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                 /* Fully-ordered if cmpxchg() succeeds, provides ACQUIRE */
  63                 if (likely(atomic_try_cmpxchg(&rwb->readers, &r, r + 1)))
  64                         return 1;
  65         }
  66         return 0;
  67 }
  68
  69 static int __sched __rwbase_read_lock(struct rwbase_rt *rwb,
  70                                       unsigned int state)
  71 {
  72         struct rt_mutex_base *rtm = &rwb->rtmutex;
  73         int ret;
  74
  75         raw_spin_lock_irq(&rtm->wait_lock);
  76         /*
  77          * Allow readers, as long as the writer has not completely
  78          * acquired the semaphore for write.
  79          */
  80         if (atomic_read(&rwb->readers) != WRITER_BIAS) {
  81                 atomic_inc(&rwb->readers);
  82                 raw_spin_unlock_irq(&rtm->wait_lock);
  83                 return 0;
  84         }
  85
  86         /*
  87          * Call into the slow lock path with the rtmutex->wait_lock
  88          * held, so this can't result in the following race:
  89          *
  90          * Reader1              Reader2         Writer
  91          *                      down_read()
  92          *                                      down_write()
  93          *                                      rtmutex_lock(m)
  94          *                                      wait()
  95          * down_read()
  96          * unlock(m->wait_lock)
  97          *                      up_read()
  98          *                      wake(Writer)
  99          *                                      lock(m->wait_lock)
 100          *                                      sem->writelocked=true
 101          *                                      unlock(m->wait_lock)
 102          *
 103          *                                      up_write()
 104          *                                      sem->writelocked=false
 105          *                                      rtmutex_unlock(m)
 106          *                      down_read()
 107          *                                      down_write()
 108          *                                      rtmutex_lock(m)
 109          *                                      wait()
 110          * rtmutex_lock(m)
 111          *
 112          * That would put Reader1 behind the writer waiting on
 113          * Reader2 to call up_read(), which might be unbound.
 114          */
 115
 116         /*
 117          * For rwlocks this returns 0 unconditionally, so the below
 118          * !ret conditionals are optimized out.
 119          */
 120         ret = rwbase_rtmutex_slowlock_locked(rtm, state);
 121
 122         /*
 123          * On success the rtmutex is held, so there can't be a writer
 124          * active. Increment the reader count and immediately drop the
 125          * rtmutex again.
 126          *
 127          * rtmutex->wait_lock has to be unlocked in any case of course.
 128          */
 129         if (!ret)
 130                 atomic_inc(&rwb->readers);
 131         raw_spin_unlock_irq(&rtm->wait_lock);
 132         if (!ret)
 133                 rwbase_rtmutex_unlock(rtm);
 134         return ret;
 135 }
 136
 137 static __always_inline int rwbase_read_lock(struct rwbase_rt *rwb,
 138                                             unsigned int state)
 139 {
 140         if (rwbase_read_trylock(rwb))
 141                 return 0;
 142
 143         return __rwbase_read_lock(rwb, state);
 144 }
 145
 146 static void __sched __rwbase_read_unlock(struct rwbase_rt *rwb,
 147                                          unsigned int state)
 148 {
 149         struct rt_mutex_base *rtm = &rwb->rtmutex;
 150         struct task_struct *owner;
 151
 152         raw_spin_lock_irq(&rtm->wait_lock);
 153         /*
 154          * Wake the writer, i.e. the rtmutex owner. It might release the
 155          * rtmutex concurrently in the fast path (due to a signal), but to
 156          * clean up rwb->readers it needs to acquire rtm->wait_lock. The
 157          * worst case which can happen is a spurious wakeup.
 158          */
 159         owner = rt_mutex_owner(rtm);
 160         if (owner)
 161                 wake_up_state(owner, state);
 162
 163         raw_spin_unlock_irq(&rtm->wait_lock);
 164 }
 165
 166 static __always_inline void rwbase_read_unlock(struct rwbase_rt *rwb,
 167                                                unsigned int state)
 168 {
 169         /*
 170          * rwb->readers can only hit 0 when a writer is waiting for the
 171          * active readers to leave the critical section.
 172          *
 173          * dec_and_test() is fully ordered, provides RELEASE.
 174          */
 175         if (unlikely(atomic_dec_and_test(&rwb->readers)))
 176                 __rwbase_read_unlock(rwb, state);
 177 }
 178
 179 static inline void __rwbase_write_unlock(struct rwbase_rt *rwb, int bias,
 180                                          unsigned long flags)
 181 {
 182         struct rt_mutex_base *rtm = &rwb->rtmutex;
 183
 184         /*
 185          * _release() is needed in case that reader is in fast path, pairing
 186          * with atomic_try_cmpxchg() in rwbase_read_trylock(), provides RELEASE
 187          */
 188         (void)atomic_add_return_release(READER_BIAS - bias, &rwb->readers);
 189         raw_spin_unlock_irqrestore(&rtm->wait_lock, flags);
 190         rwbase_rtmutex_unlock(rtm);
 191 }
 192
 193 static inline void rwbase_write_unlock(struct rwbase_rt *rwb)
 194 {
 195         struct rt_mutex_base *rtm = &rwb->rtmutex;
 196         unsigned long flags;
 197
 198         raw_spin_lock_irqsave(&rtm->wait_lock, flags);
 199         __rwbase_write_unlock(rwb, WRITER_BIAS, flags);
 200 }
 201
 202 static inline void rwbase_write_downgrade(struct rwbase_rt *rwb)
 203 {
 204         struct rt_mutex_base *rtm = &rwb->rtmutex;
 205         unsigned long flags;
 206
 207         raw_spin_lock_irqsave(&rtm->wait_lock, flags);
 208         /* Release it and account current as reader */
 209         __rwbase_write_unlock(rwb, WRITER_BIAS - 1, flags);
 210 }
 211
 212 static inline bool __rwbase_write_trylock(struct rwbase_rt *rwb)
 213 {
 214         /* Can do without CAS because we're serialized by wait_lock. */
 215         lockdep_assert_held(&rwb->rtmutex.wait_lock);
 216
 217         /*
 218          * _acquire is needed in case the reader is in the fast path, pairing
 219          * with rwbase_read_unlock(), provides ACQUIRE.
 220          */
 221         if (!atomic_read_acquire(&rwb->readers)) {
 222                 atomic_set(&rwb->readers, WRITER_BIAS);
 223                 return 1;
 224         }
 225
 226         return 0;
 227 }
 228
 229 static int __sched rwbase_write_lock(struct rwbase_rt *rwb,
 230                                      unsigned int state)
 231 {
 232         struct rt_mutex_base *rtm = &rwb->rtmutex;
 233         unsigned long flags;
 234
 235         /* Take the rtmutex as a first step */
 236         if (rwbase_rtmutex_lock_state(rtm, state))
 237                 return -EINTR;
 238
 239         /* Force readers into slow path */
 240         atomic_sub(READER_BIAS, &rwb->readers);
 241
 242         raw_spin_lock_irqsave(&rtm->wait_lock, flags);
 243         if (__rwbase_write_trylock(rwb))
 244                 goto out_unlock;
 245
 246         rwbase_set_and_save_current_state(state);
 247         for (;;) {
 248                 /* Optimized out for rwlocks */
 249                 if (rwbase_signal_pending_state(state, current)) {
 250                         rwbase_restore_current_state();
 251                         __rwbase_write_unlock(rwb, 0, flags);
 252                         return -EINTR;
 253                 }
 254
 255                 if (__rwbase_write_trylock(rwb))
 256                         break;
 257
 258                 raw_spin_unlock_irqrestore(&rtm->wait_lock, flags);
 259                 rwbase_schedule();
 260                 raw_spin_lock_irqsave(&rtm->wait_lock, flags);
 261
 262                 set_current_state(state);
 263         }
 264         rwbase_restore_current_state();
 265
 266 out_unlock:
 267         raw_spin_unlock_irqrestore(&rtm->wait_lock, flags);
 268         return 0;
 269 }
 270
 271 static inline int rwbase_write_trylock(struct rwbase_rt *rwb)
 272 {
 273         struct rt_mutex_base *rtm = &rwb->rtmutex;
 274         unsigned long flags;
 275
 276         if (!rwbase_rtmutex_trylock(rtm))
 277                 return 0;
 278
 279         atomic_sub(READER_BIAS, &rwb->readers);
 280
 281         raw_spin_lock_irqsave(&rtm->wait_lock, flags);
 282         if (__rwbase_write_trylock(rwb)) {
 283                 raw_spin_unlock_irqrestore(&rtm->wait_lock, flags);
 284                 return 1;
 285         }
 286         __rwbase_write_unlock(rwb, 0, flags);
 287         return 0;
 288 }