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  * Common code shared between RT rw_semaphore and rwlock
  45  */
  46
  47 static __always_inline int rwbase_read_trylock(struct rwbase_rt *rwb)
  48 {
  49         int r;
  50
  51         /*
  52          * Increment reader count, if sem->readers < 0, i.e. READER_BIAS is
  53          * set.
  54          */
  55         for (r = atomic_read(&rwb->readers); r < 0;) {
  56                 if (likely(atomic_try_cmpxchg(&rwb->readers, &r, r + 1)))
  57                         return 1;
  58         }
  59         return 0;
  60 }
  61
  62 static int __sched __rwbase_read_lock(struct rwbase_rt *rwb,
  63                                       unsigned int state)
  64 {
  65         struct rt_mutex_base *rtm = &rwb->rtmutex;
  66         int ret;
  67
  68         raw_spin_lock_irq(&rtm->wait_lock);
  69         /*
  70          * Allow readers, as long as the writer has not completely
  71          * acquired the semaphore for write.
  72          */
  73         if (atomic_read(&rwb->readers) != WRITER_BIAS) {
  74                 atomic_inc(&rwb->readers);
  75                 raw_spin_unlock_irq(&rtm->wait_lock);
  76                 return 0;
  77         }
  78
  79         /*
  80          * Call into the slow lock path with the rtmutex->wait_lock
  81          * held, so this can't result in the following race:
  82          *
  83          * Reader1              Reader2         Writer
  84          *                      down_read()
  85          *                                      down_write()
  86          *                                      rtmutex_lock(m)
  87          *                                      wait()
  88          * down_read()
  89          * unlock(m->wait_lock)
  90          *                      up_read()
  91          *                      wake(Writer)
  92          *                                      lock(m->wait_lock)
  93          *                                      sem->writelocked=true
  94          *                                      unlock(m->wait_lock)
  95          *
  96          *                                      up_write()
  97          *                                      sem->writelocked=false
  98          *                                      rtmutex_unlock(m)
  99          *                      down_read()
 100          *                                      down_write()
 101          *                                      rtmutex_lock(m)
 102          *                                      wait()
 103          * rtmutex_lock(m)
 104          *
 105          * That would put Reader1 behind the writer waiting on
 106          * Reader2 to call up_read(), which might be unbound.
 107          */
 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         return ret;
 128 }
 129
 130 static __always_inline int rwbase_read_lock(struct rwbase_rt *rwb,
 131                                             unsigned int state)
 132 {
 133         if (rwbase_read_trylock(rwb))
 134                 return 0;
 135
 136         return __rwbase_read_lock(rwb, state);
 137 }
 138
 139 static void __sched __rwbase_read_unlock(struct rwbase_rt *rwb,
 140                                          unsigned int state)
 141 {
 142         struct rt_mutex_base *rtm = &rwb->rtmutex;
 143         struct task_struct *owner;
 144
 145         raw_spin_lock_irq(&rtm->wait_lock);
 146         /*
 147          * Wake the writer, i.e. the rtmutex owner. It might release the
 148          * rtmutex concurrently in the fast path (due to a signal), but to
 149          * clean up rwb->readers it needs to acquire rtm->wait_lock. The
 150          * worst case which can happen is a spurious wakeup.
 151          */
 152         owner = rt_mutex_owner(rtm);
 153         if (owner)
 154                 wake_up_state(owner, state);
 155
 156         raw_spin_unlock_irq(&rtm->wait_lock);
 157 }
 158
 159 static __always_inline void rwbase_read_unlock(struct rwbase_rt *rwb,
 160                                                unsigned int state)
 161 {
 162         /*
 163          * rwb->readers can only hit 0 when a writer is waiting for the
 164          * active readers to leave the critical section.
 165          */
 166         if (unlikely(atomic_dec_and_test(&rwb->readers)))
 167                 __rwbase_read_unlock(rwb, state);
 168 }
 169
 170 static inline void __rwbase_write_unlock(struct rwbase_rt *rwb, int bias,
 171                                          unsigned long flags)
 172 {
 173         struct rt_mutex_base *rtm = &rwb->rtmutex;
 174
 175         atomic_add(READER_BIAS - bias, &rwb->readers);
 176         raw_spin_unlock_irqrestore(&rtm->wait_lock, flags);
 177         rwbase_rtmutex_unlock(rtm);
 178 }
 179
 180 static inline void rwbase_write_unlock(struct rwbase_rt *rwb)
 181 {
 182         struct rt_mutex_base *rtm = &rwb->rtmutex;
 183         unsigned long flags;
 184
 185         raw_spin_lock_irqsave(&rtm->wait_lock, flags);
 186         __rwbase_write_unlock(rwb, WRITER_BIAS, flags);
 187 }
 188
 189 static inline void rwbase_write_downgrade(struct rwbase_rt *rwb)
 190 {
 191         struct rt_mutex_base *rtm = &rwb->rtmutex;
 192         unsigned long flags;
 193
 194         raw_spin_lock_irqsave(&rtm->wait_lock, flags);
 195         /* Release it and account current as reader */
 196         __rwbase_write_unlock(rwb, WRITER_BIAS - 1, flags);
 197 }
 198
 199 static int __sched rwbase_write_lock(struct rwbase_rt *rwb,
 200                                      unsigned int state)
 201 {
 202         struct rt_mutex_base *rtm = &rwb->rtmutex;
 203         unsigned long flags;
 204
 205         /* Take the rtmutex as a first step */
 206         if (rwbase_rtmutex_lock_state(rtm, state))
 207                 return -EINTR;
 208
 209         /* Force readers into slow path */
 210         atomic_sub(READER_BIAS, &rwb->readers);
 211
 212         raw_spin_lock_irqsave(&rtm->wait_lock, flags);
 213         /*
 214          * set_current_state() for rw_semaphore
 215          * current_save_and_set_rtlock_wait_state() for rwlock
 216          */
 217         rwbase_set_and_save_current_state(state);
 218
 219         /* Block until all readers have left the critical section. */
 220         for (; atomic_read(&rwb->readers);) {
 221                 /* Optimized out for rwlocks */
 222                 if (rwbase_signal_pending_state(state, current)) {
 223                         __set_current_state(TASK_RUNNING);
 224                         __rwbase_write_unlock(rwb, 0, flags);
 225                         return -EINTR;
 226                 }
 227                 raw_spin_unlock_irqrestore(&rtm->wait_lock, flags);
 228
 229                 /*
 230                  * Schedule and wait for the readers to leave the critical
 231                  * section. The last reader leaving it wakes the waiter.
 232                  */
 233                 if (atomic_read(&rwb->readers) != 0)
 234                         rwbase_schedule();
 235                 set_current_state(state);
 236                 raw_spin_lock_irqsave(&rtm->wait_lock, flags);
 237         }
 238
 239         atomic_set(&rwb->readers, WRITER_BIAS);
 240         rwbase_restore_current_state();
 241         raw_spin_unlock_irqrestore(&rtm->wait_lock, flags);
 242         return 0;
 243 }
 244
 245 static inline int rwbase_write_trylock(struct rwbase_rt *rwb)
 246 {
 247         struct rt_mutex_base *rtm = &rwb->rtmutex;
 248         unsigned long flags;
 249
 250         if (!rwbase_rtmutex_trylock(rtm))
 251                 return 0;
 252
 253         atomic_sub(READER_BIAS, &rwb->readers);
 254
 255         raw_spin_lock_irqsave(&rtm->wait_lock, flags);
 256         if (!atomic_read(&rwb->readers)) {
 257                 atomic_set(&rwb->readers, WRITER_BIAS);
 258                 raw_spin_unlock_irqrestore(&rtm->wait_lock, flags);
 259                 return 1;
 260         }
 261         __rwbase_write_unlock(rwb, 0, flags);
 262         return 0;
 263 }