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         DEFINE_WAKE_Q(wake_q);
  73         int ret;
  74
  75         rwbase_pre_schedule();
  76         raw_spin_lock_irq(&rtm->wait_lock);
  77
  78         /*
  79          * Call into the slow lock path with the rtmutex->wait_lock
  80          * held, so this can't result in the following race:
  81          *
  82          * Reader1              Reader2         Writer
  83          *                      down_read()
  84          *                                      down_write()
  85          *                                      rtmutex_lock(m)
  86          *                                      wait()
  87          * down_read()
  88          * unlock(m->wait_lock)
  89          *                      up_read()
  90          *                      wake(Writer)
  91          *                                      lock(m->wait_lock)
  92          *                                      sem->writelocked=true
  93          *                                      unlock(m->wait_lock)
  94          *
  95          *                                      up_write()
  96          *                                      sem->writelocked=false
  97          *                                      rtmutex_unlock(m)
  98          *                      down_read()
  99          *                                      down_write()
 100          *                                      rtmutex_lock(m)
 101          *                                      wait()
 102          * rtmutex_lock(m)
 103          *
 104          * That would put Reader1 behind the writer waiting on
 105          * Reader2 to call up_read(), which might be unbound.
 106          */
 107
 108         trace_contention_begin(rwb, LCB_F_RT | LCB_F_READ);
 109
 110         /*
 111          * For rwlocks this returns 0 unconditionally, so the below
 112          * !ret conditionals are optimized out.
 113          */
 114         ret = rwbase_rtmutex_slowlock_locked(rtm, state, &wake_q);
 115
 116         /*
 117          * On success the rtmutex is held, so there can't be a writer
 118          * active. Increment the reader count and immediately drop the
 119          * rtmutex again.
 120          *
 121          * rtmutex->wait_lock has to be unlocked in any case of course.
 122          */
 123         if (!ret)
 124                 atomic_inc(&rwb->readers);
 125
 126         preempt_disable();
 127         raw_spin_unlock_irq(&rtm->wait_lock);
 128         wake_up_q(&wake_q);
 129         preempt_enable();
 130
 131         if (!ret)
 132                 rwbase_rtmutex_unlock(rtm);
 133
 134         trace_contention_end(rwb, ret);
 135         rwbase_post_schedule();
 136         return ret;
 137 }
 138
 139 static __always_inline int rwbase_read_lock(struct rwbase_rt *rwb,
 140                                             unsigned int state)
 141 {
 142         lockdep_assert(!current->pi_blocked_on);
 143
 144         if (rwbase_read_trylock(rwb))
 145                 return 0;
 146
 147         return __rwbase_read_lock(rwb, state);
 148 }
 149
 150 static void __sched __rwbase_read_unlock(struct rwbase_rt *rwb,
 151                                          unsigned int state)
 152 {
 153         struct rt_mutex_base *rtm = &rwb->rtmutex;
 154         struct task_struct *owner;
 155         DEFINE_RT_WAKE_Q(wqh);
 156
 157         raw_spin_lock_irq(&rtm->wait_lock);
 158         /*
 159          * Wake the writer, i.e. the rtmutex owner. It might release the
 160          * rtmutex concurrently in the fast path (due to a signal), but to
 161          * clean up rwb->readers it needs to acquire rtm->wait_lock. The
 162          * worst case which can happen is a spurious wakeup.
 163          */
 164         owner = rt_mutex_owner(rtm);
 165         if (owner)
 166                 rt_mutex_wake_q_add_task(&wqh, owner, state);
 167
 168         /* Pairs with the preempt_enable in rt_mutex_wake_up_q() */
 169         preempt_disable();
 170         raw_spin_unlock_irq(&rtm->wait_lock);
 171         rt_mutex_wake_up_q(&wqh);
 172 }
 173
 174 static __always_inline void rwbase_read_unlock(struct rwbase_rt *rwb,
 175                                                unsigned int state)
 176 {
 177         /*
 178          * rwb->readers can only hit 0 when a writer is waiting for the
 179          * active readers to leave the critical section.
 180          *
 181          * dec_and_test() is fully ordered, provides RELEASE.
 182          */
 183         if (unlikely(atomic_dec_and_test(&rwb->readers)))
 184                 __rwbase_read_unlock(rwb, state);
 185 }
 186
 187 static inline void __rwbase_write_unlock(struct rwbase_rt *rwb, int bias,
 188                                          unsigned long flags)
 189 {
 190         struct rt_mutex_base *rtm = &rwb->rtmutex;
 191
 192         /*
 193          * _release() is needed in case that reader is in fast path, pairing
 194          * with atomic_try_cmpxchg_acquire() in rwbase_read_trylock().
 195          */
 196         (void)atomic_add_return_release(READER_BIAS - bias, &rwb->readers);
 197         raw_spin_unlock_irqrestore(&rtm->wait_lock, flags);
 198         rwbase_rtmutex_unlock(rtm);
 199 }
 200
 201 static inline void rwbase_write_unlock(struct rwbase_rt *rwb)
 202 {
 203         struct rt_mutex_base *rtm = &rwb->rtmutex;
 204         unsigned long flags;
 205
 206         raw_spin_lock_irqsave(&rtm->wait_lock, flags);
 207         __rwbase_write_unlock(rwb, WRITER_BIAS, flags);
 208 }
 209
 210 static inline void rwbase_write_downgrade(struct rwbase_rt *rwb)
 211 {
 212         struct rt_mutex_base *rtm = &rwb->rtmutex;
 213         unsigned long flags;
 214
 215         raw_spin_lock_irqsave(&rtm->wait_lock, flags);
 216         /* Release it and account current as reader */
 217         __rwbase_write_unlock(rwb, WRITER_BIAS - 1, flags);
 218 }
 219
 220 static inline bool __rwbase_write_trylock(struct rwbase_rt *rwb)
 221 {
 222         /* Can do without CAS because we're serialized by wait_lock. */
 223         lockdep_assert_held(&rwb->rtmutex.wait_lock);
 224
 225         /*
 226          * _acquire is needed in case the reader is in the fast path, pairing
 227          * with rwbase_read_unlock(), provides ACQUIRE.
 228          */
 229         if (!atomic_read_acquire(&rwb->readers)) {
 230                 atomic_set(&rwb->readers, WRITER_BIAS);
 231                 return 1;
 232         }
 233
 234         return 0;
 235 }
 236
 237 static int __sched rwbase_write_lock(struct rwbase_rt *rwb,
 238                                      unsigned int state)
 239 {
 240         struct rt_mutex_base *rtm = &rwb->rtmutex;
 241         unsigned long flags;
 242
 243         /* Take the rtmutex as a first step */
 244         if (rwbase_rtmutex_lock_state(rtm, state))
 245                 return -EINTR;
 246
 247         /* Force readers into slow path */
 248         atomic_sub(READER_BIAS, &rwb->readers);
 249
 250         rwbase_pre_schedule();
 251
 252         raw_spin_lock_irqsave(&rtm->wait_lock, flags);
 253         if (__rwbase_write_trylock(rwb))
 254                 goto out_unlock;
 255
 256         rwbase_set_and_save_current_state(state);
 257         trace_contention_begin(rwb, LCB_F_RT | LCB_F_WRITE);
 258         for (;;) {
 259                 /* Optimized out for rwlocks */
 260                 if (rwbase_signal_pending_state(state, current)) {
 261                         rwbase_restore_current_state();
 262                         __rwbase_write_unlock(rwb, 0, flags);
 263                         rwbase_post_schedule();
 264                         trace_contention_end(rwb, -EINTR);
 265                         return -EINTR;
 266                 }
 267
 268                 if (__rwbase_write_trylock(rwb))
 269                         break;
 270
 271                 raw_spin_unlock_irqrestore(&rtm->wait_lock, flags);
 272                 rwbase_schedule();
 273                 raw_spin_lock_irqsave(&rtm->wait_lock, flags);
 274
 275                 set_current_state(state);
 276         }
 277         rwbase_restore_current_state();
 278         trace_contention_end(rwb, 0);
 279
 280 out_unlock:
 281         raw_spin_unlock_irqrestore(&rtm->wait_lock, flags);
 282         rwbase_post_schedule();
 283         return 0;
 284 }
 285
 286 static inline int rwbase_write_trylock(struct rwbase_rt *rwb)
 287 {
 288         struct rt_mutex_base *rtm = &rwb->rtmutex;
 289         unsigned long flags;
 290
 291         if (!rwbase_rtmutex_trylock(rtm))
 292                 return 0;
 293
 294         atomic_sub(READER_BIAS, &rwb->readers);
 295
 296         raw_spin_lock_irqsave(&rtm->wait_lock, flags);
 297         if (__rwbase_write_trylock(rwb)) {
 298                 raw_spin_unlock_irqrestore(&rtm->wait_lock, flags);
 299                 return 1;
 300         }
 301         __rwbase_write_unlock(rwb, 0, flags);
 302         return 0;
 303 }