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1/* rwsem.c: R/W semaphores: contention handling functions
2 *
3 * Written by David Howells (dhowells@redhat.com).
4 * Derived from arch/i386/kernel/semaphore.c
5 *
6 * Writer lock-stealing by Alex Shi <alex.shi@intel.com>
7 * and Michel Lespinasse <walken@google.com>
8 *
9 * Optimistic spinning by Tim Chen <tim.c.chen@intel.com>
10 * and Davidlohr Bueso <davidlohr@hp.com>. Based on mutexes.
11 */
12#include <linux/rwsem.h>
13#include <linux/sched.h>
14#include <linux/init.h>
15#include <linux/export.h>
16#include <linux/sched/rt.h>
17#include <linux/osq_lock.h>
18
19#include "rwsem.h"
20
21/*
22 * Guide to the rw_semaphore's count field for common values.
23 * (32-bit case illustrated, similar for 64-bit)
24 *
25 * 0x0000000X (1) X readers active or attempting lock, no writer waiting
26 * X = #active_readers + #readers attempting to lock
27 * (X*ACTIVE_BIAS)
28 *
29 * 0x00000000 rwsem is unlocked, and no one is waiting for the lock or
30 * attempting to read lock or write lock.
31 *
32 * 0xffff000X (1) X readers active or attempting lock, with waiters for lock
33 * X = #active readers + # readers attempting lock
34 * (X*ACTIVE_BIAS + WAITING_BIAS)
35 * (2) 1 writer attempting lock, no waiters for lock
36 * X-1 = #active readers + #readers attempting lock
37 * ((X-1)*ACTIVE_BIAS + ACTIVE_WRITE_BIAS)
38 * (3) 1 writer active, no waiters for lock
39 * X-1 = #active readers + #readers attempting lock
40 * ((X-1)*ACTIVE_BIAS + ACTIVE_WRITE_BIAS)
41 *
42 * 0xffff0001 (1) 1 reader active or attempting lock, waiters for lock
43 * (WAITING_BIAS + ACTIVE_BIAS)
44 * (2) 1 writer active or attempting lock, no waiters for lock
45 * (ACTIVE_WRITE_BIAS)
46 *
47 * 0xffff0000 (1) There are writers or readers queued but none active
48 * or in the process of attempting lock.
49 * (WAITING_BIAS)
50 * Note: writer can attempt to steal lock for this count by adding
51 * ACTIVE_WRITE_BIAS in cmpxchg and checking the old count
52 *
53 * 0xfffe0001 (1) 1 writer active, or attempting lock. Waiters on queue.
54 * (ACTIVE_WRITE_BIAS + WAITING_BIAS)
55 *
56 * Note: Readers attempt to lock by adding ACTIVE_BIAS in down_read and checking
57 * the count becomes more than 0 for successful lock acquisition,
58 * i.e. the case where there are only readers or nobody has lock.
59 * (1st and 2nd case above).
60 *
61 * Writers attempt to lock by adding ACTIVE_WRITE_BIAS in down_write and
62 * checking the count becomes ACTIVE_WRITE_BIAS for successful lock
63 * acquisition (i.e. nobody else has lock or attempts lock). If
64 * unsuccessful, in rwsem_down_write_failed, we'll check to see if there
65 * are only waiters but none active (5th case above), and attempt to
66 * steal the lock.
67 *
68 */
69
70/*
71 * Initialize an rwsem:
72 */
73void __init_rwsem(struct rw_semaphore *sem, const char *name,
74 struct lock_class_key *key)
75{
76#ifdef CONFIG_DEBUG_LOCK_ALLOC
77 /*
78 * Make sure we are not reinitializing a held semaphore:
79 */
80 debug_check_no_locks_freed((void *)sem, sizeof(*sem));
81 lockdep_init_map(&sem->dep_map, name, key, 0);
82#endif
83 atomic_long_set(&sem->count, RWSEM_UNLOCKED_VALUE);
84 raw_spin_lock_init(&sem->wait_lock);
85 INIT_LIST_HEAD(&sem->wait_list);
86#ifdef CONFIG_RWSEM_SPIN_ON_OWNER
87 sem->owner = NULL;
88 osq_lock_init(&sem->osq);
89#endif
90}
91
92EXPORT_SYMBOL(__init_rwsem);
93
94enum rwsem_waiter_type {
95 RWSEM_WAITING_FOR_WRITE,
96 RWSEM_WAITING_FOR_READ
97};
98
99struct rwsem_waiter {
100 struct list_head list;
101 struct task_struct *task;
102 enum rwsem_waiter_type type;
103};
104
105enum rwsem_wake_type {
106 RWSEM_WAKE_ANY, /* Wake whatever's at head of wait list */
107 RWSEM_WAKE_READERS, /* Wake readers only */
108 RWSEM_WAKE_READ_OWNED /* Waker thread holds the read lock */
109};
110
111/*
112 * handle the lock release when processes blocked on it that can now run
113 * - if we come here from up_xxxx(), then:
114 * - the 'active part' of count (&0x0000ffff) reached 0 (but may have changed)
115 * - the 'waiting part' of count (&0xffff0000) is -ve (and will still be so)
116 * - there must be someone on the queue
117 * - the wait_lock must be held by the caller
118 * - tasks are marked for wakeup, the caller must later invoke wake_up_q()
119 * to actually wakeup the blocked task(s) and drop the reference count,
120 * preferably when the wait_lock is released
121 * - woken process blocks are discarded from the list after having task zeroed
122 * - writers are only marked woken if downgrading is false
123 */
124static void __rwsem_mark_wake(struct rw_semaphore *sem,
125 enum rwsem_wake_type wake_type,
126 struct wake_q_head *wake_q)
127{
128 struct rwsem_waiter *waiter, *tmp;
129 long oldcount, woken = 0, adjustment = 0;
130
131 /*
132 * Take a peek at the queue head waiter such that we can determine
133 * the wakeup(s) to perform.
134 */
135 waiter = list_first_entry(&sem->wait_list, struct rwsem_waiter, list);
136
137 if (waiter->type == RWSEM_WAITING_FOR_WRITE) {
138 if (wake_type == RWSEM_WAKE_ANY) {
139 /*
140 * Mark writer at the front of the queue for wakeup.
141 * Until the task is actually later awoken later by
142 * the caller, other writers are able to steal it.
143 * Readers, on the other hand, will block as they
144 * will notice the queued writer.
145 */
146 wake_q_add(wake_q, waiter->task);
147 }
148
149 return;
150 }
151
152 /*
153 * Writers might steal the lock before we grant it to the next reader.
154 * We prefer to do the first reader grant before counting readers
155 * so we can bail out early if a writer stole the lock.
156 */
157 if (wake_type != RWSEM_WAKE_READ_OWNED) {
158 adjustment = RWSEM_ACTIVE_READ_BIAS;
159 try_reader_grant:
160 oldcount = atomic_long_fetch_add(adjustment, &sem->count);
161 if (unlikely(oldcount < RWSEM_WAITING_BIAS)) {
162 /*
163 * If the count is still less than RWSEM_WAITING_BIAS
164 * after removing the adjustment, it is assumed that
165 * a writer has stolen the lock. We have to undo our
166 * reader grant.
167 */
168 if (atomic_long_add_return(-adjustment, &sem->count) <
169 RWSEM_WAITING_BIAS)
170 return;
171
172 /* Last active locker left. Retry waking readers. */
173 goto try_reader_grant;
174 }
175 /*
176 * It is not really necessary to set it to reader-owned here,
177 * but it gives the spinners an early indication that the
178 * readers now have the lock.
179 */
180 rwsem_set_reader_owned(sem);
181 }
182
183 /*
184 * Grant an infinite number of read locks to the readers at the front
185 * of the queue. We know that woken will be at least 1 as we accounted
186 * for above. Note we increment the 'active part' of the count by the
187 * number of readers before waking any processes up.
188 */
189 list_for_each_entry_safe(waiter, tmp, &sem->wait_list, list) {
190 struct task_struct *tsk;
191
192 if (waiter->type == RWSEM_WAITING_FOR_WRITE)
193 break;
194
195 woken++;
196 tsk = waiter->task;
197
198 wake_q_add(wake_q, tsk);
199 list_del(&waiter->list);
200 /*
201 * Ensure that the last operation is setting the reader
202 * waiter to nil such that rwsem_down_read_failed() cannot
203 * race with do_exit() by always holding a reference count
204 * to the task to wakeup.
205 */
206 smp_store_release(&waiter->task, NULL);
207 }
208
209 adjustment = woken * RWSEM_ACTIVE_READ_BIAS - adjustment;
210 if (list_empty(&sem->wait_list)) {
211 /* hit end of list above */
212 adjustment -= RWSEM_WAITING_BIAS;
213 }
214
215 if (adjustment)
216 atomic_long_add(adjustment, &sem->count);
217}
218
219/*
220 * Wait for the read lock to be granted
221 */
222__visible
223struct rw_semaphore __sched *rwsem_down_read_failed(struct rw_semaphore *sem)
224{
225 long count, adjustment = -RWSEM_ACTIVE_READ_BIAS;
226 struct rwsem_waiter waiter;
227 struct task_struct *tsk = current;
228 DEFINE_WAKE_Q(wake_q);
229
230 waiter.task = tsk;
231 waiter.type = RWSEM_WAITING_FOR_READ;
232
233 raw_spin_lock_irq(&sem->wait_lock);
234 if (list_empty(&sem->wait_list))
235 adjustment += RWSEM_WAITING_BIAS;
236 list_add_tail(&waiter.list, &sem->wait_list);
237
238 /* we're now waiting on the lock, but no longer actively locking */
239 count = atomic_long_add_return(adjustment, &sem->count);
240
241 /*
242 * If there are no active locks, wake the front queued process(es).
243 *
244 * If there are no writers and we are first in the queue,
245 * wake our own waiter to join the existing active readers !
246 */
247 if (count == RWSEM_WAITING_BIAS ||
248 (count > RWSEM_WAITING_BIAS &&
249 adjustment != -RWSEM_ACTIVE_READ_BIAS))
250 __rwsem_mark_wake(sem, RWSEM_WAKE_ANY, &wake_q);
251
252 raw_spin_unlock_irq(&sem->wait_lock);
253 wake_up_q(&wake_q);
254
255 /* wait to be given the lock */
256 while (true) {
257 set_task_state(tsk, TASK_UNINTERRUPTIBLE);
258 if (!waiter.task)
259 break;
260 schedule();
261 }
262
263 __set_task_state(tsk, TASK_RUNNING);
264 return sem;
265}
266EXPORT_SYMBOL(rwsem_down_read_failed);
267
268/*
269 * This function must be called with the sem->wait_lock held to prevent
270 * race conditions between checking the rwsem wait list and setting the
271 * sem->count accordingly.
272 */
273static inline bool rwsem_try_write_lock(long count, struct rw_semaphore *sem)
274{
275 /*
276 * Avoid trying to acquire write lock if count isn't RWSEM_WAITING_BIAS.
277 */
278 if (count != RWSEM_WAITING_BIAS)
279 return false;
280
281 /*
282 * Acquire the lock by trying to set it to ACTIVE_WRITE_BIAS. If there
283 * are other tasks on the wait list, we need to add on WAITING_BIAS.
284 */
285 count = list_is_singular(&sem->wait_list) ?
286 RWSEM_ACTIVE_WRITE_BIAS :
287 RWSEM_ACTIVE_WRITE_BIAS + RWSEM_WAITING_BIAS;
288
289 if (atomic_long_cmpxchg_acquire(&sem->count, RWSEM_WAITING_BIAS, count)
290 == RWSEM_WAITING_BIAS) {
291 rwsem_set_owner(sem);
292 return true;
293 }
294
295 return false;
296}
297
298#ifdef CONFIG_RWSEM_SPIN_ON_OWNER
299/*
300 * Try to acquire write lock before the writer has been put on wait queue.
301 */
302static inline bool rwsem_try_write_lock_unqueued(struct rw_semaphore *sem)
303{
304 long old, count = atomic_long_read(&sem->count);
305
306 while (true) {
307 if (!(count == 0 || count == RWSEM_WAITING_BIAS))
308 return false;
309
310 old = atomic_long_cmpxchg_acquire(&sem->count, count,
311 count + RWSEM_ACTIVE_WRITE_BIAS);
312 if (old == count) {
313 rwsem_set_owner(sem);
314 return true;
315 }
316
317 count = old;
318 }
319}
320
321static inline bool rwsem_can_spin_on_owner(struct rw_semaphore *sem)
322{
323 struct task_struct *owner;
324 bool ret = true;
325
326 if (need_resched())
327 return false;
328
329 rcu_read_lock();
330 owner = READ_ONCE(sem->owner);
331 if (!rwsem_owner_is_writer(owner)) {
332 /*
333 * Don't spin if the rwsem is readers owned.
334 */
335 ret = !rwsem_owner_is_reader(owner);
336 goto done;
337 }
338
339 /*
340 * As lock holder preemption issue, we both skip spinning if task is not
341 * on cpu or its cpu is preempted
342 */
343 ret = owner->on_cpu && !vcpu_is_preempted(task_cpu(owner));
344done:
345 rcu_read_unlock();
346 return ret;
347}
348
349/*
350 * Return true only if we can still spin on the owner field of the rwsem.
351 */
352static noinline bool rwsem_spin_on_owner(struct rw_semaphore *sem)
353{
354 struct task_struct *owner = READ_ONCE(sem->owner);
355
356 if (!rwsem_owner_is_writer(owner))
357 goto out;
358
359 rcu_read_lock();
360 while (sem->owner == owner) {
361 /*
362 * Ensure we emit the owner->on_cpu, dereference _after_
363 * checking sem->owner still matches owner, if that fails,
364 * owner might point to free()d memory, if it still matches,
365 * the rcu_read_lock() ensures the memory stays valid.
366 */
367 barrier();
368
369 /*
370 * abort spinning when need_resched or owner is not running or
371 * owner's cpu is preempted.
372 */
373 if (!owner->on_cpu || need_resched() ||
374 vcpu_is_preempted(task_cpu(owner))) {
375 rcu_read_unlock();
376 return false;
377 }
378
379 cpu_relax();
380 }
381 rcu_read_unlock();
382out:
383 /*
384 * If there is a new owner or the owner is not set, we continue
385 * spinning.
386 */
387 return !rwsem_owner_is_reader(READ_ONCE(sem->owner));
388}
389
390static bool rwsem_optimistic_spin(struct rw_semaphore *sem)
391{
392 bool taken = false;
393
394 preempt_disable();
395
396 /* sem->wait_lock should not be held when doing optimistic spinning */
397 if (!rwsem_can_spin_on_owner(sem))
398 goto done;
399
400 if (!osq_lock(&sem->osq))
401 goto done;
402
403 /*
404 * Optimistically spin on the owner field and attempt to acquire the
405 * lock whenever the owner changes. Spinning will be stopped when:
406 * 1) the owning writer isn't running; or
407 * 2) readers own the lock as we can't determine if they are
408 * actively running or not.
409 */
410 while (rwsem_spin_on_owner(sem)) {
411 /*
412 * Try to acquire the lock
413 */
414 if (rwsem_try_write_lock_unqueued(sem)) {
415 taken = true;
416 break;
417 }
418
419 /*
420 * When there's no owner, we might have preempted between the
421 * owner acquiring the lock and setting the owner field. If
422 * we're an RT task that will live-lock because we won't let
423 * the owner complete.
424 */
425 if (!sem->owner && (need_resched() || rt_task(current)))
426 break;
427
428 /*
429 * The cpu_relax() call is a compiler barrier which forces
430 * everything in this loop to be re-loaded. We don't need
431 * memory barriers as we'll eventually observe the right
432 * values at the cost of a few extra spins.
433 */
434 cpu_relax();
435 }
436 osq_unlock(&sem->osq);
437done:
438 preempt_enable();
439 return taken;
440}
441
442/*
443 * Return true if the rwsem has active spinner
444 */
445static inline bool rwsem_has_spinner(struct rw_semaphore *sem)
446{
447 return osq_is_locked(&sem->osq);
448}
449
450#else
451static bool rwsem_optimistic_spin(struct rw_semaphore *sem)
452{
453 return false;
454}
455
456static inline bool rwsem_has_spinner(struct rw_semaphore *sem)
457{
458 return false;
459}
460#endif
461
462/*
463 * Wait until we successfully acquire the write lock
464 */
465static inline struct rw_semaphore *
466__rwsem_down_write_failed_common(struct rw_semaphore *sem, int state)
467{
468 long count;
469 bool waiting = true; /* any queued threads before us */
470 struct rwsem_waiter waiter;
471 struct rw_semaphore *ret = sem;
472 DEFINE_WAKE_Q(wake_q);
473
474 /* undo write bias from down_write operation, stop active locking */
475 count = atomic_long_sub_return(RWSEM_ACTIVE_WRITE_BIAS, &sem->count);
476
477 /* do optimistic spinning and steal lock if possible */
478 if (rwsem_optimistic_spin(sem))
479 return sem;
480
481 /*
482 * Optimistic spinning failed, proceed to the slowpath
483 * and block until we can acquire the sem.
484 */
485 waiter.task = current;
486 waiter.type = RWSEM_WAITING_FOR_WRITE;
487
488 raw_spin_lock_irq(&sem->wait_lock);
489
490 /* account for this before adding a new element to the list */
491 if (list_empty(&sem->wait_list))
492 waiting = false;
493
494 list_add_tail(&waiter.list, &sem->wait_list);
495
496 /* we're now waiting on the lock, but no longer actively locking */
497 if (waiting) {
498 count = atomic_long_read(&sem->count);
499
500 /*
501 * If there were already threads queued before us and there are
502 * no active writers, the lock must be read owned; so we try to
503 * wake any read locks that were queued ahead of us.
504 */
505 if (count > RWSEM_WAITING_BIAS) {
506 DEFINE_WAKE_Q(wake_q);
507
508 __rwsem_mark_wake(sem, RWSEM_WAKE_READERS, &wake_q);
509 /*
510 * The wakeup is normally called _after_ the wait_lock
511 * is released, but given that we are proactively waking
512 * readers we can deal with the wake_q overhead as it is
513 * similar to releasing and taking the wait_lock again
514 * for attempting rwsem_try_write_lock().
515 */
516 wake_up_q(&wake_q);
517 }
518
519 } else
520 count = atomic_long_add_return(RWSEM_WAITING_BIAS, &sem->count);
521
522 /* wait until we successfully acquire the lock */
523 set_current_state(state);
524 while (true) {
525 if (rwsem_try_write_lock(count, sem))
526 break;
527 raw_spin_unlock_irq(&sem->wait_lock);
528
529 /* Block until there are no active lockers. */
530 do {
531 if (signal_pending_state(state, current))
532 goto out_nolock;
533
534 schedule();
535 set_current_state(state);
536 } while ((count = atomic_long_read(&sem->count)) & RWSEM_ACTIVE_MASK);
537
538 raw_spin_lock_irq(&sem->wait_lock);
539 }
540 __set_current_state(TASK_RUNNING);
541 list_del(&waiter.list);
542 raw_spin_unlock_irq(&sem->wait_lock);
543
544 return ret;
545
546out_nolock:
547 __set_current_state(TASK_RUNNING);
548 raw_spin_lock_irq(&sem->wait_lock);
549 list_del(&waiter.list);
550 if (list_empty(&sem->wait_list))
551 atomic_long_add(-RWSEM_WAITING_BIAS, &sem->count);
552 else
553 __rwsem_mark_wake(sem, RWSEM_WAKE_ANY, &wake_q);
554 raw_spin_unlock_irq(&sem->wait_lock);
555 wake_up_q(&wake_q);
556
557 return ERR_PTR(-EINTR);
558}
559
560__visible struct rw_semaphore * __sched
561rwsem_down_write_failed(struct rw_semaphore *sem)
562{
563 return __rwsem_down_write_failed_common(sem, TASK_UNINTERRUPTIBLE);
564}
565EXPORT_SYMBOL(rwsem_down_write_failed);
566
567__visible struct rw_semaphore * __sched
568rwsem_down_write_failed_killable(struct rw_semaphore *sem)
569{
570 return __rwsem_down_write_failed_common(sem, TASK_KILLABLE);
571}
572EXPORT_SYMBOL(rwsem_down_write_failed_killable);
573
574/*
575 * handle waking up a waiter on the semaphore
576 * - up_read/up_write has decremented the active part of count if we come here
577 */
578__visible
579struct rw_semaphore *rwsem_wake(struct rw_semaphore *sem)
580{
581 unsigned long flags;
582 DEFINE_WAKE_Q(wake_q);
583
584 /*
585 * If a spinner is present, it is not necessary to do the wakeup.
586 * Try to do wakeup only if the trylock succeeds to minimize
587 * spinlock contention which may introduce too much delay in the
588 * unlock operation.
589 *
590 * spinning writer up_write/up_read caller
591 * --------------- -----------------------
592 * [S] osq_unlock() [L] osq
593 * MB RMB
594 * [RmW] rwsem_try_write_lock() [RmW] spin_trylock(wait_lock)
595 *
596 * Here, it is important to make sure that there won't be a missed
597 * wakeup while the rwsem is free and the only spinning writer goes
598 * to sleep without taking the rwsem. Even when the spinning writer
599 * is just going to break out of the waiting loop, it will still do
600 * a trylock in rwsem_down_write_failed() before sleeping. IOW, if
601 * rwsem_has_spinner() is true, it will guarantee at least one
602 * trylock attempt on the rwsem later on.
603 */
604 if (rwsem_has_spinner(sem)) {
605 /*
606 * The smp_rmb() here is to make sure that the spinner
607 * state is consulted before reading the wait_lock.
608 */
609 smp_rmb();
610 if (!raw_spin_trylock_irqsave(&sem->wait_lock, flags))
611 return sem;
612 goto locked;
613 }
614 raw_spin_lock_irqsave(&sem->wait_lock, flags);
615locked:
616
617 if (!list_empty(&sem->wait_list))
618 __rwsem_mark_wake(sem, RWSEM_WAKE_ANY, &wake_q);
619
620 raw_spin_unlock_irqrestore(&sem->wait_lock, flags);
621 wake_up_q(&wake_q);
622
623 return sem;
624}
625EXPORT_SYMBOL(rwsem_wake);
626
627/*
628 * downgrade a write lock into a read lock
629 * - caller incremented waiting part of count and discovered it still negative
630 * - just wake up any readers at the front of the queue
631 */
632__visible
633struct rw_semaphore *rwsem_downgrade_wake(struct rw_semaphore *sem)
634{
635 unsigned long flags;
636 DEFINE_WAKE_Q(wake_q);
637
638 raw_spin_lock_irqsave(&sem->wait_lock, flags);
639
640 if (!list_empty(&sem->wait_list))
641 __rwsem_mark_wake(sem, RWSEM_WAKE_READ_OWNED, &wake_q);
642
643 raw_spin_unlock_irqrestore(&sem->wait_lock, flags);
644 wake_up_q(&wake_q);
645
646 return sem;
647}
648EXPORT_SYMBOL(rwsem_downgrade_wake);
1/* rwsem.c: R/W semaphores: contention handling functions
2 *
3 * Written by David Howells (dhowells@redhat.com).
4 * Derived from arch/i386/kernel/semaphore.c
5 *
6 * Writer lock-stealing by Alex Shi <alex.shi@intel.com>
7 * and Michel Lespinasse <walken@google.com>
8 *
9 * Optimistic spinning by Tim Chen <tim.c.chen@intel.com>
10 * and Davidlohr Bueso <davidlohr@hp.com>. Based on mutexes.
11 */
12#include <linux/rwsem.h>
13#include <linux/sched.h>
14#include <linux/init.h>
15#include <linux/export.h>
16#include <linux/sched/rt.h>
17#include <linux/osq_lock.h>
18
19#include "rwsem.h"
20
21/*
22 * Guide to the rw_semaphore's count field for common values.
23 * (32-bit case illustrated, similar for 64-bit)
24 *
25 * 0x0000000X (1) X readers active or attempting lock, no writer waiting
26 * X = #active_readers + #readers attempting to lock
27 * (X*ACTIVE_BIAS)
28 *
29 * 0x00000000 rwsem is unlocked, and no one is waiting for the lock or
30 * attempting to read lock or write lock.
31 *
32 * 0xffff000X (1) X readers active or attempting lock, with waiters for lock
33 * X = #active readers + # readers attempting lock
34 * (X*ACTIVE_BIAS + WAITING_BIAS)
35 * (2) 1 writer attempting lock, no waiters for lock
36 * X-1 = #active readers + #readers attempting lock
37 * ((X-1)*ACTIVE_BIAS + ACTIVE_WRITE_BIAS)
38 * (3) 1 writer active, no waiters for lock
39 * X-1 = #active readers + #readers attempting lock
40 * ((X-1)*ACTIVE_BIAS + ACTIVE_WRITE_BIAS)
41 *
42 * 0xffff0001 (1) 1 reader active or attempting lock, waiters for lock
43 * (WAITING_BIAS + ACTIVE_BIAS)
44 * (2) 1 writer active or attempting lock, no waiters for lock
45 * (ACTIVE_WRITE_BIAS)
46 *
47 * 0xffff0000 (1) There are writers or readers queued but none active
48 * or in the process of attempting lock.
49 * (WAITING_BIAS)
50 * Note: writer can attempt to steal lock for this count by adding
51 * ACTIVE_WRITE_BIAS in cmpxchg and checking the old count
52 *
53 * 0xfffe0001 (1) 1 writer active, or attempting lock. Waiters on queue.
54 * (ACTIVE_WRITE_BIAS + WAITING_BIAS)
55 *
56 * Note: Readers attempt to lock by adding ACTIVE_BIAS in down_read and checking
57 * the count becomes more than 0 for successful lock acquisition,
58 * i.e. the case where there are only readers or nobody has lock.
59 * (1st and 2nd case above).
60 *
61 * Writers attempt to lock by adding ACTIVE_WRITE_BIAS in down_write and
62 * checking the count becomes ACTIVE_WRITE_BIAS for successful lock
63 * acquisition (i.e. nobody else has lock or attempts lock). If
64 * unsuccessful, in rwsem_down_write_failed, we'll check to see if there
65 * are only waiters but none active (5th case above), and attempt to
66 * steal the lock.
67 *
68 */
69
70/*
71 * Initialize an rwsem:
72 */
73void __init_rwsem(struct rw_semaphore *sem, const char *name,
74 struct lock_class_key *key)
75{
76#ifdef CONFIG_DEBUG_LOCK_ALLOC
77 /*
78 * Make sure we are not reinitializing a held semaphore:
79 */
80 debug_check_no_locks_freed((void *)sem, sizeof(*sem));
81 lockdep_init_map(&sem->dep_map, name, key, 0);
82#endif
83 sem->count = RWSEM_UNLOCKED_VALUE;
84 raw_spin_lock_init(&sem->wait_lock);
85 INIT_LIST_HEAD(&sem->wait_list);
86#ifdef CONFIG_RWSEM_SPIN_ON_OWNER
87 sem->owner = NULL;
88 osq_lock_init(&sem->osq);
89#endif
90}
91
92EXPORT_SYMBOL(__init_rwsem);
93
94enum rwsem_waiter_type {
95 RWSEM_WAITING_FOR_WRITE,
96 RWSEM_WAITING_FOR_READ
97};
98
99struct rwsem_waiter {
100 struct list_head list;
101 struct task_struct *task;
102 enum rwsem_waiter_type type;
103};
104
105enum rwsem_wake_type {
106 RWSEM_WAKE_ANY, /* Wake whatever's at head of wait list */
107 RWSEM_WAKE_READERS, /* Wake readers only */
108 RWSEM_WAKE_READ_OWNED /* Waker thread holds the read lock */
109};
110
111/*
112 * handle the lock release when processes blocked on it that can now run
113 * - if we come here from up_xxxx(), then:
114 * - the 'active part' of count (&0x0000ffff) reached 0 (but may have changed)
115 * - the 'waiting part' of count (&0xffff0000) is -ve (and will still be so)
116 * - there must be someone on the queue
117 * - the spinlock must be held by the caller
118 * - woken process blocks are discarded from the list after having task zeroed
119 * - writers are only woken if downgrading is false
120 */
121static struct rw_semaphore *
122__rwsem_do_wake(struct rw_semaphore *sem, enum rwsem_wake_type wake_type)
123{
124 struct rwsem_waiter *waiter;
125 struct task_struct *tsk;
126 struct list_head *next;
127 long oldcount, woken, loop, adjustment;
128
129 waiter = list_entry(sem->wait_list.next, struct rwsem_waiter, list);
130 if (waiter->type == RWSEM_WAITING_FOR_WRITE) {
131 if (wake_type == RWSEM_WAKE_ANY)
132 /* Wake writer at the front of the queue, but do not
133 * grant it the lock yet as we want other writers
134 * to be able to steal it. Readers, on the other hand,
135 * will block as they will notice the queued writer.
136 */
137 wake_up_process(waiter->task);
138 goto out;
139 }
140
141 /* Writers might steal the lock before we grant it to the next reader.
142 * We prefer to do the first reader grant before counting readers
143 * so we can bail out early if a writer stole the lock.
144 */
145 adjustment = 0;
146 if (wake_type != RWSEM_WAKE_READ_OWNED) {
147 adjustment = RWSEM_ACTIVE_READ_BIAS;
148 try_reader_grant:
149 oldcount = rwsem_atomic_update(adjustment, sem) - adjustment;
150 if (unlikely(oldcount < RWSEM_WAITING_BIAS)) {
151 /* A writer stole the lock. Undo our reader grant. */
152 if (rwsem_atomic_update(-adjustment, sem) &
153 RWSEM_ACTIVE_MASK)
154 goto out;
155 /* Last active locker left. Retry waking readers. */
156 goto try_reader_grant;
157 }
158 }
159
160 /* Grant an infinite number of read locks to the readers at the front
161 * of the queue. Note we increment the 'active part' of the count by
162 * the number of readers before waking any processes up.
163 */
164 woken = 0;
165 do {
166 woken++;
167
168 if (waiter->list.next == &sem->wait_list)
169 break;
170
171 waiter = list_entry(waiter->list.next,
172 struct rwsem_waiter, list);
173
174 } while (waiter->type != RWSEM_WAITING_FOR_WRITE);
175
176 adjustment = woken * RWSEM_ACTIVE_READ_BIAS - adjustment;
177 if (waiter->type != RWSEM_WAITING_FOR_WRITE)
178 /* hit end of list above */
179 adjustment -= RWSEM_WAITING_BIAS;
180
181 if (adjustment)
182 rwsem_atomic_add(adjustment, sem);
183
184 next = sem->wait_list.next;
185 loop = woken;
186 do {
187 waiter = list_entry(next, struct rwsem_waiter, list);
188 next = waiter->list.next;
189 tsk = waiter->task;
190 /*
191 * Make sure we do not wakeup the next reader before
192 * setting the nil condition to grant the next reader;
193 * otherwise we could miss the wakeup on the other
194 * side and end up sleeping again. See the pairing
195 * in rwsem_down_read_failed().
196 */
197 smp_mb();
198 waiter->task = NULL;
199 wake_up_process(tsk);
200 put_task_struct(tsk);
201 } while (--loop);
202
203 sem->wait_list.next = next;
204 next->prev = &sem->wait_list;
205
206 out:
207 return sem;
208}
209
210/*
211 * Wait for the read lock to be granted
212 */
213__visible
214struct rw_semaphore __sched *rwsem_down_read_failed(struct rw_semaphore *sem)
215{
216 long count, adjustment = -RWSEM_ACTIVE_READ_BIAS;
217 struct rwsem_waiter waiter;
218 struct task_struct *tsk = current;
219
220 /* set up my own style of waitqueue */
221 waiter.task = tsk;
222 waiter.type = RWSEM_WAITING_FOR_READ;
223 get_task_struct(tsk);
224
225 raw_spin_lock_irq(&sem->wait_lock);
226 if (list_empty(&sem->wait_list))
227 adjustment += RWSEM_WAITING_BIAS;
228 list_add_tail(&waiter.list, &sem->wait_list);
229
230 /* we're now waiting on the lock, but no longer actively locking */
231 count = rwsem_atomic_update(adjustment, sem);
232
233 /* If there are no active locks, wake the front queued process(es).
234 *
235 * If there are no writers and we are first in the queue,
236 * wake our own waiter to join the existing active readers !
237 */
238 if (count == RWSEM_WAITING_BIAS ||
239 (count > RWSEM_WAITING_BIAS &&
240 adjustment != -RWSEM_ACTIVE_READ_BIAS))
241 sem = __rwsem_do_wake(sem, RWSEM_WAKE_ANY);
242
243 raw_spin_unlock_irq(&sem->wait_lock);
244
245 /* wait to be given the lock */
246 while (true) {
247 set_task_state(tsk, TASK_UNINTERRUPTIBLE);
248 if (!waiter.task)
249 break;
250 schedule();
251 }
252
253 __set_task_state(tsk, TASK_RUNNING);
254 return sem;
255}
256EXPORT_SYMBOL(rwsem_down_read_failed);
257
258static inline bool rwsem_try_write_lock(long count, struct rw_semaphore *sem)
259{
260 /*
261 * Try acquiring the write lock. Check count first in order
262 * to reduce unnecessary expensive cmpxchg() operations.
263 */
264 if (count == RWSEM_WAITING_BIAS &&
265 cmpxchg_acquire(&sem->count, RWSEM_WAITING_BIAS,
266 RWSEM_ACTIVE_WRITE_BIAS) == RWSEM_WAITING_BIAS) {
267 if (!list_is_singular(&sem->wait_list))
268 rwsem_atomic_update(RWSEM_WAITING_BIAS, sem);
269 rwsem_set_owner(sem);
270 return true;
271 }
272
273 return false;
274}
275
276#ifdef CONFIG_RWSEM_SPIN_ON_OWNER
277/*
278 * Try to acquire write lock before the writer has been put on wait queue.
279 */
280static inline bool rwsem_try_write_lock_unqueued(struct rw_semaphore *sem)
281{
282 long old, count = READ_ONCE(sem->count);
283
284 while (true) {
285 if (!(count == 0 || count == RWSEM_WAITING_BIAS))
286 return false;
287
288 old = cmpxchg_acquire(&sem->count, count,
289 count + RWSEM_ACTIVE_WRITE_BIAS);
290 if (old == count) {
291 rwsem_set_owner(sem);
292 return true;
293 }
294
295 count = old;
296 }
297}
298
299static inline bool rwsem_can_spin_on_owner(struct rw_semaphore *sem)
300{
301 struct task_struct *owner;
302 bool ret = true;
303
304 if (need_resched())
305 return false;
306
307 rcu_read_lock();
308 owner = READ_ONCE(sem->owner);
309 if (!owner) {
310 long count = READ_ONCE(sem->count);
311 /*
312 * If sem->owner is not set, yet we have just recently entered the
313 * slowpath with the lock being active, then there is a possibility
314 * reader(s) may have the lock. To be safe, bail spinning in these
315 * situations.
316 */
317 if (count & RWSEM_ACTIVE_MASK)
318 ret = false;
319 goto done;
320 }
321
322 ret = owner->on_cpu;
323done:
324 rcu_read_unlock();
325 return ret;
326}
327
328static noinline
329bool rwsem_spin_on_owner(struct rw_semaphore *sem, struct task_struct *owner)
330{
331 long count;
332
333 rcu_read_lock();
334 while (sem->owner == owner) {
335 /*
336 * Ensure we emit the owner->on_cpu, dereference _after_
337 * checking sem->owner still matches owner, if that fails,
338 * owner might point to free()d memory, if it still matches,
339 * the rcu_read_lock() ensures the memory stays valid.
340 */
341 barrier();
342
343 /* abort spinning when need_resched or owner is not running */
344 if (!owner->on_cpu || need_resched()) {
345 rcu_read_unlock();
346 return false;
347 }
348
349 cpu_relax_lowlatency();
350 }
351 rcu_read_unlock();
352
353 if (READ_ONCE(sem->owner))
354 return true; /* new owner, continue spinning */
355
356 /*
357 * When the owner is not set, the lock could be free or
358 * held by readers. Check the counter to verify the
359 * state.
360 */
361 count = READ_ONCE(sem->count);
362 return (count == 0 || count == RWSEM_WAITING_BIAS);
363}
364
365static bool rwsem_optimistic_spin(struct rw_semaphore *sem)
366{
367 struct task_struct *owner;
368 bool taken = false;
369
370 preempt_disable();
371
372 /* sem->wait_lock should not be held when doing optimistic spinning */
373 if (!rwsem_can_spin_on_owner(sem))
374 goto done;
375
376 if (!osq_lock(&sem->osq))
377 goto done;
378
379 while (true) {
380 owner = READ_ONCE(sem->owner);
381 if (owner && !rwsem_spin_on_owner(sem, owner))
382 break;
383
384 /* wait_lock will be acquired if write_lock is obtained */
385 if (rwsem_try_write_lock_unqueued(sem)) {
386 taken = true;
387 break;
388 }
389
390 /*
391 * When there's no owner, we might have preempted between the
392 * owner acquiring the lock and setting the owner field. If
393 * we're an RT task that will live-lock because we won't let
394 * the owner complete.
395 */
396 if (!owner && (need_resched() || rt_task(current)))
397 break;
398
399 /*
400 * The cpu_relax() call is a compiler barrier which forces
401 * everything in this loop to be re-loaded. We don't need
402 * memory barriers as we'll eventually observe the right
403 * values at the cost of a few extra spins.
404 */
405 cpu_relax_lowlatency();
406 }
407 osq_unlock(&sem->osq);
408done:
409 preempt_enable();
410 return taken;
411}
412
413/*
414 * Return true if the rwsem has active spinner
415 */
416static inline bool rwsem_has_spinner(struct rw_semaphore *sem)
417{
418 return osq_is_locked(&sem->osq);
419}
420
421#else
422static bool rwsem_optimistic_spin(struct rw_semaphore *sem)
423{
424 return false;
425}
426
427static inline bool rwsem_has_spinner(struct rw_semaphore *sem)
428{
429 return false;
430}
431#endif
432
433/*
434 * Wait until we successfully acquire the write lock
435 */
436__visible
437struct rw_semaphore __sched *rwsem_down_write_failed(struct rw_semaphore *sem)
438{
439 long count;
440 bool waiting = true; /* any queued threads before us */
441 struct rwsem_waiter waiter;
442
443 /* undo write bias from down_write operation, stop active locking */
444 count = rwsem_atomic_update(-RWSEM_ACTIVE_WRITE_BIAS, sem);
445
446 /* do optimistic spinning and steal lock if possible */
447 if (rwsem_optimistic_spin(sem))
448 return sem;
449
450 /*
451 * Optimistic spinning failed, proceed to the slowpath
452 * and block until we can acquire the sem.
453 */
454 waiter.task = current;
455 waiter.type = RWSEM_WAITING_FOR_WRITE;
456
457 raw_spin_lock_irq(&sem->wait_lock);
458
459 /* account for this before adding a new element to the list */
460 if (list_empty(&sem->wait_list))
461 waiting = false;
462
463 list_add_tail(&waiter.list, &sem->wait_list);
464
465 /* we're now waiting on the lock, but no longer actively locking */
466 if (waiting) {
467 count = READ_ONCE(sem->count);
468
469 /*
470 * If there were already threads queued before us and there are
471 * no active writers, the lock must be read owned; so we try to
472 * wake any read locks that were queued ahead of us.
473 */
474 if (count > RWSEM_WAITING_BIAS)
475 sem = __rwsem_do_wake(sem, RWSEM_WAKE_READERS);
476
477 } else
478 count = rwsem_atomic_update(RWSEM_WAITING_BIAS, sem);
479
480 /* wait until we successfully acquire the lock */
481 set_current_state(TASK_UNINTERRUPTIBLE);
482 while (true) {
483 if (rwsem_try_write_lock(count, sem))
484 break;
485 raw_spin_unlock_irq(&sem->wait_lock);
486
487 /* Block until there are no active lockers. */
488 do {
489 schedule();
490 set_current_state(TASK_UNINTERRUPTIBLE);
491 } while ((count = sem->count) & RWSEM_ACTIVE_MASK);
492
493 raw_spin_lock_irq(&sem->wait_lock);
494 }
495 __set_current_state(TASK_RUNNING);
496
497 list_del(&waiter.list);
498 raw_spin_unlock_irq(&sem->wait_lock);
499
500 return sem;
501}
502EXPORT_SYMBOL(rwsem_down_write_failed);
503
504/*
505 * handle waking up a waiter on the semaphore
506 * - up_read/up_write has decremented the active part of count if we come here
507 */
508__visible
509struct rw_semaphore *rwsem_wake(struct rw_semaphore *sem)
510{
511 unsigned long flags;
512
513 /*
514 * If a spinner is present, it is not necessary to do the wakeup.
515 * Try to do wakeup only if the trylock succeeds to minimize
516 * spinlock contention which may introduce too much delay in the
517 * unlock operation.
518 *
519 * spinning writer up_write/up_read caller
520 * --------------- -----------------------
521 * [S] osq_unlock() [L] osq
522 * MB RMB
523 * [RmW] rwsem_try_write_lock() [RmW] spin_trylock(wait_lock)
524 *
525 * Here, it is important to make sure that there won't be a missed
526 * wakeup while the rwsem is free and the only spinning writer goes
527 * to sleep without taking the rwsem. Even when the spinning writer
528 * is just going to break out of the waiting loop, it will still do
529 * a trylock in rwsem_down_write_failed() before sleeping. IOW, if
530 * rwsem_has_spinner() is true, it will guarantee at least one
531 * trylock attempt on the rwsem later on.
532 */
533 if (rwsem_has_spinner(sem)) {
534 /*
535 * The smp_rmb() here is to make sure that the spinner
536 * state is consulted before reading the wait_lock.
537 */
538 smp_rmb();
539 if (!raw_spin_trylock_irqsave(&sem->wait_lock, flags))
540 return sem;
541 goto locked;
542 }
543 raw_spin_lock_irqsave(&sem->wait_lock, flags);
544locked:
545
546 /* do nothing if list empty */
547 if (!list_empty(&sem->wait_list))
548 sem = __rwsem_do_wake(sem, RWSEM_WAKE_ANY);
549
550 raw_spin_unlock_irqrestore(&sem->wait_lock, flags);
551
552 return sem;
553}
554EXPORT_SYMBOL(rwsem_wake);
555
556/*
557 * downgrade a write lock into a read lock
558 * - caller incremented waiting part of count and discovered it still negative
559 * - just wake up any readers at the front of the queue
560 */
561__visible
562struct rw_semaphore *rwsem_downgrade_wake(struct rw_semaphore *sem)
563{
564 unsigned long flags;
565
566 raw_spin_lock_irqsave(&sem->wait_lock, flags);
567
568 /* do nothing if list empty */
569 if (!list_empty(&sem->wait_list))
570 sem = __rwsem_do_wake(sem, RWSEM_WAKE_READ_OWNED);
571
572 raw_spin_unlock_irqrestore(&sem->wait_lock, flags);
573
574 return sem;
575}
576EXPORT_SYMBOL(rwsem_downgrade_wake);