sys_get_robust_list() - Get the robust-futex list head of a task
 * @pid:        pid of the process [zero for current task]
 * @head_ptr:   pointer to a list-head pointer, the kernel fills it in
 * @len_ptr:    pointer to a length field, the kernel fills in the header size
 */
SYSCALL_DEFINE3(get_robust_list, int, pid,
                struct robust_list_head __user * __user *, head_ptr,
                size_t __user *, len_ptr)
{
        struct robust_list_head __user *head;
        unsigned long ret;
        struct task_struct *p;

        if (!futex_cmpxchg_enabled)
                return -ENOSYS;

        rcu_read_lock();

        ret = -ESRCH;
        if (!pid)
                p = current;
        else {
                p = find_task_by_vpid(pid);
                if (!p)
                        goto err_unlock;
        }

        ret = -EPERM;
        if (!ptrace_may_access(p, PTRACE_MODE_READ_REALCREDS))
                goto err_unlock;

        head = p->robust_list;
        rcu_read_unlock();

        if (put_user(sizeof(*head), len_ptr))
                return -EFAULT;
        return put_user(head, head_ptr);

err_unlock:
        rcu_read_unlock();

        return ret;
}

/* Constants for the pending_op argument of handle_futex_death */
#define HANDLE_DEATH_PENDING    true
#define HANDLE_DEATH_LIST       false

/*
 * Process a futex-list entry, check whether it's owned by the
 * dying task, and do notification if so:
 */
static int handle_futex_death(u32 __user *uaddr, struct task_struct *curr,
                              bool pi, bool pending_op)
{
        u32 uval, nval, mval;
        int err;

        /* Futex address must be 32bit aligned */
        if ((((unsigned long)uaddr) % sizeof(*uaddr)) != 0)
                return -1;

retry:
        if (get_user(uval, uaddr))
                return -1;

        /*
         * Special case for regular (non PI) futexes. The unlock path in
         * user space has two race scenarios:
         *
         * 1. The unlock path releases the user space futex value and
         *    before it can execute the futex() syscall to wake up
         *    waiters it is killed.
         *
         * 2. A woken up waiter is killed before it can acquire the
         *    futex in user space.
         *
         * In both cases the TID validation below prevents a wakeup of
         * potential waiters which can cause these waiters to block
         * forever.
         *
         * In both cases the following conditions are met:
         *
         *      1) task->robust_list->list_op_pending != NULL
         *         @pending_op == true
         *      2) User space futex value == 0
         *      3) Regular futex: @pi == false
         *
         * If these conditions are met, it is safe to attempt waking up a
         * potential waiter without touching the user space futex value and
         * trying to set the OWNER_DIED bit. The user space futex value is
         * uncontended and the rest of the user space mutex state is
         * consistent, so a woken waiter will just take over the
         * uncontended futex. Setting the OWNER_DIED bit would create
         * inconsistent state and malfunction of the user space owner died
         * handling.
         */
        if (pending_op && !pi && !uval) {
                futex_wake(uaddr, 1, 1, FUTEX_BITSET_MATCH_ANY);
                return 0;
        }

        if ((uval & FUTEX_TID_MASK) != task_pid_vnr(curr))
                return 0;

        /*
         * Ok, this dying thread is truly holding a futex
         * of interest. Set the OWNER_DIED bit atomically
         * via cmpxchg, and if the value had FUTEX_WAITERS
         * set, wake up a waiter (if any). (We have to do a
         * futex_wake() even if OWNER_DIED is already set -
         * to handle the rare but possible case of recursive
         * thread-death.) The rest of the cleanup is done in
         * userspace.
         */
        mval = (uval & FUTEX_WAITERS) | FUTEX_OWNER_DIED;

        /*
         * We are not holding a lock here, but we want to have
         * the pagefault_disable/enable() protection because
         * we want to handle the fault gracefully. If the
         * access fails we try to fault in the futex with R/W
         * verification via get_user_pages. get_user() above
         * does not guarantee R/W access. If that fails we
         * give up and leave the futex locked.
         */
        if ((err = cmpxchg_futex_value_locked(&nval, uaddr, uval, mval))) {
                switch (err) {
                case -EFAULT:
                        if (fault_in_user_writeable(uaddr))
                                return -1;
                        goto retry;

                case -EAGAIN:
                        cond_resched();
                        goto retry;

                default:
                        WARN_ON_ONCE(1);
                        return err;
                }
        }

        if (nval != uval)
                goto retry;

        /*
         * Wake robust non-PI futexes here. The wakeup of
         * PI futexes happens in exit_pi_state():
         */
        if (!pi && (uval & FUTEX_WAITERS))
                futex_wake(uaddr, 1, 1, FUTEX_BITSET_MATCH_ANY);

        return 0;
}

/*
 * Fetch a robust-list pointer. Bit 0 signals PI futexes:
 */
static inline int fetch_robust_entry(struct robust_list __user **entry,
                                     struct robust_list __user * __user *head,
                                     unsigned int *pi)
{
        unsigned long uentry;

        if (get_user(uentry, (unsigned long __user *)head))
                return -EFAULT;

        *entry = (void __user *)(uentry & ~1UL);
        *pi = uentry & 1;

        return 0;
}

/*
 * Walk curr->robust_list (very carefully, it's a userspace list!)
 * and mark any locks found there dead, and notify any waiters.
 *
 * We silently return on any sign of list-walking problem.
 */
static void exit_robust_list(struct task_struct *curr)
{
        struct robust_list_head __user *head = curr->robust_list;
        struct robust_list __user *entry, *next_entry, *pending;
        unsigned int limit = ROBUST_LIST_LIMIT, pi, pip;
        unsigned int next_pi;
        unsigned long futex_offset;
        int rc;

        if (!futex_cmpxchg_enabled)
                return;

        /*
         * Fetch the list head (which was registered earlier, via
         * sys_set_robust_list()):
         */
        if (fetch_robust_entry(&entry, &head->list.next, &pi))
                return;
        /*
         * Fetch the relative futex offset:
         */
        if (get_user(futex_offset, &head->futex_offset))
                return;
        /*
         * Fetch any possibly pending lock-add first, and handle it
         * if it exists:
         */
        if (fetch_robust_entry(&pending, &head->list_op_pending, &pip))
                return;

        next_entry = NULL;      /* avoid warning with gcc */
        while (entry != &head->list) {
                /*
                 * Fetch the next entry in the list before calling
                 * handle_futex_death:
                 */
                rc = fetch_robust_entry(&next_entry, &entry->next, &next_pi);
                /*
                 * A pending lock might already be on the list, so
                 * don't process it twice:
                 */
                if (entry != pending) {
                        if (handle_futex_death((void __user *)entry + futex_offset,
                                                curr, pi, HANDLE_DEATH_LIST))
                                return;
                }
                if (rc)
                        return;
                entry = next_entry;
                pi = next_pi;
                /*
                 * Avoid excessively long or circular lists:
                 */
                if (!--limit)
                        break;

                cond_resched();
        }

        if (pending) {
                handle_futex_death((void __user *)pending + futex_offset,
                                   curr, pip, HANDLE_DEATH_PENDING);
        }
}

static void futex_cleanup(struct task_struct *tsk)
{
        if (unlikely(tsk->robust_list)) {
                exit_robust_list(tsk);
                tsk->robust_list = NULL;
        }

#ifdef CONFIG_COMPAT
        if (unlikely(tsk->compat_robust_list)) {
                compat_exit_robust_list(tsk);
                tsk->compat_robust_list = NULL;
        }
#endif

        if (unlikely(!list_empty(&tsk->pi_state_list)))
                exit_pi_state_list(tsk);
}

/**
 * futex_exit_recursive - Set the tasks futex state to FUTEX_STATE_DEAD
 * @tsk:        task to set the state on
 *
 * Set the futex exit state of the task lockless. The futex waiter code
 * observes that state when a task is exiting and loops until the task has
 * actually finished the futex cleanup. The worst case for this is that the
 * waiter runs through the wait loop until the state becomes visible.
 *
 * This is called from the recursive fault handling path in do_exit().
 *
 * This is best effort. Either the futex exit code has run already or
 * not. If the OWNER_DIED bit has been set on the futex then the waiter can
 * take it over. If not, the problem is pushed back to user space. If the
 * futex exit code did not run yet, then an already queued waiter might
 * block forever, but there is nothing which can be done about that.
 */
void futex_exit_recursive(struct task_struct *tsk)
{
        /* If the state is FUTEX_STATE_EXITING then futex_exit_mutex is held */
        if (tsk->futex_state == FUTEX_STATE_EXITING)
                mutex_unlock(&tsk->futex_exit_mutex);
        tsk->futex_state = FUTEX_STATE_DEAD;
}

static void futex_cleanup_begin(struct task_struct *tsk)
{
        /*
         * Prevent various race issues against a concurrent incoming waiter
         * including live locks by forcing the waiter to block on
         * tsk->futex_exit_mutex when it observes FUTEX_STATE_EXITING in
         * attach_to_pi_owner().
         */
        mutex_lock(&tsk->futex_exit_mutex);

        /*
         * Switch the state to FUTEX_STATE_EXITING under tsk->pi_lock.
         *
         * This ensures that all subsequent checks of tsk->futex_state in
         * attach_to_pi_owner() must observe FUTEX_STATE_EXITING with
         * tsk->pi_lock held.
         *
         * It guarantees also that a pi_state which was queued right before
         * the state change under tsk->pi_lock by a concurrent waiter must
         * be observed in exit_pi_state_list().
         */
        raw_spin_lock_irq(&tsk->pi_lock);
        tsk->futex_state = FUTEX_STATE_EXITING;
        raw_spin_unlock_irq(&tsk->pi_lock);
}

static void futex_cleanup_end(struct task_struct *tsk, int state)
{
        /*
         * Lockless store. The only side effect is that an observer might
         * take another loop until it becomes visible.
         */
        tsk->futex_state = state;
        /*
         * Drop the exit protection. This unblocks waiters which observed
         * FUTEX_STATE_EXITING to reevaluate the state.
         */
        mutex_unlock(&tsk->futex_exit_mutex);
}

void futex_exec_release(struct task_struct *tsk)
{
        /*
         * The state handling is done for consistency, but in the case of
         * exec() there is no way to prevent further damage as the PID stays
         * the same. But for the unlikely and arguably buggy case that a
         * futex is held on exec(), this provides at least as much state
         * consistency protection which is possible.
         */
        futex_cleanup_begin(tsk);
        futex_cleanup(tsk);
        /*
         * Reset the state to FUTEX_STATE_OK. The task is alive and about
         * exec a new binary.
         */
        futex_cleanup_end(tsk, FUTEX_STATE_OK);
}

void futex_exit_release(struct task_struct *tsk)
{
        futex_cleanup_begin(tsk);
        futex_cleanup(tsk);
        futex_cleanup_end(tsk, FUTEX_STATE_DEAD);
}

long do_futex(u32 __user *uaddr, int op, u32 val, ktime_t *timeout,
                u32 __user *uaddr2, u32 val2, u32 val3)
{
        int cmd = op & FUTEX_CMD_MASK;
        unsigned int flags = 0;

        if (!(op & FUTEX_PRIVATE_FLAG))
                flags |= FLAGS_SHARED;

        if (op & FUTEX_CLOCK_REALTIME) {
                flags |= FLAGS_CLOCKRT;
                if (cmd != FUTEX_WAIT_BITSET && cmd != FUTEX_WAIT_REQUEUE_PI &&
                    cmd != FUTEX_LOCK_PI2)
                        return -ENOSYS;
        }

        switch (cmd) {
        case FUTEX_LOCK_PI:
        case FUTEX_LOCK_PI2:
        case FUTEX_UNLOCK_PI:
        case FUTEX_TRYLOCK_PI:
        case FUTEX_WAIT_REQUEUE_PI:
        case FUTEX_CMP_REQUEUE_PI:
                if (!futex_cmpxchg_enabled)
                        return -ENOSYS;
        }

        switch (cmd) {
        case FUTEX_WAIT:
                val3 = FUTEX_BITSET_MATCH_ANY;
                fallthrough;
        case FUTEX_WAIT_BITSET:
                return futex_wait(uaddr, flags, val, timeout, val3);
        case FUTEX_WAKE:
                val3 = FUTEX_BITSET_MATCH_ANY;
                fallthrough;
        case FUTEX_WAKE_BITSET:
                return futex_wake(uaddr, flags, val, val3);
        case FUTEX_REQUEUE:
                return futex_requeue(uaddr, flags, uaddr2, val, val2, NULL, 0);
        case FUTEX_CMP_REQUEUE:
                return futex_requeue(uaddr, flags, uaddr2, val, val2, &val3, 0);
        case FUTEX_WAKE_OP:
                return futex_wake_op(uaddr, flags, uaddr2, val, val2, val3);
        case FUTEX_LOCK_PI:
                flags |= FLAGS_CLOCKRT;
                fallthrough;
        case FUTEX_LOCK_PI2:
                return futex_lock_pi(uaddr, flags, timeout, 0);
        case FUTEX_UNLOCK_PI:
                return futex_unlock_pi(uaddr, flags);
        case FUTEX_TRYLOCK_PI:
                return futex_lock_pi(uaddr, flags, NULL, 1);
        case FUTEX_WAIT_REQUEUE_PI:
                val3 = FUTEX_BITSET_MATCH_ANY;
                return futex_wait_requeue_pi(uaddr, flags, val, timeout, val3,
                                             uaddr2);
        case FUTEX_CMP_REQUEUE_PI:
                return futex_requeue(uaddr, flags, uaddr2, val, val2, &val3, 1);
        }
        return -ENOSYS;
}

static __always_inline bool futex_cmd_has_timeout(u32 cmd)
{
        switch (cmd) {
        case FUTEX_WAIT:
        case FUTEX_LOCK_PI:
        case FUTEX_LOCK_PI2:
        case FUTEX_WAIT_BITSET:
        case FUTEX_WAIT_REQUEUE_PI:
                return true;
        }
        return false;
}

static __always_inline int
futex_init_timeout(u32 cmd, u32 op, struct timespec64 *ts, ktime_t *t)
{
        if (!timespec64_valid(ts))
                return -EINVAL;

        *t = timespec64_to_ktime(*ts);
        if (cmd == FUTEX_WAIT)
                *t = ktime_add_safe(ktime_get(), *t);
        else if (cmd != FUTEX_LOCK_PI && !(op & FUTEX_CLOCK_REALTIME))
                *t = timens_ktime_to_host(CLOCK_MONOTONIC, *t);
        return 0;
}

SYSCALL_DEFINE6(futex, u32 __user *, uaddr, int, op, u32, val,
                const struct __kernel_timespec __user *, utime,
                u32 __user *, uaddr2, u32, val3)
{
        int ret, cmd = op & FUTEX_CMD_MASK;
        ktime_t t, *tp = NULL;
        struct timespec64 ts;

        if (utime && futex_cmd_has_timeout(cmd)) {
                if (unlikely(should_fail_futex(!(op & FUTEX_PRIVATE_FLAG))))
                        return -EFAULT;
                if (get_timespec64(&ts, utime))
                        return -EFAULT;
                ret = futex_init_timeout(cmd, op, &ts, &t);
                if (ret)
                        return ret;
                tp = &t;
        }

        return do_futex(uaddr, op, val, tp, uaddr2, (unsigned long)utime, val3);
}

#ifdef CONFIG_COMPAT
/*
 * Fetch a robust-list pointer. Bit 0 signals PI futexes:
 */
static inline int
compat_fetch_robust_entry(compat_uptr_t *uentry, struct robust_list __user **entry,
                   compat_uptr_t __user *head, unsigned int *pi)
{
        if (get_user(*uentry, head))
                return -EFAULT;

        *entry = compat_ptr((*uentry) & ~1);
        *pi = (unsigned int)(*uentry) & 1;

        return 0;
}

static void __user *futex_uaddr(struct robust_list __user *entry,
                                compat_long_t futex_offset)
{
        compat_uptr_t base = ptr_to_compat(entry);
        void __user *uaddr = compat_ptr(base + futex_offset);

        return uaddr;
}

/*
 * Walk curr->robust_list (very carefully, it's a userspace list!)
 * and mark any locks found there dead, and notify any waiters.
 *
 * We silently return on any sign of list-walking problem.
 */
static void compat_exit_robust_list(struct task_struct *curr)
{
        struct compat_robust_list_head __user *head = curr->compat_robust_list;
        struct robust_list __user *entry, *next_entry, *pending;
        unsigned int limit = ROBUST_LIST_LIMIT, pi, pip;
        unsigned int next_pi;
        compat_uptr_t uentry, next_uentry, upending;
        compat_long_t futex_offset;
        int rc;

        if (!futex_cmpxchg_enabled)
                return;

        /*
         * Fetch the list head (which was registered earlier, via
         * sys_set_robust_list()):
         */
        if (compat_fetch_robust_entry(&uentry, &entry, &head->list.next, &pi))
                return;
        /*
         * Fetch the relative futex offset:
         */
        if (get_user(futex_offset, &head->futex_offset))
                return;
        /*
         * Fetch any possibly pending lock-add first, and handle it
         * if it exists:
         */
        if (compat_fetch_robust_entry(&upending, &pending,
                               &head->list_op_pending, &pip))
                return;

        next_entry = NULL;      /* avoid warning with gcc */
        while (entry != (struct robust_list __user *) &head->list) {
                /*
                 * Fetch the next entry in the list before calling
                 * handle_futex_death:
                 */
                rc = compat_fetch_robust_entry(&next_uentry, &next_entry,
                        (compat_uptr_t __user *)&entry->next, &next_pi);
                /*
                 * A pending lock might already be on the list, so
                 * dont process it twice:
                 */
                if (entry != pending) {
                        void __user *uaddr = futex_uaddr(entry, futex_offset);

                        if (handle_futex_death(uaddr, curr, pi,
                                               HANDLE_DEATH_LIST))
                                return;
                }
                if (rc)
                        return;
                uentry = next_uentry;
                entry = next_entry;
                pi = next_pi;
                /*
                 * Avoid excessively long or circular lists:
                 */
                if (!--limit)
                        break;

                cond_resched();
        }
        if (pending) {
                void __user *uaddr = futex_uaddr(pending, futex_offset);

                handle_futex_death(uaddr, curr, pip, HANDLE_DEATH_PENDING);
        }
}

COMPAT_SYSCALL_DEFINE2(set_robust_list,
                struct compat_robust_list_head __user *, head,
                compat_size_t, len)
{
        if (!futex_cmpxchg_enabled)
                return -ENOSYS;

        if (unlikely(len != sizeof(*head)))
                return -EINVAL;

        current->compat_robust_list = head;

        return 0;
}

COMPAT_SYSCALL_DEFINE3(get_robust_list, int, pid,
                        compat_uptr_t __user *, head_ptr,
                        compat_size_t __user *, len_ptr)
{
        struct compat_robust_list_head __user *head;
        unsigned long ret;
        struct task_struct *p;

        if (!futex_cmpxchg_enabled)
                return -ENOSYS;

        rcu_read_lock();

        ret = -ESRCH;
        if (!pid)
                p = current;
        else {
                p = find_task_by_vpid(pid);
                if (!p)
                        goto err_unlock;
        }

        ret = -EPERM;
        if (!ptrace_may_access(p, PTRACE_MODE_READ_REALCREDS))
                goto err_unlock;

        head = p->compat_robust_list;
        rcu_read_unlock();

        if (put_user(sizeof(*head), len_ptr))
                return -EFAULT;
        return put_user(ptr_to_compat(head), head_ptr);

err_unlock:
        rcu_read_unlock();

        return ret;
}
#endif /* CONFIG_COMPAT */

#ifdef CONFIG_COMPAT_32BIT_TIME
SYSCALL_DEFINE6(futex_time32, u32 __user *, uaddr, int, op, u32, val,
                const struct old_timespec32 __user *, utime, u32 __user *, uaddr2,
                u32, val3)
{
        int ret, cmd = op & FUTEX_CMD_MASK;
        ktime_t t, *tp = NULL;
        struct timespec64 ts;

        if (utime && futex_cmd_has_timeout(cmd)) {
                if (get_old_timespec32(&ts, utime))
                        return -EFAULT;
                ret = futex_init_timeout(cmd, op, &ts, &t);
                if (ret)
                        return ret;
                tp = &t;
        }

        return do_futex(uaddr, op, val, tp, uaddr2, (unsigned long)utime, val3);
}
#endif /* CONFIG_COMPAT_32BIT_TIME */

static void __init futex_detect_cmpxchg(void)
{
#ifndef CONFIG_HAVE_FUTEX_CMPXCHG
        u32 curval;

        /*
         * This will fail and we want it. Some arch implementations do
         * runtime detection of the futex_atomic_cmpxchg_inatomic()
         * functionality. We want to know that before we call in any
         * of the complex code paths. Also we want to prevent
         * registration of robust lists in that case. NULL is
         * guaranteed to fault and we get -EFAULT on functional
         * implementation, the non-functional ones will return
         * -ENOSYS.
         */
        if (cmpxchg_futex_value_locked(&curval, NULL, 0, 0) == -EFAULT)
                futex_cmpxchg_enabled = 1;
#endif
}

static int __init futex_init(void)
{
        unsigned int futex_shift;
        unsigned long i;

#if CONFIG_BASE_SMALL
        futex_hashsize = 16;
#else
        futex_hashsize = roundup_pow_of_two(256 * num_possible_cpus());
#endif

        futex_queues = alloc_large_system_hash("futex", sizeof(*futex_queues),
                                               futex_hashsize, 0,
                                               futex_hashsize < 256 ? HASH_SMALL : 0,
                                               &futex_shift, NULL,
                                               futex_hashsize, futex_hashsize);
        futex_hashsize = 1UL << futex_shift;

        futex_detect_cmpxchg();

        for (i = 0; i < futex_hashsize; i++) {
                atomic_set(&futex_queues[i].waiters, 0);
                plist_head_init(&futex_queues[i].chain);
                spin_lock_init(&futex_queues[i].lock);
        }

        return 0;
}
core_initcall(futex_init);