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