futex: Remove {get,drop}_futex_key_refs()

Now that {get,drop}_futex_key_refs() have become a glorified NOP,
remove them entirely.

The only thing get_futex_key_refs() is still doing is an smp_mb(), and
now that we don't need to (ab)use existing atomic ops to obtain them,
we can place it explicitly where we need it.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>

diff --git a/kernel/futex.c b/kernel/futex.c
index 3463c91..b62cf94 100644 (file)
--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -135,8 +135,7 @@
  *
  * 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 -- this is done by the barriers for both
- * shared and private futexes in get_futex_key_refs().
+ * to futex and the waiters read (see hb_waiters_pending()).
  *
  * This yields the following case (where X:=waiters, Y:=futex):
  *
@@ -359,6 +358,10 @@ static inline void hb_waiters_dec(struct futex_hash_bucket *hb)
 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;
@@ -396,68 +399,6 @@ static inline int match_futex(union futex_key *key1, union futex_key *key2)
                && key1->both.offset == key2->both.offset);
 }
 
-/*
- * Take a reference to the resource addressed by a key.
- * Can be called while holding spinlocks.
- *
- */
-static void get_futex_key_refs(union futex_key *key)
-{
-       if (!key->both.ptr)
-               return;
-
-       /*
-        * On MMU less systems futexes are always "private" as there is no per
-        * process address space. We need the smp wmb nevertheless - yes,
-        * arch/blackfin has MMU less SMP ...
-        */
-       if (!IS_ENABLED(CONFIG_MMU)) {
-               smp_mb(); /* explicit smp_mb(); (B) */
-               return;
-       }
-
-       switch (key->both.offset & (FUT_OFF_INODE|FUT_OFF_MMSHARED)) {
-       case FUT_OFF_INODE:
-               smp_mb();               /* explicit smp_mb(); (B) */
-               break;
-       case FUT_OFF_MMSHARED:
-               smp_mb();               /* explicit smp_mb(); (B) */
-               break;
-       default:
-               /*
-                * Private futexes do not hold reference on an inode or
-                * mm, therefore the only purpose of calling get_futex_key_refs
-                * is because we need the barrier for the lockless waiter check.
-                */
-               smp_mb(); /* explicit smp_mb(); (B) */
-       }
-}
-
-/*
- * Drop a reference to the resource addressed by a key.
- * The hash bucket spinlock must not be held. This is
- * a no-op for private futexes, see comment in the get
- * counterpart.
- */
-static void drop_futex_key_refs(union futex_key *key)
-{
-       if (!key->both.ptr) {
-               /* If we're here then we tried to put a key we failed to get */
-               WARN_ON_ONCE(1);
-               return;
-       }
-
-       if (!IS_ENABLED(CONFIG_MMU))
-               return;
-
-       switch (key->both.offset & (FUT_OFF_INODE|FUT_OFF_MMSHARED)) {
-       case FUT_OFF_INODE:
-               break;
-       case FUT_OFF_MMSHARED:
-               break;
-       }
-}
-
 enum futex_access {
        FUTEX_READ,
        FUTEX_WRITE
@@ -589,7 +530,6 @@ get_futex_key(u32 __user *uaddr, int fshared, union futex_key *key, enum futex_a
        if (!fshared) {
                key->private.mm = mm;
                key->private.address = address;
-               get_futex_key_refs(key);  /* implies smp_mb(); (B) */
                return 0;
        }
 
@@ -729,8 +669,6 @@ again:
                rcu_read_unlock();
        }
 
-       get_futex_key_refs(key); /* implies smp_mb(); (B) */
-
 out:
        put_page(page);
        return err;
@@ -738,7 +676,6 @@ out:
 
 static inline void put_futex_key(union futex_key *key)
 {
-       drop_futex_key_refs(key);
 }
 
 /**
@@ -1873,7 +1810,6 @@ void requeue_futex(struct futex_q *q, struct futex_hash_bucket *hb1,
                plist_add(&q->list, &hb2->chain);
                q->lock_ptr = &hb2->lock;
        }
-       get_futex_key_refs(key2);
        q->key = *key2;
 }
 
@@ -1895,7 +1831,6 @@ static inline
 void requeue_pi_wake_futex(struct futex_q *q, union futex_key *key,
                           struct futex_hash_bucket *hb)
 {
-       get_futex_key_refs(key);
        q->key = *key;
 
        __unqueue_futex(q);
@@ -2006,7 +1941,7 @@ static int futex_requeue(u32 __user *uaddr1, unsigned int flags,
                         u32 *cmpval, int requeue_pi)
 {
        union futex_key key1 = FUTEX_KEY_INIT, key2 = FUTEX_KEY_INIT;
-       int drop_count = 0, task_count = 0, ret;
+       int task_count = 0, ret;
        struct futex_pi_state *pi_state = NULL;
        struct futex_hash_bucket *hb1, *hb2;
        struct futex_q *this, *next;
@@ -2127,7 +2062,6 @@ retry_private:
                 */
                if (ret > 0) {
                        WARN_ON(pi_state);
-                       drop_count++;
                        task_count++;
                        /*
                         * If we acquired the lock, then the user space value
@@ -2247,7 +2181,6 @@ retry_private:
                                 * doing so.
                                 */
                                requeue_pi_wake_futex(this, &key2, hb2);
-                               drop_count++;
                                continue;
                        } else if (ret) {
                                /*
@@ -2268,7 +2201,6 @@ retry_private:
                        }
                }
                requeue_futex(this, hb1, hb2, &key2);
-               drop_count++;
        }
 
        /*
@@ -2283,15 +2215,6 @@ out_unlock:
        wake_up_q(&wake_q);
        hb_waiters_dec(hb2);
 
-       /*
-        * drop_futex_key_refs() must be called outside the spinlocks. During
-        * the requeue we moved futex_q's from the hash bucket at key1 to the
-        * one at key2 and updated their key pointer.  We no longer need to
-        * hold the references to key1.
-        */
-       while (--drop_count >= 0)
-               drop_futex_key_refs(&key1);
-
 out_put_keys:
        put_futex_key(&key2);
 out_put_key1:
@@ -2421,7 +2344,6 @@ retry:
                ret = 1;
        }
 
-       drop_futex_key_refs(&q->key);
        return ret;
 }