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1// SPDX-License-Identifier: GPL-2.0
2/* kernel/rwsem.c: R/W semaphores, public implementation
3 *
4 * Written by David Howells ([email protected]).
5 * Derived from asm-i386/semaphore.h
6 *
7 * Writer lock-stealing by Alex Shi <[email protected]>
8 * and Michel Lespinasse <[email protected]>
9 *
10 * Optimistic spinning by Tim Chen <[email protected]>
11 * and Davidlohr Bueso <[email protected]>. Based on mutexes.
12 *
13 * Rwsem count bit fields re-definition and rwsem rearchitecture by
14 * Waiman Long <[email protected]> and
15 * Peter Zijlstra <[email protected]>.
16 */
17
18#include <linux/types.h>
19#include <linux/kernel.h>
20#include <linux/sched.h>
21#include <linux/sched/rt.h>
22#include <linux/sched/task.h>
23#include <linux/sched/debug.h>
24#include <linux/sched/wake_q.h>
25#include <linux/sched/signal.h>
26#include <linux/sched/clock.h>
27#include <linux/export.h>
28#include <linux/rwsem.h>
29#include <linux/atomic.h>
30
31#include "rwsem.h"
32#include "lock_events.h"
33
34/*
35 * The least significant 3 bits of the owner value has the following
36 * meanings when set.
37 * - Bit 0: RWSEM_READER_OWNED - The rwsem is owned by readers
38 * - Bit 1: RWSEM_RD_NONSPINNABLE - Readers cannot spin on this lock.
39 * - Bit 2: RWSEM_WR_NONSPINNABLE - Writers cannot spin on this lock.
40 *
41 * When the rwsem is either owned by an anonymous writer, or it is
42 * reader-owned, but a spinning writer has timed out, both nonspinnable
43 * bits will be set to disable optimistic spinning by readers and writers.
44 * In the later case, the last unlocking reader should then check the
45 * writer nonspinnable bit and clear it only to give writers preference
46 * to acquire the lock via optimistic spinning, but not readers. Similar
47 * action is also done in the reader slowpath.
48
49 * When a writer acquires a rwsem, it puts its task_struct pointer
50 * into the owner field. It is cleared after an unlock.
51 *
52 * When a reader acquires a rwsem, it will also puts its task_struct
53 * pointer into the owner field with the RWSEM_READER_OWNED bit set.
54 * On unlock, the owner field will largely be left untouched. So
55 * for a free or reader-owned rwsem, the owner value may contain
56 * information about the last reader that acquires the rwsem.
57 *
58 * That information may be helpful in debugging cases where the system
59 * seems to hang on a reader owned rwsem especially if only one reader
60 * is involved. Ideally we would like to track all the readers that own
61 * a rwsem, but the overhead is simply too big.
62 *
63 * Reader optimistic spinning is helpful when the reader critical section
64 * is short and there aren't that many readers around. It makes readers
65 * relatively more preferred than writers. When a writer times out spinning
66 * on a reader-owned lock and set the nospinnable bits, there are two main
67 * reasons for that.
68 *
69 * 1) The reader critical section is long, perhaps the task sleeps after
70 * acquiring the read lock.
71 * 2) There are just too many readers contending the lock causing it to
72 * take a while to service all of them.
73 *
74 * In the former case, long reader critical section will impede the progress
75 * of writers which is usually more important for system performance. In
76 * the later case, reader optimistic spinning tends to make the reader
77 * groups that contain readers that acquire the lock together smaller
78 * leading to more of them. That may hurt performance in some cases. In
79 * other words, the setting of nonspinnable bits indicates that reader
80 * optimistic spinning may not be helpful for those workloads that cause
81 * it.
82 *
83 * Therefore, any writers that had observed the setting of the writer
84 * nonspinnable bit for a given rwsem after they fail to acquire the lock
85 * via optimistic spinning will set the reader nonspinnable bit once they
86 * acquire the write lock. Similarly, readers that observe the setting
87 * of reader nonspinnable bit at slowpath entry will set the reader
88 * nonspinnable bits when they acquire the read lock via the wakeup path.
89 *
90 * Once the reader nonspinnable bit is on, it will only be reset when
91 * a writer is able to acquire the rwsem in the fast path or somehow a
92 * reader or writer in the slowpath doesn't observe the nonspinable bit.
93 *
94 * This is to discourage reader optmistic spinning on that particular
95 * rwsem and make writers more preferred. This adaptive disabling of reader
96 * optimistic spinning will alleviate the negative side effect of this
97 * feature.
98 */
99#define RWSEM_READER_OWNED (1UL << 0)
100#define RWSEM_RD_NONSPINNABLE (1UL << 1)
101#define RWSEM_WR_NONSPINNABLE (1UL << 2)
102#define RWSEM_NONSPINNABLE (RWSEM_RD_NONSPINNABLE | RWSEM_WR_NONSPINNABLE)
103#define RWSEM_OWNER_FLAGS_MASK (RWSEM_READER_OWNED | RWSEM_NONSPINNABLE)
104
105#ifdef CONFIG_DEBUG_RWSEMS
106# define DEBUG_RWSEMS_WARN_ON(c, sem) do { \
107 if (!debug_locks_silent && \
108 WARN_ONCE(c, "DEBUG_RWSEMS_WARN_ON(%s): count = 0x%lx, magic = 0x%lx, owner = 0x%lx, curr 0x%lx, list %sempty\n",\
109 #c, atomic_long_read(&(sem)->count), \
110 (unsigned long) sem->magic, \
111 atomic_long_read(&(sem)->owner), (long)current, \
112 list_empty(&(sem)->wait_list) ? "" : "not ")) \
113 debug_locks_off(); \
114 } while (0)
115#else
116# define DEBUG_RWSEMS_WARN_ON(c, sem)
117#endif
118
119/*
120 * On 64-bit architectures, the bit definitions of the count are:
121 *
122 * Bit 0 - writer locked bit
123 * Bit 1 - waiters present bit
124 * Bit 2 - lock handoff bit
125 * Bits 3-7 - reserved
126 * Bits 8-62 - 55-bit reader count
127 * Bit 63 - read fail bit
128 *
129 * On 32-bit architectures, the bit definitions of the count are:
130 *
131 * Bit 0 - writer locked bit
132 * Bit 1 - waiters present bit
133 * Bit 2 - lock handoff bit
134 * Bits 3-7 - reserved
135 * Bits 8-30 - 23-bit reader count
136 * Bit 31 - read fail bit
137 *
138 * It is not likely that the most significant bit (read fail bit) will ever
139 * be set. This guard bit is still checked anyway in the down_read() fastpath
140 * just in case we need to use up more of the reader bits for other purpose
141 * in the future.
142 *
143 * atomic_long_fetch_add() is used to obtain reader lock, whereas
144 * atomic_long_cmpxchg() will be used to obtain writer lock.
145 *
146 * There are three places where the lock handoff bit may be set or cleared.
147 * 1) rwsem_mark_wake() for readers.
148 * 2) rwsem_try_write_lock() for writers.
149 * 3) Error path of rwsem_down_write_slowpath().
150 *
151 * For all the above cases, wait_lock will be held. A writer must also
152 * be the first one in the wait_list to be eligible for setting the handoff
153 * bit. So concurrent setting/clearing of handoff bit is not possible.
154 */
155#define RWSEM_WRITER_LOCKED (1UL << 0)
156#define RWSEM_FLAG_WAITERS (1UL << 1)
157#define RWSEM_FLAG_HANDOFF (1UL << 2)
158#define RWSEM_FLAG_READFAIL (1UL << (BITS_PER_LONG - 1))
159
160#define RWSEM_READER_SHIFT 8
161#define RWSEM_READER_BIAS (1UL << RWSEM_READER_SHIFT)
162#define RWSEM_READER_MASK (~(RWSEM_READER_BIAS - 1))
163#define RWSEM_WRITER_MASK RWSEM_WRITER_LOCKED
164#define RWSEM_LOCK_MASK (RWSEM_WRITER_MASK|RWSEM_READER_MASK)
165#define RWSEM_READ_FAILED_MASK (RWSEM_WRITER_MASK|RWSEM_FLAG_WAITERS|\
166 RWSEM_FLAG_HANDOFF|RWSEM_FLAG_READFAIL)
167
168/*
169 * All writes to owner are protected by WRITE_ONCE() to make sure that
170 * store tearing can't happen as optimistic spinners may read and use
171 * the owner value concurrently without lock. Read from owner, however,
172 * may not need READ_ONCE() as long as the pointer value is only used
173 * for comparison and isn't being dereferenced.
174 */
175static inline void rwsem_set_owner(struct rw_semaphore *sem)
176{
177 atomic_long_set(&sem->owner, (long)current);
178}
179
180static inline void rwsem_clear_owner(struct rw_semaphore *sem)
181{
182 atomic_long_set(&sem->owner, 0);
183}
184
185/*
186 * Test the flags in the owner field.
187 */
188static inline bool rwsem_test_oflags(struct rw_semaphore *sem, long flags)
189{
190 return atomic_long_read(&sem->owner) & flags;
191}
192
193/*
194 * The task_struct pointer of the last owning reader will be left in
195 * the owner field.
196 *
197 * Note that the owner value just indicates the task has owned the rwsem
198 * previously, it may not be the real owner or one of the real owners
199 * anymore when that field is examined, so take it with a grain of salt.
200 *
201 * The reader non-spinnable bit is preserved.
202 */
203static inline void __rwsem_set_reader_owned(struct rw_semaphore *sem,
204 struct task_struct *owner)
205{
206 unsigned long val = (unsigned long)owner | RWSEM_READER_OWNED |
207 (atomic_long_read(&sem->owner) & RWSEM_RD_NONSPINNABLE);
208
209 atomic_long_set(&sem->owner, val);
210}
211
212static inline void rwsem_set_reader_owned(struct rw_semaphore *sem)
213{
214 __rwsem_set_reader_owned(sem, current);
215}
216
217/*
218 * Return true if the rwsem is owned by a reader.
219 */
220static inline bool is_rwsem_reader_owned(struct rw_semaphore *sem)
221{
222#ifdef CONFIG_DEBUG_RWSEMS
223 /*
224 * Check the count to see if it is write-locked.
225 */
226 long count = atomic_long_read(&sem->count);
227
228 if (count & RWSEM_WRITER_MASK)
229 return false;
230#endif
231 return rwsem_test_oflags(sem, RWSEM_READER_OWNED);
232}
233
234#ifdef CONFIG_DEBUG_RWSEMS
235/*
236 * With CONFIG_DEBUG_RWSEMS configured, it will make sure that if there
237 * is a task pointer in owner of a reader-owned rwsem, it will be the
238 * real owner or one of the real owners. The only exception is when the
239 * unlock is done by up_read_non_owner().
240 */
241static inline void rwsem_clear_reader_owned(struct rw_semaphore *sem)
242{
243 unsigned long val = atomic_long_read(&sem->owner);
244
245 while ((val & ~RWSEM_OWNER_FLAGS_MASK) == (unsigned long)current) {
246 if (atomic_long_try_cmpxchg(&sem->owner, &val,
247 val & RWSEM_OWNER_FLAGS_MASK))
248 return;
249 }
250}
251#else
252static inline void rwsem_clear_reader_owned(struct rw_semaphore *sem)
253{
254}
255#endif
256
257/*
258 * Set the RWSEM_NONSPINNABLE bits if the RWSEM_READER_OWNED flag
259 * remains set. Otherwise, the operation will be aborted.
260 */
261static inline void rwsem_set_nonspinnable(struct rw_semaphore *sem)
262{
263 unsigned long owner = atomic_long_read(&sem->owner);
264
265 do {
266 if (!(owner & RWSEM_READER_OWNED))
267 break;
268 if (owner & RWSEM_NONSPINNABLE)
269 break;
270 } while (!atomic_long_try_cmpxchg(&sem->owner, &owner,
271 owner | RWSEM_NONSPINNABLE));
272}
273
274static inline bool rwsem_read_trylock(struct rw_semaphore *sem)
275{
276 long cnt = atomic_long_add_return_acquire(RWSEM_READER_BIAS, &sem->count);
277 if (WARN_ON_ONCE(cnt < 0))
278 rwsem_set_nonspinnable(sem);
279 return !(cnt & RWSEM_READ_FAILED_MASK);
280}
281
282/*
283 * Return just the real task structure pointer of the owner
284 */
285static inline struct task_struct *rwsem_owner(struct rw_semaphore *sem)
286{
287 return (struct task_struct *)
288 (atomic_long_read(&sem->owner) & ~RWSEM_OWNER_FLAGS_MASK);
289}
290
291/*
292 * Return the real task structure pointer of the owner and the embedded
293 * flags in the owner. pflags must be non-NULL.
294 */
295static inline struct task_struct *
296rwsem_owner_flags(struct rw_semaphore *sem, unsigned long *pflags)
297{
298 unsigned long owner = atomic_long_read(&sem->owner);
299
300 *pflags = owner & RWSEM_OWNER_FLAGS_MASK;
301 return (struct task_struct *)(owner & ~RWSEM_OWNER_FLAGS_MASK);
302}
303
304/*
305 * Guide to the rw_semaphore's count field.
306 *
307 * When the RWSEM_WRITER_LOCKED bit in count is set, the lock is owned
308 * by a writer.
309 *
310 * The lock is owned by readers when
311 * (1) the RWSEM_WRITER_LOCKED isn't set in count,
312 * (2) some of the reader bits are set in count, and
313 * (3) the owner field has RWSEM_READ_OWNED bit set.
314 *
315 * Having some reader bits set is not enough to guarantee a readers owned
316 * lock as the readers may be in the process of backing out from the count
317 * and a writer has just released the lock. So another writer may steal
318 * the lock immediately after that.
319 */
320
321/*
322 * Initialize an rwsem:
323 */
324void __init_rwsem(struct rw_semaphore *sem, const char *name,
325 struct lock_class_key *key)
326{
327#ifdef CONFIG_DEBUG_LOCK_ALLOC
328 /*
329 * Make sure we are not reinitializing a held semaphore:
330 */
331 debug_check_no_locks_freed((void *)sem, sizeof(*sem));
332 lockdep_init_map(&sem->dep_map, name, key, 0);
333#endif
334#ifdef CONFIG_DEBUG_RWSEMS
335 sem->magic = sem;
336#endif
337 atomic_long_set(&sem->count, RWSEM_UNLOCKED_VALUE);
338 raw_spin_lock_init(&sem->wait_lock);
339 INIT_LIST_HEAD(&sem->wait_list);
340 atomic_long_set(&sem->owner, 0L);
341#ifdef CONFIG_RWSEM_SPIN_ON_OWNER
342 osq_lock_init(&sem->osq);
343#endif
344}
345EXPORT_SYMBOL(__init_rwsem);
346
347enum rwsem_waiter_type {
348 RWSEM_WAITING_FOR_WRITE,
349 RWSEM_WAITING_FOR_READ
350};
351
352struct rwsem_waiter {
353 struct list_head list;
354 struct task_struct *task;
355 enum rwsem_waiter_type type;
356 unsigned long timeout;
357 unsigned long last_rowner;
358};
359#define rwsem_first_waiter(sem) \
360 list_first_entry(&sem->wait_list, struct rwsem_waiter, list)
361
362enum rwsem_wake_type {
363 RWSEM_WAKE_ANY, /* Wake whatever's at head of wait list */
364 RWSEM_WAKE_READERS, /* Wake readers only */
365 RWSEM_WAKE_READ_OWNED /* Waker thread holds the read lock */
366};
367
368enum writer_wait_state {
369 WRITER_NOT_FIRST, /* Writer is not first in wait list */
370 WRITER_FIRST, /* Writer is first in wait list */
371 WRITER_HANDOFF /* Writer is first & handoff needed */
372};
373
374/*
375 * The typical HZ value is either 250 or 1000. So set the minimum waiting
376 * time to at least 4ms or 1 jiffy (if it is higher than 4ms) in the wait
377 * queue before initiating the handoff protocol.
378 */
379#define RWSEM_WAIT_TIMEOUT DIV_ROUND_UP(HZ, 250)
380
381/*
382 * Magic number to batch-wakeup waiting readers, even when writers are
383 * also present in the queue. This both limits the amount of work the
384 * waking thread must do and also prevents any potential counter overflow,
385 * however unlikely.
386 */
387#define MAX_READERS_WAKEUP 0x100
388
389/*
390 * handle the lock release when processes blocked on it that can now run
391 * - if we come here from up_xxxx(), then the RWSEM_FLAG_WAITERS bit must
392 * have been set.
393 * - there must be someone on the queue
394 * - the wait_lock must be held by the caller
395 * - tasks are marked for wakeup, the caller must later invoke wake_up_q()
396 * to actually wakeup the blocked task(s) and drop the reference count,
397 * preferably when the wait_lock is released
398 * - woken process blocks are discarded from the list after having task zeroed
399 * - writers are only marked woken if downgrading is false
400 */
401static void rwsem_mark_wake(struct rw_semaphore *sem,
402 enum rwsem_wake_type wake_type,
403 struct wake_q_head *wake_q)
404{
405 struct rwsem_waiter *waiter, *tmp;
406 long oldcount, woken = 0, adjustment = 0;
407 struct list_head wlist;
408
409 lockdep_assert_held(&sem->wait_lock);
410
411 /*
412 * Take a peek at the queue head waiter such that we can determine
413 * the wakeup(s) to perform.
414 */
415 waiter = rwsem_first_waiter(sem);
416
417 if (waiter->type == RWSEM_WAITING_FOR_WRITE) {
418 if (wake_type == RWSEM_WAKE_ANY) {
419 /*
420 * Mark writer at the front of the queue for wakeup.
421 * Until the task is actually later awoken later by
422 * the caller, other writers are able to steal it.
423 * Readers, on the other hand, will block as they
424 * will notice the queued writer.
425 */
426 wake_q_add(wake_q, waiter->task);
427 lockevent_inc(rwsem_wake_writer);
428 }
429
430 return;
431 }
432
433 /*
434 * No reader wakeup if there are too many of them already.
435 */
436 if (unlikely(atomic_long_read(&sem->count) < 0))
437 return;
438
439 /*
440 * Writers might steal the lock before we grant it to the next reader.
441 * We prefer to do the first reader grant before counting readers
442 * so we can bail out early if a writer stole the lock.
443 */
444 if (wake_type != RWSEM_WAKE_READ_OWNED) {
445 struct task_struct *owner;
446
447 adjustment = RWSEM_READER_BIAS;
448 oldcount = atomic_long_fetch_add(adjustment, &sem->count);
449 if (unlikely(oldcount & RWSEM_WRITER_MASK)) {
450 /*
451 * When we've been waiting "too" long (for writers
452 * to give up the lock), request a HANDOFF to
453 * force the issue.
454 */
455 if (!(oldcount & RWSEM_FLAG_HANDOFF) &&
456 time_after(jiffies, waiter->timeout)) {
457 adjustment -= RWSEM_FLAG_HANDOFF;
458 lockevent_inc(rwsem_rlock_handoff);
459 }
460
461 atomic_long_add(-adjustment, &sem->count);
462 return;
463 }
464 /*
465 * Set it to reader-owned to give spinners an early
466 * indication that readers now have the lock.
467 * The reader nonspinnable bit seen at slowpath entry of
468 * the reader is copied over.
469 */
470 owner = waiter->task;
471 if (waiter->last_rowner & RWSEM_RD_NONSPINNABLE) {
472 owner = (void *)((unsigned long)owner | RWSEM_RD_NONSPINNABLE);
473 lockevent_inc(rwsem_opt_norspin);
474 }
475 __rwsem_set_reader_owned(sem, owner);
476 }
477
478 /*
479 * Grant up to MAX_READERS_WAKEUP read locks to all the readers in the
480 * queue. We know that the woken will be at least 1 as we accounted
481 * for above. Note we increment the 'active part' of the count by the
482 * number of readers before waking any processes up.
483 *
484 * This is an adaptation of the phase-fair R/W locks where at the
485 * reader phase (first waiter is a reader), all readers are eligible
486 * to acquire the lock at the same time irrespective of their order
487 * in the queue. The writers acquire the lock according to their
488 * order in the queue.
489 *
490 * We have to do wakeup in 2 passes to prevent the possibility that
491 * the reader count may be decremented before it is incremented. It
492 * is because the to-be-woken waiter may not have slept yet. So it
493 * may see waiter->task got cleared, finish its critical section and
494 * do an unlock before the reader count increment.
495 *
496 * 1) Collect the read-waiters in a separate list, count them and
497 * fully increment the reader count in rwsem.
498 * 2) For each waiters in the new list, clear waiter->task and
499 * put them into wake_q to be woken up later.
500 */
501 INIT_LIST_HEAD(&wlist);
502 list_for_each_entry_safe(waiter, tmp, &sem->wait_list, list) {
503 if (waiter->type == RWSEM_WAITING_FOR_WRITE)
504 continue;
505
506 woken++;
507 list_move_tail(&waiter->list, &wlist);
508
509 /*
510 * Limit # of readers that can be woken up per wakeup call.
511 */
512 if (woken >= MAX_READERS_WAKEUP)
513 break;
514 }
515
516 adjustment = woken * RWSEM_READER_BIAS - adjustment;
517 lockevent_cond_inc(rwsem_wake_reader, woken);
518 if (list_empty(&sem->wait_list)) {
519 /* hit end of list above */
520 adjustment -= RWSEM_FLAG_WAITERS;
521 }
522
523 /*
524 * When we've woken a reader, we no longer need to force writers
525 * to give up the lock and we can clear HANDOFF.
526 */
527 if (woken && (atomic_long_read(&sem->count) & RWSEM_FLAG_HANDOFF))
528 adjustment -= RWSEM_FLAG_HANDOFF;
529
530 if (adjustment)
531 atomic_long_add(adjustment, &sem->count);
532
533 /* 2nd pass */
534 list_for_each_entry_safe(waiter, tmp, &wlist, list) {
535 struct task_struct *tsk;
536
537 tsk = waiter->task;
538 get_task_struct(tsk);
539
540 /*
541 * Ensure calling get_task_struct() before setting the reader
542 * waiter to nil such that rwsem_down_read_slowpath() cannot
543 * race with do_exit() by always holding a reference count
544 * to the task to wakeup.
545 */
546 smp_store_release(&waiter->task, NULL);
547 /*
548 * Ensure issuing the wakeup (either by us or someone else)
549 * after setting the reader waiter to nil.
550 */
551 wake_q_add_safe(wake_q, tsk);
552 }
553}
554
555/*
556 * This function must be called with the sem->wait_lock held to prevent
557 * race conditions between checking the rwsem wait list and setting the
558 * sem->count accordingly.
559 *
560 * If wstate is WRITER_HANDOFF, it will make sure that either the handoff
561 * bit is set or the lock is acquired with handoff bit cleared.
562 */
563static inline bool rwsem_try_write_lock(struct rw_semaphore *sem,
564 enum writer_wait_state wstate)
565{
566 long count, new;
567
568 lockdep_assert_held(&sem->wait_lock);
569
570 count = atomic_long_read(&sem->count);
571 do {
572 bool has_handoff = !!(count & RWSEM_FLAG_HANDOFF);
573
574 if (has_handoff && wstate == WRITER_NOT_FIRST)
575 return false;
576
577 new = count;
578
579 if (count & RWSEM_LOCK_MASK) {
580 if (has_handoff || (wstate != WRITER_HANDOFF))
581 return false;
582
583 new |= RWSEM_FLAG_HANDOFF;
584 } else {
585 new |= RWSEM_WRITER_LOCKED;
586 new &= ~RWSEM_FLAG_HANDOFF;
587
588 if (list_is_singular(&sem->wait_list))
589 new &= ~RWSEM_FLAG_WAITERS;
590 }
591 } while (!atomic_long_try_cmpxchg_acquire(&sem->count, &count, new));
592
593 /*
594 * We have either acquired the lock with handoff bit cleared or
595 * set the handoff bit.
596 */
597 if (new & RWSEM_FLAG_HANDOFF)
598 return false;
599
600 rwsem_set_owner(sem);
601 return true;
602}
603
604#ifdef CONFIG_RWSEM_SPIN_ON_OWNER
605/*
606 * Try to acquire read lock before the reader is put on wait queue.
607 * Lock acquisition isn't allowed if the rwsem is locked or a writer handoff
608 * is ongoing.
609 */
610static inline bool rwsem_try_read_lock_unqueued(struct rw_semaphore *sem)
611{
612 long count = atomic_long_read(&sem->count);
613
614 if (count & (RWSEM_WRITER_MASK | RWSEM_FLAG_HANDOFF))
615 return false;
616
617 count = atomic_long_fetch_add_acquire(RWSEM_READER_BIAS, &sem->count);
618 if (!(count & (RWSEM_WRITER_MASK | RWSEM_FLAG_HANDOFF))) {
619 rwsem_set_reader_owned(sem);
620 lockevent_inc(rwsem_opt_rlock);
621 return true;
622 }
623
624 /* Back out the change */
625 atomic_long_add(-RWSEM_READER_BIAS, &sem->count);
626 return false;
627}
628
629/*
630 * Try to acquire write lock before the writer has been put on wait queue.
631 */
632static inline bool rwsem_try_write_lock_unqueued(struct rw_semaphore *sem)
633{
634 long count = atomic_long_read(&sem->count);
635
636 while (!(count & (RWSEM_LOCK_MASK|RWSEM_FLAG_HANDOFF))) {
637 if (atomic_long_try_cmpxchg_acquire(&sem->count, &count,
638 count | RWSEM_WRITER_LOCKED)) {
639 rwsem_set_owner(sem);
640 lockevent_inc(rwsem_opt_wlock);
641 return true;
642 }
643 }
644 return false;
645}
646
647static inline bool owner_on_cpu(struct task_struct *owner)
648{
649 /*
650 * As lock holder preemption issue, we both skip spinning if
651 * task is not on cpu or its cpu is preempted
652 */
653 return owner->on_cpu && !vcpu_is_preempted(task_cpu(owner));
654}
655
656static inline bool rwsem_can_spin_on_owner(struct rw_semaphore *sem,
657 unsigned long nonspinnable)
658{
659 struct task_struct *owner;
660 unsigned long flags;
661 bool ret = true;
662
663 BUILD_BUG_ON(!(RWSEM_OWNER_UNKNOWN & RWSEM_NONSPINNABLE));
664
665 if (need_resched()) {
666 lockevent_inc(rwsem_opt_fail);
667 return false;
668 }
669
670 preempt_disable();
671 rcu_read_lock();
672 owner = rwsem_owner_flags(sem, &flags);
673 /*
674 * Don't check the read-owner as the entry may be stale.
675 */
676 if ((flags & nonspinnable) ||
677 (owner && !(flags & RWSEM_READER_OWNED) && !owner_on_cpu(owner)))
678 ret = false;
679 rcu_read_unlock();
680 preempt_enable();
681
682 lockevent_cond_inc(rwsem_opt_fail, !ret);
683 return ret;
684}
685
686/*
687 * The rwsem_spin_on_owner() function returns the folowing 4 values
688 * depending on the lock owner state.
689 * OWNER_NULL : owner is currently NULL
690 * OWNER_WRITER: when owner changes and is a writer
691 * OWNER_READER: when owner changes and the new owner may be a reader.
692 * OWNER_NONSPINNABLE:
693 * when optimistic spinning has to stop because either the
694 * owner stops running, is unknown, or its timeslice has
695 * been used up.
696 */
697enum owner_state {
698 OWNER_NULL = 1 << 0,
699 OWNER_WRITER = 1 << 1,
700 OWNER_READER = 1 << 2,
701 OWNER_NONSPINNABLE = 1 << 3,
702};
703#define OWNER_SPINNABLE (OWNER_NULL | OWNER_WRITER | OWNER_READER)
704
705static inline enum owner_state
706rwsem_owner_state(struct task_struct *owner, unsigned long flags, unsigned long nonspinnable)
707{
708 if (flags & nonspinnable)
709 return OWNER_NONSPINNABLE;
710
711 if (flags & RWSEM_READER_OWNED)
712 return OWNER_READER;
713
714 return owner ? OWNER_WRITER : OWNER_NULL;
715}
716
717static noinline enum owner_state
718rwsem_spin_on_owner(struct rw_semaphore *sem, unsigned long nonspinnable)
719{
720 struct task_struct *new, *owner;
721 unsigned long flags, new_flags;
722 enum owner_state state;
723
724 owner = rwsem_owner_flags(sem, &flags);
725 state = rwsem_owner_state(owner, flags, nonspinnable);
726 if (state != OWNER_WRITER)
727 return state;
728
729 rcu_read_lock();
730 for (;;) {
731 /*
732 * When a waiting writer set the handoff flag, it may spin
733 * on the owner as well. Once that writer acquires the lock,
734 * we can spin on it. So we don't need to quit even when the
735 * handoff bit is set.
736 */
737 new = rwsem_owner_flags(sem, &new_flags);
738 if ((new != owner) || (new_flags != flags)) {
739 state = rwsem_owner_state(new, new_flags, nonspinnable);
740 break;
741 }
742
743 /*
744 * Ensure we emit the owner->on_cpu, dereference _after_
745 * checking sem->owner still matches owner, if that fails,
746 * owner might point to free()d memory, if it still matches,
747 * the rcu_read_lock() ensures the memory stays valid.
748 */
749 barrier();
750
751 if (need_resched() || !owner_on_cpu(owner)) {
752 state = OWNER_NONSPINNABLE;
753 break;
754 }
755
756 cpu_relax();
757 }
758 rcu_read_unlock();
759
760 return state;
761}
762
763/*
764 * Calculate reader-owned rwsem spinning threshold for writer
765 *
766 * The more readers own the rwsem, the longer it will take for them to
767 * wind down and free the rwsem. So the empirical formula used to
768 * determine the actual spinning time limit here is:
769 *
770 * Spinning threshold = (10 + nr_readers/2)us
771 *
772 * The limit is capped to a maximum of 25us (30 readers). This is just
773 * a heuristic and is subjected to change in the future.
774 */
775static inline u64 rwsem_rspin_threshold(struct rw_semaphore *sem)
776{
777 long count = atomic_long_read(&sem->count);
778 int readers = count >> RWSEM_READER_SHIFT;
779 u64 delta;
780
781 if (readers > 30)
782 readers = 30;
783 delta = (20 + readers) * NSEC_PER_USEC / 2;
784
785 return sched_clock() + delta;
786}
787
788static bool rwsem_optimistic_spin(struct rw_semaphore *sem, bool wlock)
789{
790 bool taken = false;
791 int prev_owner_state = OWNER_NULL;
792 int loop = 0;
793 u64 rspin_threshold = 0;
794 unsigned long nonspinnable = wlock ? RWSEM_WR_NONSPINNABLE
795 : RWSEM_RD_NONSPINNABLE;
796
797 preempt_disable();
798
799 /* sem->wait_lock should not be held when doing optimistic spinning */
800 if (!osq_lock(&sem->osq))
801 goto done;
802
803 /*
804 * Optimistically spin on the owner field and attempt to acquire the
805 * lock whenever the owner changes. Spinning will be stopped when:
806 * 1) the owning writer isn't running; or
807 * 2) readers own the lock and spinning time has exceeded limit.
808 */
809 for (;;) {
810 enum owner_state owner_state;
811
812 owner_state = rwsem_spin_on_owner(sem, nonspinnable);
813 if (!(owner_state & OWNER_SPINNABLE))
814 break;
815
816 /*
817 * Try to acquire the lock
818 */
819 taken = wlock ? rwsem_try_write_lock_unqueued(sem)
820 : rwsem_try_read_lock_unqueued(sem);
821
822 if (taken)
823 break;
824
825 /*
826 * Time-based reader-owned rwsem optimistic spinning
827 */
828 if (wlock && (owner_state == OWNER_READER)) {
829 /*
830 * Re-initialize rspin_threshold every time when
831 * the owner state changes from non-reader to reader.
832 * This allows a writer to steal the lock in between
833 * 2 reader phases and have the threshold reset at
834 * the beginning of the 2nd reader phase.
835 */
836 if (prev_owner_state != OWNER_READER) {
837 if (rwsem_test_oflags(sem, nonspinnable))
838 break;
839 rspin_threshold = rwsem_rspin_threshold(sem);
840 loop = 0;
841 }
842
843 /*
844 * Check time threshold once every 16 iterations to
845 * avoid calling sched_clock() too frequently so
846 * as to reduce the average latency between the times
847 * when the lock becomes free and when the spinner
848 * is ready to do a trylock.
849 */
850 else if (!(++loop & 0xf) && (sched_clock() > rspin_threshold)) {
851 rwsem_set_nonspinnable(sem);
852 lockevent_inc(rwsem_opt_nospin);
853 break;
854 }
855 }
856
857 /*
858 * An RT task cannot do optimistic spinning if it cannot
859 * be sure the lock holder is running or live-lock may
860 * happen if the current task and the lock holder happen
861 * to run in the same CPU. However, aborting optimistic
862 * spinning while a NULL owner is detected may miss some
863 * opportunity where spinning can continue without causing
864 * problem.
865 *
866 * There are 2 possible cases where an RT task may be able
867 * to continue spinning.
868 *
869 * 1) The lock owner is in the process of releasing the
870 * lock, sem->owner is cleared but the lock has not
871 * been released yet.
872 * 2) The lock was free and owner cleared, but another
873 * task just comes in and acquire the lock before
874 * we try to get it. The new owner may be a spinnable
875 * writer.
876 *
877 * To take advantage of two scenarios listed agove, the RT
878 * task is made to retry one more time to see if it can
879 * acquire the lock or continue spinning on the new owning
880 * writer. Of course, if the time lag is long enough or the
881 * new owner is not a writer or spinnable, the RT task will
882 * quit spinning.
883 *
884 * If the owner is a writer, the need_resched() check is
885 * done inside rwsem_spin_on_owner(). If the owner is not
886 * a writer, need_resched() check needs to be done here.
887 */
888 if (owner_state != OWNER_WRITER) {
889 if (need_resched())
890 break;
891 if (rt_task(current) &&
892 (prev_owner_state != OWNER_WRITER))
893 break;
894 }
895 prev_owner_state = owner_state;
896
897 /*
898 * The cpu_relax() call is a compiler barrier which forces
899 * everything in this loop to be re-loaded. We don't need
900 * memory barriers as we'll eventually observe the right
901 * values at the cost of a few extra spins.
902 */
903 cpu_relax();
904 }
905 osq_unlock(&sem->osq);
906done:
907 preempt_enable();
908 lockevent_cond_inc(rwsem_opt_fail, !taken);
909 return taken;
910}
911
912/*
913 * Clear the owner's RWSEM_WR_NONSPINNABLE bit if it is set. This should
914 * only be called when the reader count reaches 0.
915 *
916 * This give writers better chance to acquire the rwsem first before
917 * readers when the rwsem was being held by readers for a relatively long
918 * period of time. Race can happen that an optimistic spinner may have
919 * just stolen the rwsem and set the owner, but just clearing the
920 * RWSEM_WR_NONSPINNABLE bit will do no harm anyway.
921 */
922static inline void clear_wr_nonspinnable(struct rw_semaphore *sem)
923{
924 if (rwsem_test_oflags(sem, RWSEM_WR_NONSPINNABLE))
925 atomic_long_andnot(RWSEM_WR_NONSPINNABLE, &sem->owner);
926}
927
928/*
929 * This function is called when the reader fails to acquire the lock via
930 * optimistic spinning. In this case we will still attempt to do a trylock
931 * when comparing the rwsem state right now with the state when entering
932 * the slowpath indicates that the reader is still in a valid reader phase.
933 * This happens when the following conditions are true:
934 *
935 * 1) The lock is currently reader owned, and
936 * 2) The lock is previously not reader-owned or the last read owner changes.
937 *
938 * In the former case, we have transitioned from a writer phase to a
939 * reader-phase while spinning. In the latter case, it means the reader
940 * phase hasn't ended when we entered the optimistic spinning loop. In
941 * both cases, the reader is eligible to acquire the lock. This is the
942 * secondary path where a read lock is acquired optimistically.
943 *
944 * The reader non-spinnable bit wasn't set at time of entry or it will
945 * not be here at all.
946 */
947static inline bool rwsem_reader_phase_trylock(struct rw_semaphore *sem,
948 unsigned long last_rowner)
949{
950 unsigned long owner = atomic_long_read(&sem->owner);
951
952 if (!(owner & RWSEM_READER_OWNED))
953 return false;
954
955 if (((owner ^ last_rowner) & ~RWSEM_OWNER_FLAGS_MASK) &&
956 rwsem_try_read_lock_unqueued(sem)) {
957 lockevent_inc(rwsem_opt_rlock2);
958 lockevent_add(rwsem_opt_fail, -1);
959 return true;
960 }
961 return false;
962}
963#else
964static inline bool rwsem_can_spin_on_owner(struct rw_semaphore *sem,
965 unsigned long nonspinnable)
966{
967 return false;
968}
969
970static inline bool rwsem_optimistic_spin(struct rw_semaphore *sem, bool wlock)
971{
972 return false;
973}
974
975static inline void clear_wr_nonspinnable(struct rw_semaphore *sem) { }
976
977static inline bool rwsem_reader_phase_trylock(struct rw_semaphore *sem,
978 unsigned long last_rowner)
979{
980 return false;
981}
982
983static inline int
984rwsem_spin_on_owner(struct rw_semaphore *sem, unsigned long nonspinnable)
985{
986 return 0;
987}
988#define OWNER_NULL 1
989#endif
990
991/*
992 * Wait for the read lock to be granted
993 */
994static struct rw_semaphore __sched *
995rwsem_down_read_slowpath(struct rw_semaphore *sem, int state)
996{
997 long count, adjustment = -RWSEM_READER_BIAS;
998 struct rwsem_waiter waiter;
999 DEFINE_WAKE_Q(wake_q);
1000 bool wake = false;
1001
1002 /*
1003 * Save the current read-owner of rwsem, if available, and the
1004 * reader nonspinnable bit.
1005 */
1006 waiter.last_rowner = atomic_long_read(&sem->owner);
1007 if (!(waiter.last_rowner & RWSEM_READER_OWNED))
1008 waiter.last_rowner &= RWSEM_RD_NONSPINNABLE;
1009
1010 if (!rwsem_can_spin_on_owner(sem, RWSEM_RD_NONSPINNABLE))
1011 goto queue;
1012
1013 /*
1014 * Undo read bias from down_read() and do optimistic spinning.
1015 */
1016 atomic_long_add(-RWSEM_READER_BIAS, &sem->count);
1017 adjustment = 0;
1018 if (rwsem_optimistic_spin(sem, false)) {
1019 /* rwsem_optimistic_spin() implies ACQUIRE on success */
1020 /*
1021 * Wake up other readers in the wait list if the front
1022 * waiter is a reader.
1023 */
1024 if ((atomic_long_read(&sem->count) & RWSEM_FLAG_WAITERS)) {
1025 raw_spin_lock_irq(&sem->wait_lock);
1026 if (!list_empty(&sem->wait_list))
1027 rwsem_mark_wake(sem, RWSEM_WAKE_READ_OWNED,
1028 &wake_q);
1029 raw_spin_unlock_irq(&sem->wait_lock);
1030 wake_up_q(&wake_q);
1031 }
1032 return sem;
1033 } else if (rwsem_reader_phase_trylock(sem, waiter.last_rowner)) {
1034 /* rwsem_reader_phase_trylock() implies ACQUIRE on success */
1035 return sem;
1036 }
1037
1038queue:
1039 waiter.task = current;
1040 waiter.type = RWSEM_WAITING_FOR_READ;
1041 waiter.timeout = jiffies + RWSEM_WAIT_TIMEOUT;
1042
1043 raw_spin_lock_irq(&sem->wait_lock);
1044 if (list_empty(&sem->wait_list)) {
1045 /*
1046 * In case the wait queue is empty and the lock isn't owned
1047 * by a writer or has the handoff bit set, this reader can
1048 * exit the slowpath and return immediately as its
1049 * RWSEM_READER_BIAS has already been set in the count.
1050 */
1051 if (adjustment && !(atomic_long_read(&sem->count) &
1052 (RWSEM_WRITER_MASK | RWSEM_FLAG_HANDOFF))) {
1053 /* Provide lock ACQUIRE */
1054 smp_acquire__after_ctrl_dep();
1055 raw_spin_unlock_irq(&sem->wait_lock);
1056 rwsem_set_reader_owned(sem);
1057 lockevent_inc(rwsem_rlock_fast);
1058 return sem;
1059 }
1060 adjustment += RWSEM_FLAG_WAITERS;
1061 }
1062 list_add_tail(&waiter.list, &sem->wait_list);
1063
1064 /* we're now waiting on the lock, but no longer actively locking */
1065 if (adjustment)
1066 count = atomic_long_add_return(adjustment, &sem->count);
1067 else
1068 count = atomic_long_read(&sem->count);
1069
1070 /*
1071 * If there are no active locks, wake the front queued process(es).
1072 *
1073 * If there are no writers and we are first in the queue,
1074 * wake our own waiter to join the existing active readers !
1075 */
1076 if (!(count & RWSEM_LOCK_MASK)) {
1077 clear_wr_nonspinnable(sem);
1078 wake = true;
1079 }
1080 if (wake || (!(count & RWSEM_WRITER_MASK) &&
1081 (adjustment & RWSEM_FLAG_WAITERS)))
1082 rwsem_mark_wake(sem, RWSEM_WAKE_ANY, &wake_q);
1083
1084 raw_spin_unlock_irq(&sem->wait_lock);
1085 wake_up_q(&wake_q);
1086
1087 /* wait to be given the lock */
1088 for (;;) {
1089 set_current_state(state);
1090 if (!smp_load_acquire(&waiter.task)) {
1091 /* Matches rwsem_mark_wake()'s smp_store_release(). */
1092 break;
1093 }
1094 if (signal_pending_state(state, current)) {
1095 raw_spin_lock_irq(&sem->wait_lock);
1096 if (waiter.task)
1097 goto out_nolock;
1098 raw_spin_unlock_irq(&sem->wait_lock);
1099 /* Ordered by sem->wait_lock against rwsem_mark_wake(). */
1100 break;
1101 }
1102 schedule();
1103 lockevent_inc(rwsem_sleep_reader);
1104 }
1105
1106 __set_current_state(TASK_RUNNING);
1107 lockevent_inc(rwsem_rlock);
1108 return sem;
1109
1110out_nolock:
1111 list_del(&waiter.list);
1112 if (list_empty(&sem->wait_list)) {
1113 atomic_long_andnot(RWSEM_FLAG_WAITERS|RWSEM_FLAG_HANDOFF,
1114 &sem->count);
1115 }
1116 raw_spin_unlock_irq(&sem->wait_lock);
1117 __set_current_state(TASK_RUNNING);
1118 lockevent_inc(rwsem_rlock_fail);
1119 return ERR_PTR(-EINTR);
1120}
1121
1122/*
1123 * This function is called by the a write lock owner. So the owner value
1124 * won't get changed by others.
1125 */
1126static inline void rwsem_disable_reader_optspin(struct rw_semaphore *sem,
1127 bool disable)
1128{
1129 if (unlikely(disable)) {
1130 atomic_long_or(RWSEM_RD_NONSPINNABLE, &sem->owner);
1131 lockevent_inc(rwsem_opt_norspin);
1132 }
1133}
1134
1135/*
1136 * Wait until we successfully acquire the write lock
1137 */
1138static struct rw_semaphore *
1139rwsem_down_write_slowpath(struct rw_semaphore *sem, int state)
1140{
1141 long count;
1142 bool disable_rspin;
1143 enum writer_wait_state wstate;
1144 struct rwsem_waiter waiter;
1145 struct rw_semaphore *ret = sem;
1146 DEFINE_WAKE_Q(wake_q);
1147
1148 /* do optimistic spinning and steal lock if possible */
1149 if (rwsem_can_spin_on_owner(sem, RWSEM_WR_NONSPINNABLE) &&
1150 rwsem_optimistic_spin(sem, true)) {
1151 /* rwsem_optimistic_spin() implies ACQUIRE on success */
1152 return sem;
1153 }
1154
1155 /*
1156 * Disable reader optimistic spinning for this rwsem after
1157 * acquiring the write lock when the setting of the nonspinnable
1158 * bits are observed.
1159 */
1160 disable_rspin = atomic_long_read(&sem->owner) & RWSEM_NONSPINNABLE;
1161
1162 /*
1163 * Optimistic spinning failed, proceed to the slowpath
1164 * and block until we can acquire the sem.
1165 */
1166 waiter.task = current;
1167 waiter.type = RWSEM_WAITING_FOR_WRITE;
1168 waiter.timeout = jiffies + RWSEM_WAIT_TIMEOUT;
1169
1170 raw_spin_lock_irq(&sem->wait_lock);
1171
1172 /* account for this before adding a new element to the list */
1173 wstate = list_empty(&sem->wait_list) ? WRITER_FIRST : WRITER_NOT_FIRST;
1174
1175 list_add_tail(&waiter.list, &sem->wait_list);
1176
1177 /* we're now waiting on the lock */
1178 if (wstate == WRITER_NOT_FIRST) {
1179 count = atomic_long_read(&sem->count);
1180
1181 /*
1182 * If there were already threads queued before us and:
1183 * 1) there are no no active locks, wake the front
1184 * queued process(es) as the handoff bit might be set.
1185 * 2) there are no active writers and some readers, the lock
1186 * must be read owned; so we try to wake any read lock
1187 * waiters that were queued ahead of us.
1188 */
1189 if (count & RWSEM_WRITER_MASK)
1190 goto wait;
1191
1192 rwsem_mark_wake(sem, (count & RWSEM_READER_MASK)
1193 ? RWSEM_WAKE_READERS
1194 : RWSEM_WAKE_ANY, &wake_q);
1195
1196 if (!wake_q_empty(&wake_q)) {
1197 /*
1198 * We want to minimize wait_lock hold time especially
1199 * when a large number of readers are to be woken up.
1200 */
1201 raw_spin_unlock_irq(&sem->wait_lock);
1202 wake_up_q(&wake_q);
1203 wake_q_init(&wake_q); /* Used again, reinit */
1204 raw_spin_lock_irq(&sem->wait_lock);
1205 }
1206 } else {
1207 atomic_long_or(RWSEM_FLAG_WAITERS, &sem->count);
1208 }
1209
1210wait:
1211 /* wait until we successfully acquire the lock */
1212 set_current_state(state);
1213 for (;;) {
1214 if (rwsem_try_write_lock(sem, wstate)) {
1215 /* rwsem_try_write_lock() implies ACQUIRE on success */
1216 break;
1217 }
1218
1219 raw_spin_unlock_irq(&sem->wait_lock);
1220
1221 /*
1222 * After setting the handoff bit and failing to acquire
1223 * the lock, attempt to spin on owner to accelerate lock
1224 * transfer. If the previous owner is a on-cpu writer and it
1225 * has just released the lock, OWNER_NULL will be returned.
1226 * In this case, we attempt to acquire the lock again
1227 * without sleeping.
1228 */
1229 if (wstate == WRITER_HANDOFF &&
1230 rwsem_spin_on_owner(sem, RWSEM_NONSPINNABLE) == OWNER_NULL)
1231 goto trylock_again;
1232
1233 /* Block until there are no active lockers. */
1234 for (;;) {
1235 if (signal_pending_state(state, current))
1236 goto out_nolock;
1237
1238 schedule();
1239 lockevent_inc(rwsem_sleep_writer);
1240 set_current_state(state);
1241 /*
1242 * If HANDOFF bit is set, unconditionally do
1243 * a trylock.
1244 */
1245 if (wstate == WRITER_HANDOFF)
1246 break;
1247
1248 if ((wstate == WRITER_NOT_FIRST) &&
1249 (rwsem_first_waiter(sem) == &waiter))
1250 wstate = WRITER_FIRST;
1251
1252 count = atomic_long_read(&sem->count);
1253 if (!(count & RWSEM_LOCK_MASK))
1254 break;
1255
1256 /*
1257 * The setting of the handoff bit is deferred
1258 * until rwsem_try_write_lock() is called.
1259 */
1260 if ((wstate == WRITER_FIRST) && (rt_task(current) ||
1261 time_after(jiffies, waiter.timeout))) {
1262 wstate = WRITER_HANDOFF;
1263 lockevent_inc(rwsem_wlock_handoff);
1264 break;
1265 }
1266 }
1267trylock_again:
1268 raw_spin_lock_irq(&sem->wait_lock);
1269 }
1270 __set_current_state(TASK_RUNNING);
1271 list_del(&waiter.list);
1272 rwsem_disable_reader_optspin(sem, disable_rspin);
1273 raw_spin_unlock_irq(&sem->wait_lock);
1274 lockevent_inc(rwsem_wlock);
1275
1276 return ret;
1277
1278out_nolock:
1279 __set_current_state(TASK_RUNNING);
1280 raw_spin_lock_irq(&sem->wait_lock);
1281 list_del(&waiter.list);
1282
1283 if (unlikely(wstate == WRITER_HANDOFF))
1284 atomic_long_add(-RWSEM_FLAG_HANDOFF, &sem->count);
1285
1286 if (list_empty(&sem->wait_list))
1287 atomic_long_andnot(RWSEM_FLAG_WAITERS, &sem->count);
1288 else
1289 rwsem_mark_wake(sem, RWSEM_WAKE_ANY, &wake_q);
1290 raw_spin_unlock_irq(&sem->wait_lock);
1291 wake_up_q(&wake_q);
1292 lockevent_inc(rwsem_wlock_fail);
1293
1294 return ERR_PTR(-EINTR);
1295}
1296
1297/*
1298 * handle waking up a waiter on the semaphore
1299 * - up_read/up_write has decremented the active part of count if we come here
1300 */
1301static struct rw_semaphore *rwsem_wake(struct rw_semaphore *sem, long count)
1302{
1303 unsigned long flags;
1304 DEFINE_WAKE_Q(wake_q);
1305
1306 raw_spin_lock_irqsave(&sem->wait_lock, flags);
1307
1308 if (!list_empty(&sem->wait_list))
1309 rwsem_mark_wake(sem, RWSEM_WAKE_ANY, &wake_q);
1310
1311 raw_spin_unlock_irqrestore(&sem->wait_lock, flags);
1312 wake_up_q(&wake_q);
1313
1314 return sem;
1315}
1316
1317/*
1318 * downgrade a write lock into a read lock
1319 * - caller incremented waiting part of count and discovered it still negative
1320 * - just wake up any readers at the front of the queue
1321 */
1322static struct rw_semaphore *rwsem_downgrade_wake(struct rw_semaphore *sem)
1323{
1324 unsigned long flags;
1325 DEFINE_WAKE_Q(wake_q);
1326
1327 raw_spin_lock_irqsave(&sem->wait_lock, flags);
1328
1329 if (!list_empty(&sem->wait_list))
1330 rwsem_mark_wake(sem, RWSEM_WAKE_READ_OWNED, &wake_q);
1331
1332 raw_spin_unlock_irqrestore(&sem->wait_lock, flags);
1333 wake_up_q(&wake_q);
1334
1335 return sem;
1336}
1337
1338/*
1339 * lock for reading
1340 */
1341inline void __down_read(struct rw_semaphore *sem)
1342{
1343 if (!rwsem_read_trylock(sem)) {
1344 rwsem_down_read_slowpath(sem, TASK_UNINTERRUPTIBLE);
1345 DEBUG_RWSEMS_WARN_ON(!is_rwsem_reader_owned(sem), sem);
1346 } else {
1347 rwsem_set_reader_owned(sem);
1348 }
1349}
1350
1351static inline int __down_read_killable(struct rw_semaphore *sem)
1352{
1353 if (!rwsem_read_trylock(sem)) {
1354 if (IS_ERR(rwsem_down_read_slowpath(sem, TASK_KILLABLE)))
1355 return -EINTR;
1356 DEBUG_RWSEMS_WARN_ON(!is_rwsem_reader_owned(sem), sem);
1357 } else {
1358 rwsem_set_reader_owned(sem);
1359 }
1360 return 0;
1361}
1362
1363static inline int __down_read_trylock(struct rw_semaphore *sem)
1364{
1365 long tmp;
1366
1367 DEBUG_RWSEMS_WARN_ON(sem->magic != sem, sem);
1368
1369 /*
1370 * Optimize for the case when the rwsem is not locked at all.
1371 */
1372 tmp = RWSEM_UNLOCKED_VALUE;
1373 do {
1374 if (atomic_long_try_cmpxchg_acquire(&sem->count, &tmp,
1375 tmp + RWSEM_READER_BIAS)) {
1376 rwsem_set_reader_owned(sem);
1377 return 1;
1378 }
1379 } while (!(tmp & RWSEM_READ_FAILED_MASK));
1380 return 0;
1381}
1382
1383/*
1384 * lock for writing
1385 */
1386static inline void __down_write(struct rw_semaphore *sem)
1387{
1388 long tmp = RWSEM_UNLOCKED_VALUE;
1389
1390 if (unlikely(!atomic_long_try_cmpxchg_acquire(&sem->count, &tmp,
1391 RWSEM_WRITER_LOCKED)))
1392 rwsem_down_write_slowpath(sem, TASK_UNINTERRUPTIBLE);
1393 else
1394 rwsem_set_owner(sem);
1395}
1396
1397static inline int __down_write_killable(struct rw_semaphore *sem)
1398{
1399 long tmp = RWSEM_UNLOCKED_VALUE;
1400
1401 if (unlikely(!atomic_long_try_cmpxchg_acquire(&sem->count, &tmp,
1402 RWSEM_WRITER_LOCKED))) {
1403 if (IS_ERR(rwsem_down_write_slowpath(sem, TASK_KILLABLE)))
1404 return -EINTR;
1405 } else {
1406 rwsem_set_owner(sem);
1407 }
1408 return 0;
1409}
1410
1411static inline int __down_write_trylock(struct rw_semaphore *sem)
1412{
1413 long tmp;
1414
1415 DEBUG_RWSEMS_WARN_ON(sem->magic != sem, sem);
1416
1417 tmp = RWSEM_UNLOCKED_VALUE;
1418 if (atomic_long_try_cmpxchg_acquire(&sem->count, &tmp,
1419 RWSEM_WRITER_LOCKED)) {
1420 rwsem_set_owner(sem);
1421 return true;
1422 }
1423 return false;
1424}
1425
1426/*
1427 * unlock after reading
1428 */
1429inline void __up_read(struct rw_semaphore *sem)
1430{
1431 long tmp;
1432
1433 DEBUG_RWSEMS_WARN_ON(sem->magic != sem, sem);
1434 DEBUG_RWSEMS_WARN_ON(!is_rwsem_reader_owned(sem), sem);
1435
1436 rwsem_clear_reader_owned(sem);
1437 tmp = atomic_long_add_return_release(-RWSEM_READER_BIAS, &sem->count);
1438 DEBUG_RWSEMS_WARN_ON(tmp < 0, sem);
1439 if (unlikely((tmp & (RWSEM_LOCK_MASK|RWSEM_FLAG_WAITERS)) ==
1440 RWSEM_FLAG_WAITERS)) {
1441 clear_wr_nonspinnable(sem);
1442 rwsem_wake(sem, tmp);
1443 }
1444}
1445
1446/*
1447 * unlock after writing
1448 */
1449static inline void __up_write(struct rw_semaphore *sem)
1450{
1451 long tmp;
1452
1453 DEBUG_RWSEMS_WARN_ON(sem->magic != sem, sem);
1454 /*
1455 * sem->owner may differ from current if the ownership is transferred
1456 * to an anonymous writer by setting the RWSEM_NONSPINNABLE bits.
1457 */
1458 DEBUG_RWSEMS_WARN_ON((rwsem_owner(sem) != current) &&
1459 !rwsem_test_oflags(sem, RWSEM_NONSPINNABLE), sem);
1460
1461 rwsem_clear_owner(sem);
1462 tmp = atomic_long_fetch_add_release(-RWSEM_WRITER_LOCKED, &sem->count);
1463 if (unlikely(tmp & RWSEM_FLAG_WAITERS))
1464 rwsem_wake(sem, tmp);
1465}
1466
1467/*
1468 * downgrade write lock to read lock
1469 */
1470static inline void __downgrade_write(struct rw_semaphore *sem)
1471{
1472 long tmp;
1473
1474 /*
1475 * When downgrading from exclusive to shared ownership,
1476 * anything inside the write-locked region cannot leak
1477 * into the read side. In contrast, anything in the
1478 * read-locked region is ok to be re-ordered into the
1479 * write side. As such, rely on RELEASE semantics.
1480 */
1481 DEBUG_RWSEMS_WARN_ON(rwsem_owner(sem) != current, sem);
1482 tmp = atomic_long_fetch_add_release(
1483 -RWSEM_WRITER_LOCKED+RWSEM_READER_BIAS, &sem->count);
1484 rwsem_set_reader_owned(sem);
1485 if (tmp & RWSEM_FLAG_WAITERS)
1486 rwsem_downgrade_wake(sem);
1487}
1488
1489/*
1490 * lock for reading
1491 */
1492void __sched down_read(struct rw_semaphore *sem)
1493{
1494 might_sleep();
1495 rwsem_acquire_read(&sem->dep_map, 0, 0, _RET_IP_);
1496
1497 LOCK_CONTENDED(sem, __down_read_trylock, __down_read);
1498}
1499EXPORT_SYMBOL(down_read);
1500
1501int __sched down_read_killable(struct rw_semaphore *sem)
1502{
1503 might_sleep();
1504 rwsem_acquire_read(&sem->dep_map, 0, 0, _RET_IP_);
1505
1506 if (LOCK_CONTENDED_RETURN(sem, __down_read_trylock, __down_read_killable)) {
1507 rwsem_release(&sem->dep_map, _RET_IP_);
1508 return -EINTR;
1509 }
1510
1511 return 0;
1512}
1513EXPORT_SYMBOL(down_read_killable);
1514
1515/*
1516 * trylock for reading -- returns 1 if successful, 0 if contention
1517 */
1518int down_read_trylock(struct rw_semaphore *sem)
1519{
1520 int ret = __down_read_trylock(sem);
1521
1522 if (ret == 1)
1523 rwsem_acquire_read(&sem->dep_map, 0, 1, _RET_IP_);
1524 return ret;
1525}
1526EXPORT_SYMBOL(down_read_trylock);
1527
1528/*
1529 * lock for writing
1530 */
1531void __sched down_write(struct rw_semaphore *sem)
1532{
1533 might_sleep();
1534 rwsem_acquire(&sem->dep_map, 0, 0, _RET_IP_);
1535 LOCK_CONTENDED(sem, __down_write_trylock, __down_write);
1536}
1537EXPORT_SYMBOL(down_write);
1538
1539/*
1540 * lock for writing
1541 */
1542int __sched down_write_killable(struct rw_semaphore *sem)
1543{
1544 might_sleep();
1545 rwsem_acquire(&sem->dep_map, 0, 0, _RET_IP_);
1546
1547 if (LOCK_CONTENDED_RETURN(sem, __down_write_trylock,
1548 __down_write_killable)) {
1549 rwsem_release(&sem->dep_map, _RET_IP_);
1550 return -EINTR;
1551 }
1552
1553 return 0;
1554}
1555EXPORT_SYMBOL(down_write_killable);
1556
1557/*
1558 * trylock for writing -- returns 1 if successful, 0 if contention
1559 */
1560int down_write_trylock(struct rw_semaphore *sem)
1561{
1562 int ret = __down_write_trylock(sem);
1563
1564 if (ret == 1)
1565 rwsem_acquire(&sem->dep_map, 0, 1, _RET_IP_);
1566
1567 return ret;
1568}
1569EXPORT_SYMBOL(down_write_trylock);
1570
1571/*
1572 * release a read lock
1573 */
1574void up_read(struct rw_semaphore *sem)
1575{
1576 rwsem_release(&sem->dep_map, _RET_IP_);
1577 __up_read(sem);
1578}
1579EXPORT_SYMBOL(up_read);
1580
1581/*
1582 * release a write lock
1583 */
1584void up_write(struct rw_semaphore *sem)
1585{
1586 rwsem_release(&sem->dep_map, _RET_IP_);
1587 __up_write(sem);
1588}
1589EXPORT_SYMBOL(up_write);
1590
1591/*
1592 * downgrade write lock to read lock
1593 */
1594void downgrade_write(struct rw_semaphore *sem)
1595{
1596 lock_downgrade(&sem->dep_map, _RET_IP_);
1597 __downgrade_write(sem);
1598}
1599EXPORT_SYMBOL(downgrade_write);
1600
1601#ifdef CONFIG_DEBUG_LOCK_ALLOC
1602
1603void down_read_nested(struct rw_semaphore *sem, int subclass)
1604{
1605 might_sleep();
1606 rwsem_acquire_read(&sem->dep_map, subclass, 0, _RET_IP_);
1607 LOCK_CONTENDED(sem, __down_read_trylock, __down_read);
1608}
1609EXPORT_SYMBOL(down_read_nested);
1610
1611void _down_write_nest_lock(struct rw_semaphore *sem, struct lockdep_map *nest)
1612{
1613 might_sleep();
1614 rwsem_acquire_nest(&sem->dep_map, 0, 0, nest, _RET_IP_);
1615 LOCK_CONTENDED(sem, __down_write_trylock, __down_write);
1616}
1617EXPORT_SYMBOL(_down_write_nest_lock);
1618
1619void down_read_non_owner(struct rw_semaphore *sem)
1620{
1621 might_sleep();
1622 __down_read(sem);
1623 __rwsem_set_reader_owned(sem, NULL);
1624}
1625EXPORT_SYMBOL(down_read_non_owner);
1626
1627void down_write_nested(struct rw_semaphore *sem, int subclass)
1628{
1629 might_sleep();
1630 rwsem_acquire(&sem->dep_map, subclass, 0, _RET_IP_);
1631 LOCK_CONTENDED(sem, __down_write_trylock, __down_write);
1632}
1633EXPORT_SYMBOL(down_write_nested);
1634
1635int __sched down_write_killable_nested(struct rw_semaphore *sem, int subclass)
1636{
1637 might_sleep();
1638 rwsem_acquire(&sem->dep_map, subclass, 0, _RET_IP_);
1639
1640 if (LOCK_CONTENDED_RETURN(sem, __down_write_trylock,
1641 __down_write_killable)) {
1642 rwsem_release(&sem->dep_map, _RET_IP_);
1643 return -EINTR;
1644 }
1645
1646 return 0;
1647}
1648EXPORT_SYMBOL(down_write_killable_nested);
1649
1650void up_read_non_owner(struct rw_semaphore *sem)
1651{
1652 DEBUG_RWSEMS_WARN_ON(!is_rwsem_reader_owned(sem), sem);
1653 __up_read(sem);
1654}
1655EXPORT_SYMBOL(up_read_non_owner);
1656
1657#endif