1 // SPDX-License-Identifier: GPL-2.0-only
4 * RT-specific reader/writer semaphores and reader/writer locks
6 * down_write/write_lock()
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
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
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)
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
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.
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.
41 * The risk of writer starvation is there, but the pathological use cases
42 * which trigger it are not necessarily the typical RT workloads.
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).
50 * Common code shared between RT rw_semaphore and rwlock
53 static __always_inline int rwbase_read_trylock(struct rwbase_rt *rwb)
58 * Increment reader count, if sem->readers < 0, i.e. READER_BIAS is
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)))
69 static int __sched __rwbase_read_lock(struct rwbase_rt *rwb,
72 struct rt_mutex_base *rtm = &rwb->rtmutex;
75 raw_spin_lock_irq(&rtm->wait_lock);
77 * Allow readers, as long as the writer has not completely
78 * acquired the semaphore for write.
80 if (atomic_read(&rwb->readers) != WRITER_BIAS) {
81 atomic_inc(&rwb->readers);
82 raw_spin_unlock_irq(&rtm->wait_lock);
87 * Call into the slow lock path with the rtmutex->wait_lock
88 * held, so this can't result in the following race:
90 * Reader1 Reader2 Writer
96 * unlock(m->wait_lock)
100 * sem->writelocked=true
101 * unlock(m->wait_lock)
104 * sem->writelocked=false
112 * That would put Reader1 behind the writer waiting on
113 * Reader2 to call up_read(), which might be unbound.
117 * For rwlocks this returns 0 unconditionally, so the below
118 * !ret conditionals are optimized out.
120 ret = rwbase_rtmutex_slowlock_locked(rtm, state);
123 * On success the rtmutex is held, so there can't be a writer
124 * active. Increment the reader count and immediately drop the
127 * rtmutex->wait_lock has to be unlocked in any case of course.
130 atomic_inc(&rwb->readers);
131 raw_spin_unlock_irq(&rtm->wait_lock);
133 rwbase_rtmutex_unlock(rtm);
137 static __always_inline int rwbase_read_lock(struct rwbase_rt *rwb,
140 if (rwbase_read_trylock(rwb))
143 return __rwbase_read_lock(rwb, state);
146 static void __sched __rwbase_read_unlock(struct rwbase_rt *rwb,
149 struct rt_mutex_base *rtm = &rwb->rtmutex;
150 struct task_struct *owner;
152 raw_spin_lock_irq(&rtm->wait_lock);
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.
159 owner = rt_mutex_owner(rtm);
161 wake_up_state(owner, state);
163 raw_spin_unlock_irq(&rtm->wait_lock);
166 static __always_inline void rwbase_read_unlock(struct rwbase_rt *rwb,
170 * rwb->readers can only hit 0 when a writer is waiting for the
171 * active readers to leave the critical section.
173 * dec_and_test() is fully ordered, provides RELEASE.
175 if (unlikely(atomic_dec_and_test(&rwb->readers)))
176 __rwbase_read_unlock(rwb, state);
179 static inline void __rwbase_write_unlock(struct rwbase_rt *rwb, int bias,
182 struct rt_mutex_base *rtm = &rwb->rtmutex;
185 * _release() is needed in case that reader is in fast path, pairing
186 * with atomic_try_cmpxchg() in rwbase_read_trylock(), provides RELEASE
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);
193 static inline void rwbase_write_unlock(struct rwbase_rt *rwb)
195 struct rt_mutex_base *rtm = &rwb->rtmutex;
198 raw_spin_lock_irqsave(&rtm->wait_lock, flags);
199 __rwbase_write_unlock(rwb, WRITER_BIAS, flags);
202 static inline void rwbase_write_downgrade(struct rwbase_rt *rwb)
204 struct rt_mutex_base *rtm = &rwb->rtmutex;
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);
212 static inline bool __rwbase_write_trylock(struct rwbase_rt *rwb)
214 /* Can do without CAS because we're serialized by wait_lock. */
215 lockdep_assert_held(&rwb->rtmutex.wait_lock);
218 * _acquire is needed in case the reader is in the fast path, pairing
219 * with rwbase_read_unlock(), provides ACQUIRE.
221 if (!atomic_read_acquire(&rwb->readers)) {
222 atomic_set(&rwb->readers, WRITER_BIAS);
229 static int __sched rwbase_write_lock(struct rwbase_rt *rwb,
232 struct rt_mutex_base *rtm = &rwb->rtmutex;
235 /* Take the rtmutex as a first step */
236 if (rwbase_rtmutex_lock_state(rtm, state))
239 /* Force readers into slow path */
240 atomic_sub(READER_BIAS, &rwb->readers);
242 raw_spin_lock_irqsave(&rtm->wait_lock, flags);
243 if (__rwbase_write_trylock(rwb))
246 rwbase_set_and_save_current_state(state);
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);
255 if (__rwbase_write_trylock(rwb))
258 raw_spin_unlock_irqrestore(&rtm->wait_lock, flags);
260 raw_spin_lock_irqsave(&rtm->wait_lock, flags);
262 set_current_state(state);
264 rwbase_restore_current_state();
267 raw_spin_unlock_irqrestore(&rtm->wait_lock, flags);
271 static inline int rwbase_write_trylock(struct rwbase_rt *rwb)
273 struct rt_mutex_base *rtm = &rwb->rtmutex;
276 if (!rwbase_rtmutex_trylock(rtm))
279 atomic_sub(READER_BIAS, &rwb->readers);
281 raw_spin_lock_irqsave(&rtm->wait_lock, flags);
282 if (__rwbase_write_trylock(rwb)) {
283 raw_spin_unlock_irqrestore(&rtm->wait_lock, flags);
286 __rwbase_write_unlock(rwb, 0, flags);