kernel/locking/rwbase_rt.c

   1 // SPDX-License-Identifier: GPL-2.0-only
   2
   3 /*
   4  * RT-specific reader/writer semaphores and reader/writer locks
   5  *
   6  * down_write/write_lock()
   7  *  1) Lock rtmutex
   8  *  2) Remove the reader BIAS to force readers into the slow path
   9  *  3) Wait until all readers have left the critical section
  10  *  4) Mark it write locked
  11  *
  12  * up_write/write_unlock()
  13  *  1) Remove the write locked marker
  14  *  2) Set the reader BIAS, so readers can use the fast path again
  15  *  3) Unlock rtmutex, to release blocked readers
  16  *
  17  * down_read/read_lock()
  18  *  1) Try fast path acquisition (reader BIAS is set)
  19  *  2) Take tmutex::wait_lock, which protects the writelocked flag
  20  *  3) If !writelocked, acquire it for read
  21  *  4) If writelocked, block on tmutex
  22  *  5) unlock rtmutex, goto 1)
  23  *
  24  * up_read/read_unlock()
  25  *  1) Try fast path release (reader count != 1)
  26  *  2) Wake the writer waiting in down_write()/write_lock() #3
  27  *
  28  * down_read/read_lock()#3 has the consequence, that rw semaphores and rw
  29  * locks on RT are not writer fair, but writers, which should be avoided in
  30  * RT tasks (think mmap_sem), are subject to the rtmutex priority/DL
  31  * inheritance mechanism.
  32  *
  33  * It's possible to make the rw primitives writer fair by keeping a list of
  34  * active readers. A blocked writer would force all newly incoming readers
  35  * to block on the rtmutex, but the rtmutex would have to be proxy locked
  36  * for one reader after the other. We can't use multi-reader inheritance
  37  * because there is no way to support that with SCHED_DEADLINE.
  38  * Implementing the one by one reader boosting/handover mechanism is a
  39  * major surgery for a very dubious value.
  40  *
  41  * The risk of writer starvation is there, but the pathological use cases
  42  * which trigger it are not necessarily the typical RT workloads.
  43  *
  44  * Fast-path orderings:
  45  * The lock/unlock of readers can run in fast paths: lock and unlock are only
  46  * atomic ops, and there is no inner lock to provide ACQUIRE and RELEASE
  47  * semantics of rwbase_rt. Atomic ops should thus provide _acquire()
  48  * and _release() (or stronger).
  49  *
  50  * Common code shared between RT rw_semaphore and rwlock
  51  */
  52
  53 static __always_inline int rwbase_read_trylock(struct rwbase_rt *rwb)
  54 {
  55         int r;
  56
  57         /*
  58          * Increment reader count, if sem->readers < 0, i.e. READER_BIAS is
  59          * set.
  60          */
  61         for (r = atomic_read(&rwb->readers); r < 0;) {
  62                 if (likely(atomic_try_cmpxchg_acquire(&rwb->readers, &r, r + 1)))
  63                         return 1;
  64         }
  65         return 0;
  66 }
  67
  68 static int __sched __rwbase_read_lock(struct rwbase_rt *rwb,
  69                                       unsigned int state)
  70 {
  71         struct rt_mutex_base *rtm = &rwb->rtmutex;
  72         int ret;
  73
  74         raw_spin_lock_irq(&rtm->wait_lock);
  75         /*
  76          * Allow readers, as long as the writer has not completely
  77          * acquired the semaphore for write.
  78          */
  79         if (atomic_read(&rwb->readers) != WRITER_BIAS) {
  80                 atomic_inc(&rwb->readers);
  81                 raw_spin_unlock_irq(&rtm->wait_lock);
  82                 return 0;
  83         }
  84
  85         /*
  86          * Call into the slow lock path with the rtmutex->wait_lock
  87          * held, so this can't result in the following race:
  88          *
  89          * Reader1              Reader2         Writer
  90          *                      down_read()
  91          *                                      down_write()
  92          *                                      rtmutex_lock(m)
  93          *                                      wait()
  94          * down_read()
  95          * unlock(m->wait_lock)
  96          *                      up_read()
  97          *                      wake(Writer)
  98          *                                      lock(m->wait_lock)
  99          *                                      sem->writelocked=true
 100          *                                      unlock(m->wait_lock)
 101          *
 102          *                                      up_write()
 103          *                                      sem->writelocked=false
 104          *                                      rtmutex_unlock(m)
 105          *                      down_read()
 106          *                                      down_write()
 107          *                                      rtmutex_lock(m)
 108          *                                      wait()
 109          * rtmutex_lock(m)
 110          *
 111          * That would put Reader1 behind the writer waiting on
 112          * Reader2 to call up_read(), which might be unbound.
 113          */
 114
 115         trace_contention_begin(rwb, LCB_F_RT | LCB_F_READ);
 116
 117         /*
 118          * For rwlocks this returns 0 unconditionally, so the below
 119          * !ret conditionals are optimized out.
 120          */
 121         ret = rwbase_rtmutex_slowlock_locked(rtm, state);
 122
 123         /*
 124          * On success the rtmutex is held, so there can't be a writer
 125          * active. Increment the reader count and immediately drop the
 126          * rtmutex again.
 127          *
 128          * rtmutex->wait_lock has to be unlocked in any case of course.
 129          */
 130         if (!ret)
 131                 atomic_inc(&rwb->readers);
 132         raw_spin_unlock_irq(&rtm->wait_lock);
 133         if (!ret)
 134                 rwbase_rtmutex_unlock(rtm);
 135
 136         trace_contention_end(rwb, ret);
 137         return ret;
 138 }
 139
 140 static __always_inline int rwbase_read_lock(struct rwbase_rt *rwb,
 141                                             unsigned int state)
 142 {
 143         if (rwbase_read_trylock(rwb))
 144                 return 0;
 145
 146         return __rwbase_read_lock(rwb, state);
 147 }
 148
 149 static void __sched __rwbase_read_unlock(struct rwbase_rt *rwb,
 150                                          unsigned int state)
 151 {
 152         struct rt_mutex_base *rtm = &rwb->rtmutex;
 153         struct task_struct *owner;
 154         DEFINE_RT_WAKE_Q(wqh);
 155
 156         raw_spin_lock_irq(&rtm->wait_lock);
 157         /*
 158          * Wake the writer, i.e. the rtmutex owner. It might release the
 159          * rtmutex concurrently in the fast path (due to a signal), but to
 160          * clean up rwb->readers it needs to acquire rtm->wait_lock. The
 161          * worst case which can happen is a spurious wakeup.
 162          */
 163         owner = rt_mutex_owner(rtm);
 164         if (owner)
 165                 rt_mutex_wake_q_add_task(&wqh, owner, state);
 166
 167         /* Pairs with the preempt_enable in rt_mutex_wake_up_q() */
 168         preempt_disable();
 169         raw_spin_unlock_irq(&rtm->wait_lock);
 170         rt_mutex_wake_up_q(&wqh);
 171 }
 172
 173 static __always_inline void rwbase_read_unlock(struct rwbase_rt *rwb,
 174                                                unsigned int state)
 175 {
 176         /*
 177          * rwb->readers can only hit 0 when a writer is waiting for the
 178          * active readers to leave the critical section.
 179          *
 180          * dec_and_test() is fully ordered, provides RELEASE.
 181          */
 182         if (unlikely(atomic_dec_and_test(&rwb->readers)))
 183                 __rwbase_read_unlock(rwb, state);
 184 }
 185
 186 static inline void __rwbase_write_unlock(struct rwbase_rt *rwb, int bias,
 187                                          unsigned long flags)
 188 {
 189         struct rt_mutex_base *rtm = &rwb->rtmutex;
 190
 191         /*
 192          * _release() is needed in case that reader is in fast path, pairing
 193          * with atomic_try_cmpxchg_acquire() in rwbase_read_trylock().
 194          */
 195         (void)atomic_add_return_release(READER_BIAS - bias, &rwb->readers);
 196         raw_spin_unlock_irqrestore(&rtm->wait_lock, flags);
 197         rwbase_rtmutex_unlock(rtm);
 198 }
 199
 200 static inline void rwbase_write_unlock(struct rwbase_rt *rwb)
 201 {
 202         struct rt_mutex_base *rtm = &rwb->rtmutex;
 203         unsigned long flags;
 204
 205         raw_spin_lock_irqsave(&rtm->wait_lock, flags);
 206         __rwbase_write_unlock(rwb, WRITER_BIAS, flags);
 207 }
 208
 209 static inline void rwbase_write_downgrade(struct rwbase_rt *rwb)
 210 {
 211         struct rt_mutex_base *rtm = &rwb->rtmutex;
 212         unsigned long flags;
 213
 214         raw_spin_lock_irqsave(&rtm->wait_lock, flags);
 215         /* Release it and account current as reader */
 216         __rwbase_write_unlock(rwb, WRITER_BIAS - 1, flags);
 217 }
 218
 219 static inline bool __rwbase_write_trylock(struct rwbase_rt *rwb)
 220 {
 221         /* Can do without CAS because we're serialized by wait_lock. */
 222         lockdep_assert_held(&rwb->rtmutex.wait_lock);
 223
 224         /*
 225          * _acquire is needed in case the reader is in the fast path, pairing
 226          * with rwbase_read_unlock(), provides ACQUIRE.
 227          */
 228         if (!atomic_read_acquire(&rwb->readers)) {
 229                 atomic_set(&rwb->readers, WRITER_BIAS);
 230                 return 1;
 231         }
 232
 233         return 0;
 234 }
 235
 236 static int __sched rwbase_write_lock(struct rwbase_rt *rwb,
 237                                      unsigned int state)
 238 {
 239         struct rt_mutex_base *rtm = &rwb->rtmutex;
 240         unsigned long flags;
 241
 242         /* Take the rtmutex as a first step */
 243         if (rwbase_rtmutex_lock_state(rtm, state))
 244                 return -EINTR;
 245
 246         /* Force readers into slow path */
 247         atomic_sub(READER_BIAS, &rwb->readers);
 248
 249         raw_spin_lock_irqsave(&rtm->wait_lock, flags);
 250         if (__rwbase_write_trylock(rwb))
 251                 goto out_unlock;
 252
 253         rwbase_set_and_save_current_state(state);
 254         trace_contention_begin(rwb, LCB_F_RT | LCB_F_WRITE);
 255         for (;;) {
 256                 /* Optimized out for rwlocks */
 257                 if (rwbase_signal_pending_state(state, current)) {
 258                         rwbase_restore_current_state();
 259                         __rwbase_write_unlock(rwb, 0, flags);
 260                         trace_contention_end(rwb, -EINTR);
 261                         return -EINTR;
 262                 }
 263
 264                 if (__rwbase_write_trylock(rwb))
 265                         break;
 266
 267                 raw_spin_unlock_irqrestore(&rtm->wait_lock, flags);
 268                 rwbase_schedule();
 269                 raw_spin_lock_irqsave(&rtm->wait_lock, flags);
 270
 271                 set_current_state(state);
 272         }
 273         rwbase_restore_current_state();
 274         trace_contention_end(rwb, 0);
 275
 276 out_unlock:
 277         raw_spin_unlock_irqrestore(&rtm->wait_lock, flags);
 278         return 0;
 279 }
 280
 281 static inline int rwbase_write_trylock(struct rwbase_rt *rwb)
 282 {
 283         struct rt_mutex_base *rtm = &rwb->rtmutex;
 284         unsigned long flags;
 285
 286         if (!rwbase_rtmutex_trylock(rtm))
 287                 return 0;
 288
 289         atomic_sub(READER_BIAS, &rwb->readers);
 290
 291         raw_spin_lock_irqsave(&rtm->wait_lock, flags);
 292         if (__rwbase_write_trylock(rwb)) {
 293                 raw_spin_unlock_irqrestore(&rtm->wait_lock, flags);
 294                 return 1;
 295         }
 296         __rwbase_write_unlock(rwb, 0, flags);
 297         return 0;
 298 }