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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 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);
1// SPDX-License-Identifier: GPL-2.0
2/* rwsem.c: R/W semaphores: contention handling functions
3 *
4 * Written by David Howells (dhowells@redhat.com).
5 * Derived from arch/i386/kernel/semaphore.c
6 *
7 * Writer lock-stealing by Alex Shi <alex.shi@intel.com>
8 * and Michel Lespinasse <walken@google.com>
9 *
10 * Optimistic spinning by Tim Chen <tim.c.chen@intel.com>
11 * and Davidlohr Bueso <davidlohr@hp.com>. Based on mutexes.
12 */
13#include <linux/rwsem.h>
14#include <linux/init.h>
15#include <linux/export.h>
16#include <linux/sched/signal.h>
17#include <linux/sched/rt.h>
18#include <linux/sched/wake_q.h>
19#include <linux/sched/debug.h>
20#include <linux/osq_lock.h>
21
22#include "rwsem.h"
23
24/*
25 * Guide to the rw_semaphore's count field for common values.
26 * (32-bit case illustrated, similar for 64-bit)
27 *
28 * 0x0000000X (1) X readers active or attempting lock, no writer waiting
29 * X = #active_readers + #readers attempting to lock
30 * (X*ACTIVE_BIAS)
31 *
32 * 0x00000000 rwsem is unlocked, and no one is waiting for the lock or
33 * attempting to read lock or write lock.
34 *
35 * 0xffff000X (1) X readers active or attempting lock, with waiters for lock
36 * X = #active readers + # readers attempting lock
37 * (X*ACTIVE_BIAS + WAITING_BIAS)
38 * (2) 1 writer attempting lock, no waiters for lock
39 * X-1 = #active readers + #readers attempting lock
40 * ((X-1)*ACTIVE_BIAS + ACTIVE_WRITE_BIAS)
41 * (3) 1 writer active, no waiters for lock
42 * X-1 = #active readers + #readers attempting lock
43 * ((X-1)*ACTIVE_BIAS + ACTIVE_WRITE_BIAS)
44 *
45 * 0xffff0001 (1) 1 reader active or attempting lock, waiters for lock
46 * (WAITING_BIAS + ACTIVE_BIAS)
47 * (2) 1 writer active or attempting lock, no waiters for lock
48 * (ACTIVE_WRITE_BIAS)
49 *
50 * 0xffff0000 (1) There are writers or readers queued but none active
51 * or in the process of attempting lock.
52 * (WAITING_BIAS)
53 * Note: writer can attempt to steal lock for this count by adding
54 * ACTIVE_WRITE_BIAS in cmpxchg and checking the old count
55 *
56 * 0xfffe0001 (1) 1 writer active, or attempting lock. Waiters on queue.
57 * (ACTIVE_WRITE_BIAS + WAITING_BIAS)
58 *
59 * Note: Readers attempt to lock by adding ACTIVE_BIAS in down_read and checking
60 * the count becomes more than 0 for successful lock acquisition,
61 * i.e. the case where there are only readers or nobody has lock.
62 * (1st and 2nd case above).
63 *
64 * Writers attempt to lock by adding ACTIVE_WRITE_BIAS in down_write and
65 * checking the count becomes ACTIVE_WRITE_BIAS for successful lock
66 * acquisition (i.e. nobody else has lock or attempts lock). If
67 * unsuccessful, in rwsem_down_write_failed, we'll check to see if there
68 * are only waiters but none active (5th case above), and attempt to
69 * steal the lock.
70 *
71 */
72
73/*
74 * Initialize an rwsem:
75 */
76void __init_rwsem(struct rw_semaphore *sem, const char *name,
77 struct lock_class_key *key)
78{
79#ifdef CONFIG_DEBUG_LOCK_ALLOC
80 /*
81 * Make sure we are not reinitializing a held semaphore:
82 */
83 debug_check_no_locks_freed((void *)sem, sizeof(*sem));
84 lockdep_init_map(&sem->dep_map, name, key, 0);
85#endif
86 atomic_long_set(&sem->count, RWSEM_UNLOCKED_VALUE);
87 raw_spin_lock_init(&sem->wait_lock);
88 INIT_LIST_HEAD(&sem->wait_list);
89#ifdef CONFIG_RWSEM_SPIN_ON_OWNER
90 sem->owner = NULL;
91 osq_lock_init(&sem->osq);
92#endif
93}
94
95EXPORT_SYMBOL(__init_rwsem);
96
97enum rwsem_waiter_type {
98 RWSEM_WAITING_FOR_WRITE,
99 RWSEM_WAITING_FOR_READ
100};
101
102struct rwsem_waiter {
103 struct list_head list;
104 struct task_struct *task;
105 enum rwsem_waiter_type type;
106};
107
108enum rwsem_wake_type {
109 RWSEM_WAKE_ANY, /* Wake whatever's at head of wait list */
110 RWSEM_WAKE_READERS, /* Wake readers only */
111 RWSEM_WAKE_READ_OWNED /* Waker thread holds the read lock */
112};
113
114/*
115 * handle the lock release when processes blocked on it that can now run
116 * - if we come here from up_xxxx(), then:
117 * - the 'active part' of count (&0x0000ffff) reached 0 (but may have changed)
118 * - the 'waiting part' of count (&0xffff0000) is -ve (and will still be so)
119 * - there must be someone on the queue
120 * - the wait_lock must be held by the caller
121 * - tasks are marked for wakeup, the caller must later invoke wake_up_q()
122 * to actually wakeup the blocked task(s) and drop the reference count,
123 * preferably when the wait_lock is released
124 * - woken process blocks are discarded from the list after having task zeroed
125 * - writers are only marked woken if downgrading is false
126 */
127static void __rwsem_mark_wake(struct rw_semaphore *sem,
128 enum rwsem_wake_type wake_type,
129 struct wake_q_head *wake_q)
130{
131 struct rwsem_waiter *waiter, *tmp;
132 long oldcount, woken = 0, adjustment = 0;
133
134 /*
135 * Take a peek at the queue head waiter such that we can determine
136 * the wakeup(s) to perform.
137 */
138 waiter = list_first_entry(&sem->wait_list, struct rwsem_waiter, list);
139
140 if (waiter->type == RWSEM_WAITING_FOR_WRITE) {
141 if (wake_type == RWSEM_WAKE_ANY) {
142 /*
143 * Mark writer at the front of the queue for wakeup.
144 * Until the task is actually later awoken later by
145 * the caller, other writers are able to steal it.
146 * Readers, on the other hand, will block as they
147 * will notice the queued writer.
148 */
149 wake_q_add(wake_q, waiter->task);
150 }
151
152 return;
153 }
154
155 /*
156 * Writers might steal the lock before we grant it to the next reader.
157 * We prefer to do the first reader grant before counting readers
158 * so we can bail out early if a writer stole the lock.
159 */
160 if (wake_type != RWSEM_WAKE_READ_OWNED) {
161 adjustment = RWSEM_ACTIVE_READ_BIAS;
162 try_reader_grant:
163 oldcount = atomic_long_fetch_add(adjustment, &sem->count);
164 if (unlikely(oldcount < RWSEM_WAITING_BIAS)) {
165 /*
166 * If the count is still less than RWSEM_WAITING_BIAS
167 * after removing the adjustment, it is assumed that
168 * a writer has stolen the lock. We have to undo our
169 * reader grant.
170 */
171 if (atomic_long_add_return(-adjustment, &sem->count) <
172 RWSEM_WAITING_BIAS)
173 return;
174
175 /* Last active locker left. Retry waking readers. */
176 goto try_reader_grant;
177 }
178 /*
179 * It is not really necessary to set it to reader-owned here,
180 * but it gives the spinners an early indication that the
181 * readers now have the lock.
182 */
183 rwsem_set_reader_owned(sem);
184 }
185
186 /*
187 * Grant an infinite number of read locks to the readers at the front
188 * of the queue. We know that woken will be at least 1 as we accounted
189 * for above. Note we increment the 'active part' of the count by the
190 * number of readers before waking any processes up.
191 */
192 list_for_each_entry_safe(waiter, tmp, &sem->wait_list, list) {
193 struct task_struct *tsk;
194
195 if (waiter->type == RWSEM_WAITING_FOR_WRITE)
196 break;
197
198 woken++;
199 tsk = waiter->task;
200
201 wake_q_add(wake_q, tsk);
202 list_del(&waiter->list);
203 /*
204 * Ensure that the last operation is setting the reader
205 * waiter to nil such that rwsem_down_read_failed() cannot
206 * race with do_exit() by always holding a reference count
207 * to the task to wakeup.
208 */
209 smp_store_release(&waiter->task, NULL);
210 }
211
212 adjustment = woken * RWSEM_ACTIVE_READ_BIAS - adjustment;
213 if (list_empty(&sem->wait_list)) {
214 /* hit end of list above */
215 adjustment -= RWSEM_WAITING_BIAS;
216 }
217
218 if (adjustment)
219 atomic_long_add(adjustment, &sem->count);
220}
221
222/*
223 * Wait for the read lock to be granted
224 */
225static inline struct rw_semaphore __sched *
226__rwsem_down_read_failed_common(struct rw_semaphore *sem, int state)
227{
228 long count, adjustment = -RWSEM_ACTIVE_READ_BIAS;
229 struct rwsem_waiter waiter;
230 DEFINE_WAKE_Q(wake_q);
231
232 waiter.task = current;
233 waiter.type = RWSEM_WAITING_FOR_READ;
234
235 raw_spin_lock_irq(&sem->wait_lock);
236 if (list_empty(&sem->wait_list))
237 adjustment += RWSEM_WAITING_BIAS;
238 list_add_tail(&waiter.list, &sem->wait_list);
239
240 /* we're now waiting on the lock, but no longer actively locking */
241 count = atomic_long_add_return(adjustment, &sem->count);
242
243 /*
244 * If there are no active locks, wake the front queued process(es).
245 *
246 * If there are no writers and we are first in the queue,
247 * wake our own waiter to join the existing active readers !
248 */
249 if (count == RWSEM_WAITING_BIAS ||
250 (count > RWSEM_WAITING_BIAS &&
251 adjustment != -RWSEM_ACTIVE_READ_BIAS))
252 __rwsem_mark_wake(sem, RWSEM_WAKE_ANY, &wake_q);
253
254 raw_spin_unlock_irq(&sem->wait_lock);
255 wake_up_q(&wake_q);
256
257 /* wait to be given the lock */
258 while (true) {
259 set_current_state(state);
260 if (!waiter.task)
261 break;
262 if (signal_pending_state(state, current)) {
263 raw_spin_lock_irq(&sem->wait_lock);
264 if (waiter.task)
265 goto out_nolock;
266 raw_spin_unlock_irq(&sem->wait_lock);
267 break;
268 }
269 schedule();
270 }
271
272 __set_current_state(TASK_RUNNING);
273 return sem;
274out_nolock:
275 list_del(&waiter.list);
276 if (list_empty(&sem->wait_list))
277 atomic_long_add(-RWSEM_WAITING_BIAS, &sem->count);
278 raw_spin_unlock_irq(&sem->wait_lock);
279 __set_current_state(TASK_RUNNING);
280 return ERR_PTR(-EINTR);
281}
282
283__visible struct rw_semaphore * __sched
284rwsem_down_read_failed(struct rw_semaphore *sem)
285{
286 return __rwsem_down_read_failed_common(sem, TASK_UNINTERRUPTIBLE);
287}
288EXPORT_SYMBOL(rwsem_down_read_failed);
289
290__visible struct rw_semaphore * __sched
291rwsem_down_read_failed_killable(struct rw_semaphore *sem)
292{
293 return __rwsem_down_read_failed_common(sem, TASK_KILLABLE);
294}
295EXPORT_SYMBOL(rwsem_down_read_failed_killable);
296
297/*
298 * This function must be called with the sem->wait_lock held to prevent
299 * race conditions between checking the rwsem wait list and setting the
300 * sem->count accordingly.
301 */
302static inline bool rwsem_try_write_lock(long count, struct rw_semaphore *sem)
303{
304 /*
305 * Avoid trying to acquire write lock if count isn't RWSEM_WAITING_BIAS.
306 */
307 if (count != RWSEM_WAITING_BIAS)
308 return false;
309
310 /*
311 * Acquire the lock by trying to set it to ACTIVE_WRITE_BIAS. If there
312 * are other tasks on the wait list, we need to add on WAITING_BIAS.
313 */
314 count = list_is_singular(&sem->wait_list) ?
315 RWSEM_ACTIVE_WRITE_BIAS :
316 RWSEM_ACTIVE_WRITE_BIAS + RWSEM_WAITING_BIAS;
317
318 if (atomic_long_cmpxchg_acquire(&sem->count, RWSEM_WAITING_BIAS, count)
319 == RWSEM_WAITING_BIAS) {
320 rwsem_set_owner(sem);
321 return true;
322 }
323
324 return false;
325}
326
327#ifdef CONFIG_RWSEM_SPIN_ON_OWNER
328/*
329 * Try to acquire write lock before the writer has been put on wait queue.
330 */
331static inline bool rwsem_try_write_lock_unqueued(struct rw_semaphore *sem)
332{
333 long old, count = atomic_long_read(&sem->count);
334
335 while (true) {
336 if (!(count == 0 || count == RWSEM_WAITING_BIAS))
337 return false;
338
339 old = atomic_long_cmpxchg_acquire(&sem->count, count,
340 count + RWSEM_ACTIVE_WRITE_BIAS);
341 if (old == count) {
342 rwsem_set_owner(sem);
343 return true;
344 }
345
346 count = old;
347 }
348}
349
350static inline bool rwsem_can_spin_on_owner(struct rw_semaphore *sem)
351{
352 struct task_struct *owner;
353 bool ret = true;
354
355 BUILD_BUG_ON(!rwsem_has_anonymous_owner(RWSEM_OWNER_UNKNOWN));
356
357 if (need_resched())
358 return false;
359
360 rcu_read_lock();
361 owner = READ_ONCE(sem->owner);
362 if (!owner || !is_rwsem_owner_spinnable(owner)) {
363 ret = !owner; /* !owner is spinnable */
364 goto done;
365 }
366
367 /*
368 * As lock holder preemption issue, we both skip spinning if task is not
369 * on cpu or its cpu is preempted
370 */
371 ret = owner->on_cpu && !vcpu_is_preempted(task_cpu(owner));
372done:
373 rcu_read_unlock();
374 return ret;
375}
376
377/*
378 * Return true only if we can still spin on the owner field of the rwsem.
379 */
380static noinline bool rwsem_spin_on_owner(struct rw_semaphore *sem)
381{
382 struct task_struct *owner = READ_ONCE(sem->owner);
383
384 if (!is_rwsem_owner_spinnable(owner))
385 return false;
386
387 rcu_read_lock();
388 while (owner && (READ_ONCE(sem->owner) == owner)) {
389 /*
390 * Ensure we emit the owner->on_cpu, dereference _after_
391 * checking sem->owner still matches owner, if that fails,
392 * owner might point to free()d memory, if it still matches,
393 * the rcu_read_lock() ensures the memory stays valid.
394 */
395 barrier();
396
397 /*
398 * abort spinning when need_resched or owner is not running or
399 * owner's cpu is preempted.
400 */
401 if (!owner->on_cpu || need_resched() ||
402 vcpu_is_preempted(task_cpu(owner))) {
403 rcu_read_unlock();
404 return false;
405 }
406
407 cpu_relax();
408 }
409 rcu_read_unlock();
410
411 /*
412 * If there is a new owner or the owner is not set, we continue
413 * spinning.
414 */
415 return is_rwsem_owner_spinnable(READ_ONCE(sem->owner));
416}
417
418static bool rwsem_optimistic_spin(struct rw_semaphore *sem)
419{
420 bool taken = false;
421
422 preempt_disable();
423
424 /* sem->wait_lock should not be held when doing optimistic spinning */
425 if (!rwsem_can_spin_on_owner(sem))
426 goto done;
427
428 if (!osq_lock(&sem->osq))
429 goto done;
430
431 /*
432 * Optimistically spin on the owner field and attempt to acquire the
433 * lock whenever the owner changes. Spinning will be stopped when:
434 * 1) the owning writer isn't running; or
435 * 2) readers own the lock as we can't determine if they are
436 * actively running or not.
437 */
438 while (rwsem_spin_on_owner(sem)) {
439 /*
440 * Try to acquire the lock
441 */
442 if (rwsem_try_write_lock_unqueued(sem)) {
443 taken = true;
444 break;
445 }
446
447 /*
448 * When there's no owner, we might have preempted between the
449 * owner acquiring the lock and setting the owner field. If
450 * we're an RT task that will live-lock because we won't let
451 * the owner complete.
452 */
453 if (!sem->owner && (need_resched() || rt_task(current)))
454 break;
455
456 /*
457 * The cpu_relax() call is a compiler barrier which forces
458 * everything in this loop to be re-loaded. We don't need
459 * memory barriers as we'll eventually observe the right
460 * values at the cost of a few extra spins.
461 */
462 cpu_relax();
463 }
464 osq_unlock(&sem->osq);
465done:
466 preempt_enable();
467 return taken;
468}
469
470/*
471 * Return true if the rwsem has active spinner
472 */
473static inline bool rwsem_has_spinner(struct rw_semaphore *sem)
474{
475 return osq_is_locked(&sem->osq);
476}
477
478#else
479static bool rwsem_optimistic_spin(struct rw_semaphore *sem)
480{
481 return false;
482}
483
484static inline bool rwsem_has_spinner(struct rw_semaphore *sem)
485{
486 return false;
487}
488#endif
489
490/*
491 * Wait until we successfully acquire the write lock
492 */
493static inline struct rw_semaphore *
494__rwsem_down_write_failed_common(struct rw_semaphore *sem, int state)
495{
496 long count;
497 bool waiting = true; /* any queued threads before us */
498 struct rwsem_waiter waiter;
499 struct rw_semaphore *ret = sem;
500 DEFINE_WAKE_Q(wake_q);
501
502 /* undo write bias from down_write operation, stop active locking */
503 count = atomic_long_sub_return(RWSEM_ACTIVE_WRITE_BIAS, &sem->count);
504
505 /* do optimistic spinning and steal lock if possible */
506 if (rwsem_optimistic_spin(sem))
507 return sem;
508
509 /*
510 * Optimistic spinning failed, proceed to the slowpath
511 * and block until we can acquire the sem.
512 */
513 waiter.task = current;
514 waiter.type = RWSEM_WAITING_FOR_WRITE;
515
516 raw_spin_lock_irq(&sem->wait_lock);
517
518 /* account for this before adding a new element to the list */
519 if (list_empty(&sem->wait_list))
520 waiting = false;
521
522 list_add_tail(&waiter.list, &sem->wait_list);
523
524 /* we're now waiting on the lock, but no longer actively locking */
525 if (waiting) {
526 count = atomic_long_read(&sem->count);
527
528 /*
529 * If there were already threads queued before us and there are
530 * no active writers, the lock must be read owned; so we try to
531 * wake any read locks that were queued ahead of us.
532 */
533 if (count > RWSEM_WAITING_BIAS) {
534 __rwsem_mark_wake(sem, RWSEM_WAKE_READERS, &wake_q);
535 /*
536 * The wakeup is normally called _after_ the wait_lock
537 * is released, but given that we are proactively waking
538 * readers we can deal with the wake_q overhead as it is
539 * similar to releasing and taking the wait_lock again
540 * for attempting rwsem_try_write_lock().
541 */
542 wake_up_q(&wake_q);
543
544 /*
545 * Reinitialize wake_q after use.
546 */
547 wake_q_init(&wake_q);
548 }
549
550 } else
551 count = atomic_long_add_return(RWSEM_WAITING_BIAS, &sem->count);
552
553 /* wait until we successfully acquire the lock */
554 set_current_state(state);
555 while (true) {
556 if (rwsem_try_write_lock(count, sem))
557 break;
558 raw_spin_unlock_irq(&sem->wait_lock);
559
560 /* Block until there are no active lockers. */
561 do {
562 if (signal_pending_state(state, current))
563 goto out_nolock;
564
565 schedule();
566 set_current_state(state);
567 } while ((count = atomic_long_read(&sem->count)) & RWSEM_ACTIVE_MASK);
568
569 raw_spin_lock_irq(&sem->wait_lock);
570 }
571 __set_current_state(TASK_RUNNING);
572 list_del(&waiter.list);
573 raw_spin_unlock_irq(&sem->wait_lock);
574
575 return ret;
576
577out_nolock:
578 __set_current_state(TASK_RUNNING);
579 raw_spin_lock_irq(&sem->wait_lock);
580 list_del(&waiter.list);
581 if (list_empty(&sem->wait_list))
582 atomic_long_add(-RWSEM_WAITING_BIAS, &sem->count);
583 else
584 __rwsem_mark_wake(sem, RWSEM_WAKE_ANY, &wake_q);
585 raw_spin_unlock_irq(&sem->wait_lock);
586 wake_up_q(&wake_q);
587
588 return ERR_PTR(-EINTR);
589}
590
591__visible struct rw_semaphore * __sched
592rwsem_down_write_failed(struct rw_semaphore *sem)
593{
594 return __rwsem_down_write_failed_common(sem, TASK_UNINTERRUPTIBLE);
595}
596EXPORT_SYMBOL(rwsem_down_write_failed);
597
598__visible struct rw_semaphore * __sched
599rwsem_down_write_failed_killable(struct rw_semaphore *sem)
600{
601 return __rwsem_down_write_failed_common(sem, TASK_KILLABLE);
602}
603EXPORT_SYMBOL(rwsem_down_write_failed_killable);
604
605/*
606 * handle waking up a waiter on the semaphore
607 * - up_read/up_write has decremented the active part of count if we come here
608 */
609__visible
610struct rw_semaphore *rwsem_wake(struct rw_semaphore *sem)
611{
612 unsigned long flags;
613 DEFINE_WAKE_Q(wake_q);
614
615 /*
616 * __rwsem_down_write_failed_common(sem)
617 * rwsem_optimistic_spin(sem)
618 * osq_unlock(sem->osq)
619 * ...
620 * atomic_long_add_return(&sem->count)
621 *
622 * - VS -
623 *
624 * __up_write()
625 * if (atomic_long_sub_return_release(&sem->count) < 0)
626 * rwsem_wake(sem)
627 * osq_is_locked(&sem->osq)
628 *
629 * And __up_write() must observe !osq_is_locked() when it observes the
630 * atomic_long_add_return() in order to not miss a wakeup.
631 *
632 * This boils down to:
633 *
634 * [S.rel] X = 1 [RmW] r0 = (Y += 0)
635 * MB RMB
636 * [RmW] Y += 1 [L] r1 = X
637 *
638 * exists (r0=1 /\ r1=0)
639 */
640 smp_rmb();
641
642 /*
643 * If a spinner is present, it is not necessary to do the wakeup.
644 * Try to do wakeup only if the trylock succeeds to minimize
645 * spinlock contention which may introduce too much delay in the
646 * unlock operation.
647 *
648 * spinning writer up_write/up_read caller
649 * --------------- -----------------------
650 * [S] osq_unlock() [L] osq
651 * MB RMB
652 * [RmW] rwsem_try_write_lock() [RmW] spin_trylock(wait_lock)
653 *
654 * Here, it is important to make sure that there won't be a missed
655 * wakeup while the rwsem is free and the only spinning writer goes
656 * to sleep without taking the rwsem. Even when the spinning writer
657 * is just going to break out of the waiting loop, it will still do
658 * a trylock in rwsem_down_write_failed() before sleeping. IOW, if
659 * rwsem_has_spinner() is true, it will guarantee at least one
660 * trylock attempt on the rwsem later on.
661 */
662 if (rwsem_has_spinner(sem)) {
663 /*
664 * The smp_rmb() here is to make sure that the spinner
665 * state is consulted before reading the wait_lock.
666 */
667 smp_rmb();
668 if (!raw_spin_trylock_irqsave(&sem->wait_lock, flags))
669 return sem;
670 goto locked;
671 }
672 raw_spin_lock_irqsave(&sem->wait_lock, flags);
673locked:
674
675 if (!list_empty(&sem->wait_list))
676 __rwsem_mark_wake(sem, RWSEM_WAKE_ANY, &wake_q);
677
678 raw_spin_unlock_irqrestore(&sem->wait_lock, flags);
679 wake_up_q(&wake_q);
680
681 return sem;
682}
683EXPORT_SYMBOL(rwsem_wake);
684
685/*
686 * downgrade a write lock into a read lock
687 * - caller incremented waiting part of count and discovered it still negative
688 * - just wake up any readers at the front of the queue
689 */
690__visible
691struct rw_semaphore *rwsem_downgrade_wake(struct rw_semaphore *sem)
692{
693 unsigned long flags;
694 DEFINE_WAKE_Q(wake_q);
695
696 raw_spin_lock_irqsave(&sem->wait_lock, flags);
697
698 if (!list_empty(&sem->wait_list))
699 __rwsem_mark_wake(sem, RWSEM_WAKE_READ_OWNED, &wake_q);
700
701 raw_spin_unlock_irqrestore(&sem->wait_lock, flags);
702 wake_up_q(&wake_q);
703
704 return sem;
705}
706EXPORT_SYMBOL(rwsem_downgrade_wake);