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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#include <trace/events/lock.h>
31
32#ifndef CONFIG_PREEMPT_RT
33#include "lock_events.h"
34
35/*
36 * The least significant 2 bits of the owner value has the following
37 * meanings when set.
38 * - Bit 0: RWSEM_READER_OWNED - The rwsem is owned by readers
39 * - Bit 1: RWSEM_NONSPINNABLE - Cannot spin on a reader-owned lock
40 *
41 * When the rwsem is reader-owned and a spinning writer has timed out,
42 * the nonspinnable bit will be set to disable optimistic spinning.
43
44 * When a writer acquires a rwsem, it puts its task_struct pointer
45 * into the owner field. It is cleared after an unlock.
46 *
47 * When a reader acquires a rwsem, it will also puts its task_struct
48 * pointer into the owner field with the RWSEM_READER_OWNED bit set.
49 * On unlock, the owner field will largely be left untouched. So
50 * for a free or reader-owned rwsem, the owner value may contain
51 * information about the last reader that acquires the rwsem.
52 *
53 * That information may be helpful in debugging cases where the system
54 * seems to hang on a reader owned rwsem especially if only one reader
55 * is involved. Ideally we would like to track all the readers that own
56 * a rwsem, but the overhead is simply too big.
57 *
58 * A fast path reader optimistic lock stealing is supported when the rwsem
59 * is previously owned by a writer and the following conditions are met:
60 * - rwsem is not currently writer owned
61 * - the handoff isn't set.
62 */
63#define RWSEM_READER_OWNED (1UL << 0)
64#define RWSEM_NONSPINNABLE (1UL << 1)
65#define RWSEM_OWNER_FLAGS_MASK (RWSEM_READER_OWNED | RWSEM_NONSPINNABLE)
66
67#ifdef CONFIG_DEBUG_RWSEMS
68# define DEBUG_RWSEMS_WARN_ON(c, sem) do { \
69 if (!debug_locks_silent && \
70 WARN_ONCE(c, "DEBUG_RWSEMS_WARN_ON(%s): count = 0x%lx, magic = 0x%lx, owner = 0x%lx, curr 0x%lx, list %sempty\n",\
71 #c, atomic_long_read(&(sem)->count), \
72 (unsigned long) sem->magic, \
73 atomic_long_read(&(sem)->owner), (long)current, \
74 list_empty(&(sem)->wait_list) ? "" : "not ")) \
75 debug_locks_off(); \
76 } while (0)
77#else
78# define DEBUG_RWSEMS_WARN_ON(c, sem)
79#endif
80
81/*
82 * On 64-bit architectures, the bit definitions of the count are:
83 *
84 * Bit 0 - writer locked bit
85 * Bit 1 - waiters present bit
86 * Bit 2 - lock handoff bit
87 * Bits 3-7 - reserved
88 * Bits 8-62 - 55-bit reader count
89 * Bit 63 - read fail bit
90 *
91 * On 32-bit architectures, the bit definitions of the count are:
92 *
93 * Bit 0 - writer locked bit
94 * Bit 1 - waiters present bit
95 * Bit 2 - lock handoff bit
96 * Bits 3-7 - reserved
97 * Bits 8-30 - 23-bit reader count
98 * Bit 31 - read fail bit
99 *
100 * It is not likely that the most significant bit (read fail bit) will ever
101 * be set. This guard bit is still checked anyway in the down_read() fastpath
102 * just in case we need to use up more of the reader bits for other purpose
103 * in the future.
104 *
105 * atomic_long_fetch_add() is used to obtain reader lock, whereas
106 * atomic_long_cmpxchg() will be used to obtain writer lock.
107 *
108 * There are three places where the lock handoff bit may be set or cleared.
109 * 1) rwsem_mark_wake() for readers -- set, clear
110 * 2) rwsem_try_write_lock() for writers -- set, clear
111 * 3) rwsem_del_waiter() -- clear
112 *
113 * For all the above cases, wait_lock will be held. A writer must also
114 * be the first one in the wait_list to be eligible for setting the handoff
115 * bit. So concurrent setting/clearing of handoff bit is not possible.
116 */
117#define RWSEM_WRITER_LOCKED (1UL << 0)
118#define RWSEM_FLAG_WAITERS (1UL << 1)
119#define RWSEM_FLAG_HANDOFF (1UL << 2)
120#define RWSEM_FLAG_READFAIL (1UL << (BITS_PER_LONG - 1))
121
122#define RWSEM_READER_SHIFT 8
123#define RWSEM_READER_BIAS (1UL << RWSEM_READER_SHIFT)
124#define RWSEM_READER_MASK (~(RWSEM_READER_BIAS - 1))
125#define RWSEM_WRITER_MASK RWSEM_WRITER_LOCKED
126#define RWSEM_LOCK_MASK (RWSEM_WRITER_MASK|RWSEM_READER_MASK)
127#define RWSEM_READ_FAILED_MASK (RWSEM_WRITER_MASK|RWSEM_FLAG_WAITERS|\
128 RWSEM_FLAG_HANDOFF|RWSEM_FLAG_READFAIL)
129
130/*
131 * All writes to owner are protected by WRITE_ONCE() to make sure that
132 * store tearing can't happen as optimistic spinners may read and use
133 * the owner value concurrently without lock. Read from owner, however,
134 * may not need READ_ONCE() as long as the pointer value is only used
135 * for comparison and isn't being dereferenced.
136 *
137 * Both rwsem_{set,clear}_owner() functions should be in the same
138 * preempt disable section as the atomic op that changes sem->count.
139 */
140static inline void rwsem_set_owner(struct rw_semaphore *sem)
141{
142 lockdep_assert_preemption_disabled();
143 atomic_long_set(&sem->owner, (long)current);
144}
145
146static inline void rwsem_clear_owner(struct rw_semaphore *sem)
147{
148 lockdep_assert_preemption_disabled();
149 atomic_long_set(&sem->owner, 0);
150}
151
152/*
153 * Test the flags in the owner field.
154 */
155static inline bool rwsem_test_oflags(struct rw_semaphore *sem, long flags)
156{
157 return atomic_long_read(&sem->owner) & flags;
158}
159
160/*
161 * The task_struct pointer of the last owning reader will be left in
162 * the owner field.
163 *
164 * Note that the owner value just indicates the task has owned the rwsem
165 * previously, it may not be the real owner or one of the real owners
166 * anymore when that field is examined, so take it with a grain of salt.
167 *
168 * The reader non-spinnable bit is preserved.
169 */
170static inline void __rwsem_set_reader_owned(struct rw_semaphore *sem,
171 struct task_struct *owner)
172{
173 unsigned long val = (unsigned long)owner | RWSEM_READER_OWNED |
174 (atomic_long_read(&sem->owner) & RWSEM_NONSPINNABLE);
175
176 atomic_long_set(&sem->owner, val);
177}
178
179static inline void rwsem_set_reader_owned(struct rw_semaphore *sem)
180{
181 __rwsem_set_reader_owned(sem, current);
182}
183
184/*
185 * Return true if the rwsem is owned by a reader.
186 */
187static inline bool is_rwsem_reader_owned(struct rw_semaphore *sem)
188{
189#ifdef CONFIG_DEBUG_RWSEMS
190 /*
191 * Check the count to see if it is write-locked.
192 */
193 long count = atomic_long_read(&sem->count);
194
195 if (count & RWSEM_WRITER_MASK)
196 return false;
197#endif
198 return rwsem_test_oflags(sem, RWSEM_READER_OWNED);
199}
200
201#ifdef CONFIG_DEBUG_RWSEMS
202/*
203 * With CONFIG_DEBUG_RWSEMS configured, it will make sure that if there
204 * is a task pointer in owner of a reader-owned rwsem, it will be the
205 * real owner or one of the real owners. The only exception is when the
206 * unlock is done by up_read_non_owner().
207 */
208static inline void rwsem_clear_reader_owned(struct rw_semaphore *sem)
209{
210 unsigned long val = atomic_long_read(&sem->owner);
211
212 while ((val & ~RWSEM_OWNER_FLAGS_MASK) == (unsigned long)current) {
213 if (atomic_long_try_cmpxchg(&sem->owner, &val,
214 val & RWSEM_OWNER_FLAGS_MASK))
215 return;
216 }
217}
218#else
219static inline void rwsem_clear_reader_owned(struct rw_semaphore *sem)
220{
221}
222#endif
223
224/*
225 * Set the RWSEM_NONSPINNABLE bits if the RWSEM_READER_OWNED flag
226 * remains set. Otherwise, the operation will be aborted.
227 */
228static inline void rwsem_set_nonspinnable(struct rw_semaphore *sem)
229{
230 unsigned long owner = atomic_long_read(&sem->owner);
231
232 do {
233 if (!(owner & RWSEM_READER_OWNED))
234 break;
235 if (owner & RWSEM_NONSPINNABLE)
236 break;
237 } while (!atomic_long_try_cmpxchg(&sem->owner, &owner,
238 owner | RWSEM_NONSPINNABLE));
239}
240
241static inline bool rwsem_read_trylock(struct rw_semaphore *sem, long *cntp)
242{
243 *cntp = atomic_long_add_return_acquire(RWSEM_READER_BIAS, &sem->count);
244
245 if (WARN_ON_ONCE(*cntp < 0))
246 rwsem_set_nonspinnable(sem);
247
248 if (!(*cntp & RWSEM_READ_FAILED_MASK)) {
249 rwsem_set_reader_owned(sem);
250 return true;
251 }
252
253 return false;
254}
255
256static inline bool rwsem_write_trylock(struct rw_semaphore *sem)
257{
258 long tmp = RWSEM_UNLOCKED_VALUE;
259
260 if (atomic_long_try_cmpxchg_acquire(&sem->count, &tmp, RWSEM_WRITER_LOCKED)) {
261 rwsem_set_owner(sem);
262 return true;
263 }
264
265 return false;
266}
267
268/*
269 * Return just the real task structure pointer of the owner
270 */
271static inline struct task_struct *rwsem_owner(struct rw_semaphore *sem)
272{
273 return (struct task_struct *)
274 (atomic_long_read(&sem->owner) & ~RWSEM_OWNER_FLAGS_MASK);
275}
276
277/*
278 * Return the real task structure pointer of the owner and the embedded
279 * flags in the owner. pflags must be non-NULL.
280 */
281static inline struct task_struct *
282rwsem_owner_flags(struct rw_semaphore *sem, unsigned long *pflags)
283{
284 unsigned long owner = atomic_long_read(&sem->owner);
285
286 *pflags = owner & RWSEM_OWNER_FLAGS_MASK;
287 return (struct task_struct *)(owner & ~RWSEM_OWNER_FLAGS_MASK);
288}
289
290/*
291 * Guide to the rw_semaphore's count field.
292 *
293 * When the RWSEM_WRITER_LOCKED bit in count is set, the lock is owned
294 * by a writer.
295 *
296 * The lock is owned by readers when
297 * (1) the RWSEM_WRITER_LOCKED isn't set in count,
298 * (2) some of the reader bits are set in count, and
299 * (3) the owner field has RWSEM_READ_OWNED bit set.
300 *
301 * Having some reader bits set is not enough to guarantee a readers owned
302 * lock as the readers may be in the process of backing out from the count
303 * and a writer has just released the lock. So another writer may steal
304 * the lock immediately after that.
305 */
306
307/*
308 * Initialize an rwsem:
309 */
310void __init_rwsem(struct rw_semaphore *sem, const char *name,
311 struct lock_class_key *key)
312{
313#ifdef CONFIG_DEBUG_LOCK_ALLOC
314 /*
315 * Make sure we are not reinitializing a held semaphore:
316 */
317 debug_check_no_locks_freed((void *)sem, sizeof(*sem));
318 lockdep_init_map_wait(&sem->dep_map, name, key, 0, LD_WAIT_SLEEP);
319#endif
320#ifdef CONFIG_DEBUG_RWSEMS
321 sem->magic = sem;
322#endif
323 atomic_long_set(&sem->count, RWSEM_UNLOCKED_VALUE);
324 raw_spin_lock_init(&sem->wait_lock);
325 INIT_LIST_HEAD(&sem->wait_list);
326 atomic_long_set(&sem->owner, 0L);
327#ifdef CONFIG_RWSEM_SPIN_ON_OWNER
328 osq_lock_init(&sem->osq);
329#endif
330}
331EXPORT_SYMBOL(__init_rwsem);
332
333enum rwsem_waiter_type {
334 RWSEM_WAITING_FOR_WRITE,
335 RWSEM_WAITING_FOR_READ
336};
337
338struct rwsem_waiter {
339 struct list_head list;
340 struct task_struct *task;
341 enum rwsem_waiter_type type;
342 unsigned long timeout;
343 bool handoff_set;
344};
345#define rwsem_first_waiter(sem) \
346 list_first_entry(&sem->wait_list, struct rwsem_waiter, list)
347
348enum rwsem_wake_type {
349 RWSEM_WAKE_ANY, /* Wake whatever's at head of wait list */
350 RWSEM_WAKE_READERS, /* Wake readers only */
351 RWSEM_WAKE_READ_OWNED /* Waker thread holds the read lock */
352};
353
354/*
355 * The typical HZ value is either 250 or 1000. So set the minimum waiting
356 * time to at least 4ms or 1 jiffy (if it is higher than 4ms) in the wait
357 * queue before initiating the handoff protocol.
358 */
359#define RWSEM_WAIT_TIMEOUT DIV_ROUND_UP(HZ, 250)
360
361/*
362 * Magic number to batch-wakeup waiting readers, even when writers are
363 * also present in the queue. This both limits the amount of work the
364 * waking thread must do and also prevents any potential counter overflow,
365 * however unlikely.
366 */
367#define MAX_READERS_WAKEUP 0x100
368
369static inline void
370rwsem_add_waiter(struct rw_semaphore *sem, struct rwsem_waiter *waiter)
371{
372 lockdep_assert_held(&sem->wait_lock);
373 list_add_tail(&waiter->list, &sem->wait_list);
374 /* caller will set RWSEM_FLAG_WAITERS */
375}
376
377/*
378 * Remove a waiter from the wait_list and clear flags.
379 *
380 * Both rwsem_mark_wake() and rwsem_try_write_lock() contain a full 'copy' of
381 * this function. Modify with care.
382 *
383 * Return: true if wait_list isn't empty and false otherwise
384 */
385static inline bool
386rwsem_del_waiter(struct rw_semaphore *sem, struct rwsem_waiter *waiter)
387{
388 lockdep_assert_held(&sem->wait_lock);
389 list_del(&waiter->list);
390 if (likely(!list_empty(&sem->wait_list)))
391 return true;
392
393 atomic_long_andnot(RWSEM_FLAG_HANDOFF | RWSEM_FLAG_WAITERS, &sem->count);
394 return false;
395}
396
397/*
398 * handle the lock release when processes blocked on it that can now run
399 * - if we come here from up_xxxx(), then the RWSEM_FLAG_WAITERS bit must
400 * have been set.
401 * - there must be someone on the queue
402 * - the wait_lock must be held by the caller
403 * - tasks are marked for wakeup, the caller must later invoke wake_up_q()
404 * to actually wakeup the blocked task(s) and drop the reference count,
405 * preferably when the wait_lock is released
406 * - woken process blocks are discarded from the list after having task zeroed
407 * - writers are only marked woken if downgrading is false
408 *
409 * Implies rwsem_del_waiter() for all woken readers.
410 */
411static void rwsem_mark_wake(struct rw_semaphore *sem,
412 enum rwsem_wake_type wake_type,
413 struct wake_q_head *wake_q)
414{
415 struct rwsem_waiter *waiter, *tmp;
416 long oldcount, woken = 0, adjustment = 0;
417 struct list_head wlist;
418
419 lockdep_assert_held(&sem->wait_lock);
420
421 /*
422 * Take a peek at the queue head waiter such that we can determine
423 * the wakeup(s) to perform.
424 */
425 waiter = rwsem_first_waiter(sem);
426
427 if (waiter->type == RWSEM_WAITING_FOR_WRITE) {
428 if (wake_type == RWSEM_WAKE_ANY) {
429 /*
430 * Mark writer at the front of the queue for wakeup.
431 * Until the task is actually later awoken later by
432 * the caller, other writers are able to steal it.
433 * Readers, on the other hand, will block as they
434 * will notice the queued writer.
435 */
436 wake_q_add(wake_q, waiter->task);
437 lockevent_inc(rwsem_wake_writer);
438 }
439
440 return;
441 }
442
443 /*
444 * No reader wakeup if there are too many of them already.
445 */
446 if (unlikely(atomic_long_read(&sem->count) < 0))
447 return;
448
449 /*
450 * Writers might steal the lock before we grant it to the next reader.
451 * We prefer to do the first reader grant before counting readers
452 * so we can bail out early if a writer stole the lock.
453 */
454 if (wake_type != RWSEM_WAKE_READ_OWNED) {
455 struct task_struct *owner;
456
457 adjustment = RWSEM_READER_BIAS;
458 oldcount = atomic_long_fetch_add(adjustment, &sem->count);
459 if (unlikely(oldcount & RWSEM_WRITER_MASK)) {
460 /*
461 * When we've been waiting "too" long (for writers
462 * to give up the lock), request a HANDOFF to
463 * force the issue.
464 */
465 if (time_after(jiffies, waiter->timeout)) {
466 if (!(oldcount & RWSEM_FLAG_HANDOFF)) {
467 adjustment -= RWSEM_FLAG_HANDOFF;
468 lockevent_inc(rwsem_rlock_handoff);
469 }
470 waiter->handoff_set = true;
471 }
472
473 atomic_long_add(-adjustment, &sem->count);
474 return;
475 }
476 /*
477 * Set it to reader-owned to give spinners an early
478 * indication that readers now have the lock.
479 * The reader nonspinnable bit seen at slowpath entry of
480 * the reader is copied over.
481 */
482 owner = waiter->task;
483 __rwsem_set_reader_owned(sem, owner);
484 }
485
486 /*
487 * Grant up to MAX_READERS_WAKEUP read locks to all the readers in the
488 * queue. We know that the woken will be at least 1 as we accounted
489 * for above. Note we increment the 'active part' of the count by the
490 * number of readers before waking any processes up.
491 *
492 * This is an adaptation of the phase-fair R/W locks where at the
493 * reader phase (first waiter is a reader), all readers are eligible
494 * to acquire the lock at the same time irrespective of their order
495 * in the queue. The writers acquire the lock according to their
496 * order in the queue.
497 *
498 * We have to do wakeup in 2 passes to prevent the possibility that
499 * the reader count may be decremented before it is incremented. It
500 * is because the to-be-woken waiter may not have slept yet. So it
501 * may see waiter->task got cleared, finish its critical section and
502 * do an unlock before the reader count increment.
503 *
504 * 1) Collect the read-waiters in a separate list, count them and
505 * fully increment the reader count in rwsem.
506 * 2) For each waiters in the new list, clear waiter->task and
507 * put them into wake_q to be woken up later.
508 */
509 INIT_LIST_HEAD(&wlist);
510 list_for_each_entry_safe(waiter, tmp, &sem->wait_list, list) {
511 if (waiter->type == RWSEM_WAITING_FOR_WRITE)
512 continue;
513
514 woken++;
515 list_move_tail(&waiter->list, &wlist);
516
517 /*
518 * Limit # of readers that can be woken up per wakeup call.
519 */
520 if (unlikely(woken >= MAX_READERS_WAKEUP))
521 break;
522 }
523
524 adjustment = woken * RWSEM_READER_BIAS - adjustment;
525 lockevent_cond_inc(rwsem_wake_reader, woken);
526
527 oldcount = atomic_long_read(&sem->count);
528 if (list_empty(&sem->wait_list)) {
529 /*
530 * Combined with list_move_tail() above, this implies
531 * rwsem_del_waiter().
532 */
533 adjustment -= RWSEM_FLAG_WAITERS;
534 if (oldcount & RWSEM_FLAG_HANDOFF)
535 adjustment -= RWSEM_FLAG_HANDOFF;
536 } else if (woken) {
537 /*
538 * When we've woken a reader, we no longer need to force
539 * writers to give up the lock and we can clear HANDOFF.
540 */
541 if (oldcount & RWSEM_FLAG_HANDOFF)
542 adjustment -= RWSEM_FLAG_HANDOFF;
543 }
544
545 if (adjustment)
546 atomic_long_add(adjustment, &sem->count);
547
548 /* 2nd pass */
549 list_for_each_entry_safe(waiter, tmp, &wlist, list) {
550 struct task_struct *tsk;
551
552 tsk = waiter->task;
553 get_task_struct(tsk);
554
555 /*
556 * Ensure calling get_task_struct() before setting the reader
557 * waiter to nil such that rwsem_down_read_slowpath() cannot
558 * race with do_exit() by always holding a reference count
559 * to the task to wakeup.
560 */
561 smp_store_release(&waiter->task, NULL);
562 /*
563 * Ensure issuing the wakeup (either by us or someone else)
564 * after setting the reader waiter to nil.
565 */
566 wake_q_add_safe(wake_q, tsk);
567 }
568}
569
570/*
571 * Remove a waiter and try to wake up other waiters in the wait queue
572 * This function is called from the out_nolock path of both the reader and
573 * writer slowpaths with wait_lock held. It releases the wait_lock and
574 * optionally wake up waiters before it returns.
575 */
576static inline void
577rwsem_del_wake_waiter(struct rw_semaphore *sem, struct rwsem_waiter *waiter,
578 struct wake_q_head *wake_q)
579 __releases(&sem->wait_lock)
580{
581 bool first = rwsem_first_waiter(sem) == waiter;
582
583 wake_q_init(wake_q);
584
585 /*
586 * If the wait_list isn't empty and the waiter to be deleted is
587 * the first waiter, we wake up the remaining waiters as they may
588 * be eligible to acquire or spin on the lock.
589 */
590 if (rwsem_del_waiter(sem, waiter) && first)
591 rwsem_mark_wake(sem, RWSEM_WAKE_ANY, wake_q);
592 raw_spin_unlock_irq(&sem->wait_lock);
593 if (!wake_q_empty(wake_q))
594 wake_up_q(wake_q);
595}
596
597/*
598 * This function must be called with the sem->wait_lock held to prevent
599 * race conditions between checking the rwsem wait list and setting the
600 * sem->count accordingly.
601 *
602 * Implies rwsem_del_waiter() on success.
603 */
604static inline bool rwsem_try_write_lock(struct rw_semaphore *sem,
605 struct rwsem_waiter *waiter)
606{
607 struct rwsem_waiter *first = rwsem_first_waiter(sem);
608 long count, new;
609
610 lockdep_assert_held(&sem->wait_lock);
611
612 count = atomic_long_read(&sem->count);
613 do {
614 bool has_handoff = !!(count & RWSEM_FLAG_HANDOFF);
615
616 if (has_handoff) {
617 /*
618 * Honor handoff bit and yield only when the first
619 * waiter is the one that set it. Otherwisee, we
620 * still try to acquire the rwsem.
621 */
622 if (first->handoff_set && (waiter != first))
623 return false;
624 }
625
626 new = count;
627
628 if (count & RWSEM_LOCK_MASK) {
629 /*
630 * A waiter (first or not) can set the handoff bit
631 * if it is an RT task or wait in the wait queue
632 * for too long.
633 */
634 if (has_handoff || (!rt_task(waiter->task) &&
635 !time_after(jiffies, waiter->timeout)))
636 return false;
637
638 new |= RWSEM_FLAG_HANDOFF;
639 } else {
640 new |= RWSEM_WRITER_LOCKED;
641 new &= ~RWSEM_FLAG_HANDOFF;
642
643 if (list_is_singular(&sem->wait_list))
644 new &= ~RWSEM_FLAG_WAITERS;
645 }
646 } while (!atomic_long_try_cmpxchg_acquire(&sem->count, &count, new));
647
648 /*
649 * We have either acquired the lock with handoff bit cleared or set
650 * the handoff bit. Only the first waiter can have its handoff_set
651 * set here to enable optimistic spinning in slowpath loop.
652 */
653 if (new & RWSEM_FLAG_HANDOFF) {
654 first->handoff_set = true;
655 lockevent_inc(rwsem_wlock_handoff);
656 return false;
657 }
658
659 /*
660 * Have rwsem_try_write_lock() fully imply rwsem_del_waiter() on
661 * success.
662 */
663 list_del(&waiter->list);
664 rwsem_set_owner(sem);
665 return true;
666}
667
668/*
669 * The rwsem_spin_on_owner() function returns the following 4 values
670 * depending on the lock owner state.
671 * OWNER_NULL : owner is currently NULL
672 * OWNER_WRITER: when owner changes and is a writer
673 * OWNER_READER: when owner changes and the new owner may be a reader.
674 * OWNER_NONSPINNABLE:
675 * when optimistic spinning has to stop because either the
676 * owner stops running, is unknown, or its timeslice has
677 * been used up.
678 */
679enum owner_state {
680 OWNER_NULL = 1 << 0,
681 OWNER_WRITER = 1 << 1,
682 OWNER_READER = 1 << 2,
683 OWNER_NONSPINNABLE = 1 << 3,
684};
685
686#ifdef CONFIG_RWSEM_SPIN_ON_OWNER
687/*
688 * Try to acquire write lock before the writer has been put on wait queue.
689 */
690static inline bool rwsem_try_write_lock_unqueued(struct rw_semaphore *sem)
691{
692 long count = atomic_long_read(&sem->count);
693
694 while (!(count & (RWSEM_LOCK_MASK|RWSEM_FLAG_HANDOFF))) {
695 if (atomic_long_try_cmpxchg_acquire(&sem->count, &count,
696 count | RWSEM_WRITER_LOCKED)) {
697 rwsem_set_owner(sem);
698 lockevent_inc(rwsem_opt_lock);
699 return true;
700 }
701 }
702 return false;
703}
704
705static inline bool rwsem_can_spin_on_owner(struct rw_semaphore *sem)
706{
707 struct task_struct *owner;
708 unsigned long flags;
709 bool ret = true;
710
711 if (need_resched()) {
712 lockevent_inc(rwsem_opt_fail);
713 return false;
714 }
715
716 /*
717 * Disable preemption is equal to the RCU read-side crital section,
718 * thus the task_strcut structure won't go away.
719 */
720 owner = rwsem_owner_flags(sem, &flags);
721 /*
722 * Don't check the read-owner as the entry may be stale.
723 */
724 if ((flags & RWSEM_NONSPINNABLE) ||
725 (owner && !(flags & RWSEM_READER_OWNED) && !owner_on_cpu(owner)))
726 ret = false;
727
728 lockevent_cond_inc(rwsem_opt_fail, !ret);
729 return ret;
730}
731
732#define OWNER_SPINNABLE (OWNER_NULL | OWNER_WRITER | OWNER_READER)
733
734static inline enum owner_state
735rwsem_owner_state(struct task_struct *owner, unsigned long flags)
736{
737 if (flags & RWSEM_NONSPINNABLE)
738 return OWNER_NONSPINNABLE;
739
740 if (flags & RWSEM_READER_OWNED)
741 return OWNER_READER;
742
743 return owner ? OWNER_WRITER : OWNER_NULL;
744}
745
746static noinline enum owner_state
747rwsem_spin_on_owner(struct rw_semaphore *sem)
748{
749 struct task_struct *new, *owner;
750 unsigned long flags, new_flags;
751 enum owner_state state;
752
753 lockdep_assert_preemption_disabled();
754
755 owner = rwsem_owner_flags(sem, &flags);
756 state = rwsem_owner_state(owner, flags);
757 if (state != OWNER_WRITER)
758 return state;
759
760 for (;;) {
761 /*
762 * When a waiting writer set the handoff flag, it may spin
763 * on the owner as well. Once that writer acquires the lock,
764 * we can spin on it. So we don't need to quit even when the
765 * handoff bit is set.
766 */
767 new = rwsem_owner_flags(sem, &new_flags);
768 if ((new != owner) || (new_flags != flags)) {
769 state = rwsem_owner_state(new, new_flags);
770 break;
771 }
772
773 /*
774 * Ensure we emit the owner->on_cpu, dereference _after_
775 * checking sem->owner still matches owner, if that fails,
776 * owner might point to free()d memory, if it still matches,
777 * our spinning context already disabled preemption which is
778 * equal to RCU read-side crital section ensures the memory
779 * stays valid.
780 */
781 barrier();
782
783 if (need_resched() || !owner_on_cpu(owner)) {
784 state = OWNER_NONSPINNABLE;
785 break;
786 }
787
788 cpu_relax();
789 }
790
791 return state;
792}
793
794/*
795 * Calculate reader-owned rwsem spinning threshold for writer
796 *
797 * The more readers own the rwsem, the longer it will take for them to
798 * wind down and free the rwsem. So the empirical formula used to
799 * determine the actual spinning time limit here is:
800 *
801 * Spinning threshold = (10 + nr_readers/2)us
802 *
803 * The limit is capped to a maximum of 25us (30 readers). This is just
804 * a heuristic and is subjected to change in the future.
805 */
806static inline u64 rwsem_rspin_threshold(struct rw_semaphore *sem)
807{
808 long count = atomic_long_read(&sem->count);
809 int readers = count >> RWSEM_READER_SHIFT;
810 u64 delta;
811
812 if (readers > 30)
813 readers = 30;
814 delta = (20 + readers) * NSEC_PER_USEC / 2;
815
816 return sched_clock() + delta;
817}
818
819static bool rwsem_optimistic_spin(struct rw_semaphore *sem)
820{
821 bool taken = false;
822 int prev_owner_state = OWNER_NULL;
823 int loop = 0;
824 u64 rspin_threshold = 0;
825
826 /* sem->wait_lock should not be held when doing optimistic spinning */
827 if (!osq_lock(&sem->osq))
828 goto done;
829
830 /*
831 * Optimistically spin on the owner field and attempt to acquire the
832 * lock whenever the owner changes. Spinning will be stopped when:
833 * 1) the owning writer isn't running; or
834 * 2) readers own the lock and spinning time has exceeded limit.
835 */
836 for (;;) {
837 enum owner_state owner_state;
838
839 owner_state = rwsem_spin_on_owner(sem);
840 if (!(owner_state & OWNER_SPINNABLE))
841 break;
842
843 /*
844 * Try to acquire the lock
845 */
846 taken = rwsem_try_write_lock_unqueued(sem);
847
848 if (taken)
849 break;
850
851 /*
852 * Time-based reader-owned rwsem optimistic spinning
853 */
854 if (owner_state == OWNER_READER) {
855 /*
856 * Re-initialize rspin_threshold every time when
857 * the owner state changes from non-reader to reader.
858 * This allows a writer to steal the lock in between
859 * 2 reader phases and have the threshold reset at
860 * the beginning of the 2nd reader phase.
861 */
862 if (prev_owner_state != OWNER_READER) {
863 if (rwsem_test_oflags(sem, RWSEM_NONSPINNABLE))
864 break;
865 rspin_threshold = rwsem_rspin_threshold(sem);
866 loop = 0;
867 }
868
869 /*
870 * Check time threshold once every 16 iterations to
871 * avoid calling sched_clock() too frequently so
872 * as to reduce the average latency between the times
873 * when the lock becomes free and when the spinner
874 * is ready to do a trylock.
875 */
876 else if (!(++loop & 0xf) && (sched_clock() > rspin_threshold)) {
877 rwsem_set_nonspinnable(sem);
878 lockevent_inc(rwsem_opt_nospin);
879 break;
880 }
881 }
882
883 /*
884 * An RT task cannot do optimistic spinning if it cannot
885 * be sure the lock holder is running or live-lock may
886 * happen if the current task and the lock holder happen
887 * to run in the same CPU. However, aborting optimistic
888 * spinning while a NULL owner is detected may miss some
889 * opportunity where spinning can continue without causing
890 * problem.
891 *
892 * There are 2 possible cases where an RT task may be able
893 * to continue spinning.
894 *
895 * 1) The lock owner is in the process of releasing the
896 * lock, sem->owner is cleared but the lock has not
897 * been released yet.
898 * 2) The lock was free and owner cleared, but another
899 * task just comes in and acquire the lock before
900 * we try to get it. The new owner may be a spinnable
901 * writer.
902 *
903 * To take advantage of two scenarios listed above, the RT
904 * task is made to retry one more time to see if it can
905 * acquire the lock or continue spinning on the new owning
906 * writer. Of course, if the time lag is long enough or the
907 * new owner is not a writer or spinnable, the RT task will
908 * quit spinning.
909 *
910 * If the owner is a writer, the need_resched() check is
911 * done inside rwsem_spin_on_owner(). If the owner is not
912 * a writer, need_resched() check needs to be done here.
913 */
914 if (owner_state != OWNER_WRITER) {
915 if (need_resched())
916 break;
917 if (rt_task(current) &&
918 (prev_owner_state != OWNER_WRITER))
919 break;
920 }
921 prev_owner_state = owner_state;
922
923 /*
924 * The cpu_relax() call is a compiler barrier which forces
925 * everything in this loop to be re-loaded. We don't need
926 * memory barriers as we'll eventually observe the right
927 * values at the cost of a few extra spins.
928 */
929 cpu_relax();
930 }
931 osq_unlock(&sem->osq);
932done:
933 lockevent_cond_inc(rwsem_opt_fail, !taken);
934 return taken;
935}
936
937/*
938 * Clear the owner's RWSEM_NONSPINNABLE bit if it is set. This should
939 * only be called when the reader count reaches 0.
940 */
941static inline void clear_nonspinnable(struct rw_semaphore *sem)
942{
943 if (unlikely(rwsem_test_oflags(sem, RWSEM_NONSPINNABLE)))
944 atomic_long_andnot(RWSEM_NONSPINNABLE, &sem->owner);
945}
946
947#else
948static inline bool rwsem_can_spin_on_owner(struct rw_semaphore *sem)
949{
950 return false;
951}
952
953static inline bool rwsem_optimistic_spin(struct rw_semaphore *sem)
954{
955 return false;
956}
957
958static inline void clear_nonspinnable(struct rw_semaphore *sem) { }
959
960static inline enum owner_state
961rwsem_spin_on_owner(struct rw_semaphore *sem)
962{
963 return OWNER_NONSPINNABLE;
964}
965#endif
966
967/*
968 * Prepare to wake up waiter(s) in the wait queue by putting them into the
969 * given wake_q if the rwsem lock owner isn't a writer. If rwsem is likely
970 * reader-owned, wake up read lock waiters in queue front or wake up any
971 * front waiter otherwise.
972
973 * This is being called from both reader and writer slow paths.
974 */
975static inline void rwsem_cond_wake_waiter(struct rw_semaphore *sem, long count,
976 struct wake_q_head *wake_q)
977{
978 enum rwsem_wake_type wake_type;
979
980 if (count & RWSEM_WRITER_MASK)
981 return;
982
983 if (count & RWSEM_READER_MASK) {
984 wake_type = RWSEM_WAKE_READERS;
985 } else {
986 wake_type = RWSEM_WAKE_ANY;
987 clear_nonspinnable(sem);
988 }
989 rwsem_mark_wake(sem, wake_type, wake_q);
990}
991
992/*
993 * Wait for the read lock to be granted
994 */
995static struct rw_semaphore __sched *
996rwsem_down_read_slowpath(struct rw_semaphore *sem, long count, unsigned int state)
997{
998 long adjustment = -RWSEM_READER_BIAS;
999 long rcnt = (count >> RWSEM_READER_SHIFT);
1000 struct rwsem_waiter waiter;
1001 DEFINE_WAKE_Q(wake_q);
1002
1003 /*
1004 * To prevent a constant stream of readers from starving a sleeping
1005 * waiter, don't attempt optimistic lock stealing if the lock is
1006 * currently owned by readers.
1007 */
1008 if ((atomic_long_read(&sem->owner) & RWSEM_READER_OWNED) &&
1009 (rcnt > 1) && !(count & RWSEM_WRITER_LOCKED))
1010 goto queue;
1011
1012 /*
1013 * Reader optimistic lock stealing.
1014 */
1015 if (!(count & (RWSEM_WRITER_LOCKED | RWSEM_FLAG_HANDOFF))) {
1016 rwsem_set_reader_owned(sem);
1017 lockevent_inc(rwsem_rlock_steal);
1018
1019 /*
1020 * Wake up other readers in the wait queue if it is
1021 * the first reader.
1022 */
1023 if ((rcnt == 1) && (count & RWSEM_FLAG_WAITERS)) {
1024 raw_spin_lock_irq(&sem->wait_lock);
1025 if (!list_empty(&sem->wait_list))
1026 rwsem_mark_wake(sem, RWSEM_WAKE_READ_OWNED,
1027 &wake_q);
1028 raw_spin_unlock_irq(&sem->wait_lock);
1029 wake_up_q(&wake_q);
1030 }
1031 return sem;
1032 }
1033
1034queue:
1035 waiter.task = current;
1036 waiter.type = RWSEM_WAITING_FOR_READ;
1037 waiter.timeout = jiffies + RWSEM_WAIT_TIMEOUT;
1038 waiter.handoff_set = false;
1039
1040 raw_spin_lock_irq(&sem->wait_lock);
1041 if (list_empty(&sem->wait_list)) {
1042 /*
1043 * In case the wait queue is empty and the lock isn't owned
1044 * by a writer, this reader can exit the slowpath and return
1045 * immediately as its RWSEM_READER_BIAS has already been set
1046 * in the count.
1047 */
1048 if (!(atomic_long_read(&sem->count) & RWSEM_WRITER_MASK)) {
1049 /* Provide lock ACQUIRE */
1050 smp_acquire__after_ctrl_dep();
1051 raw_spin_unlock_irq(&sem->wait_lock);
1052 rwsem_set_reader_owned(sem);
1053 lockevent_inc(rwsem_rlock_fast);
1054 return sem;
1055 }
1056 adjustment += RWSEM_FLAG_WAITERS;
1057 }
1058 rwsem_add_waiter(sem, &waiter);
1059
1060 /* we're now waiting on the lock, but no longer actively locking */
1061 count = atomic_long_add_return(adjustment, &sem->count);
1062
1063 rwsem_cond_wake_waiter(sem, count, &wake_q);
1064 raw_spin_unlock_irq(&sem->wait_lock);
1065
1066 if (!wake_q_empty(&wake_q))
1067 wake_up_q(&wake_q);
1068
1069 trace_contention_begin(sem, LCB_F_READ);
1070
1071 /* wait to be given the lock */
1072 for (;;) {
1073 set_current_state(state);
1074 if (!smp_load_acquire(&waiter.task)) {
1075 /* Matches rwsem_mark_wake()'s smp_store_release(). */
1076 break;
1077 }
1078 if (signal_pending_state(state, current)) {
1079 raw_spin_lock_irq(&sem->wait_lock);
1080 if (waiter.task)
1081 goto out_nolock;
1082 raw_spin_unlock_irq(&sem->wait_lock);
1083 /* Ordered by sem->wait_lock against rwsem_mark_wake(). */
1084 break;
1085 }
1086 schedule_preempt_disabled();
1087 lockevent_inc(rwsem_sleep_reader);
1088 }
1089
1090 __set_current_state(TASK_RUNNING);
1091 lockevent_inc(rwsem_rlock);
1092 trace_contention_end(sem, 0);
1093 return sem;
1094
1095out_nolock:
1096 rwsem_del_wake_waiter(sem, &waiter, &wake_q);
1097 __set_current_state(TASK_RUNNING);
1098 lockevent_inc(rwsem_rlock_fail);
1099 trace_contention_end(sem, -EINTR);
1100 return ERR_PTR(-EINTR);
1101}
1102
1103/*
1104 * Wait until we successfully acquire the write lock
1105 */
1106static struct rw_semaphore __sched *
1107rwsem_down_write_slowpath(struct rw_semaphore *sem, int state)
1108{
1109 struct rwsem_waiter waiter;
1110 DEFINE_WAKE_Q(wake_q);
1111
1112 /* do optimistic spinning and steal lock if possible */
1113 if (rwsem_can_spin_on_owner(sem) && rwsem_optimistic_spin(sem)) {
1114 /* rwsem_optimistic_spin() implies ACQUIRE on success */
1115 return sem;
1116 }
1117
1118 /*
1119 * Optimistic spinning failed, proceed to the slowpath
1120 * and block until we can acquire the sem.
1121 */
1122 waiter.task = current;
1123 waiter.type = RWSEM_WAITING_FOR_WRITE;
1124 waiter.timeout = jiffies + RWSEM_WAIT_TIMEOUT;
1125 waiter.handoff_set = false;
1126
1127 raw_spin_lock_irq(&sem->wait_lock);
1128 rwsem_add_waiter(sem, &waiter);
1129
1130 /* we're now waiting on the lock */
1131 if (rwsem_first_waiter(sem) != &waiter) {
1132 rwsem_cond_wake_waiter(sem, atomic_long_read(&sem->count),
1133 &wake_q);
1134 if (!wake_q_empty(&wake_q)) {
1135 /*
1136 * We want to minimize wait_lock hold time especially
1137 * when a large number of readers are to be woken up.
1138 */
1139 raw_spin_unlock_irq(&sem->wait_lock);
1140 wake_up_q(&wake_q);
1141 raw_spin_lock_irq(&sem->wait_lock);
1142 }
1143 } else {
1144 atomic_long_or(RWSEM_FLAG_WAITERS, &sem->count);
1145 }
1146
1147 /* wait until we successfully acquire the lock */
1148 set_current_state(state);
1149 trace_contention_begin(sem, LCB_F_WRITE);
1150
1151 for (;;) {
1152 if (rwsem_try_write_lock(sem, &waiter)) {
1153 /* rwsem_try_write_lock() implies ACQUIRE on success */
1154 break;
1155 }
1156
1157 raw_spin_unlock_irq(&sem->wait_lock);
1158
1159 if (signal_pending_state(state, current))
1160 goto out_nolock;
1161
1162 /*
1163 * After setting the handoff bit and failing to acquire
1164 * the lock, attempt to spin on owner to accelerate lock
1165 * transfer. If the previous owner is a on-cpu writer and it
1166 * has just released the lock, OWNER_NULL will be returned.
1167 * In this case, we attempt to acquire the lock again
1168 * without sleeping.
1169 */
1170 if (waiter.handoff_set) {
1171 enum owner_state owner_state;
1172
1173 owner_state = rwsem_spin_on_owner(sem);
1174 if (owner_state == OWNER_NULL)
1175 goto trylock_again;
1176 }
1177
1178 schedule_preempt_disabled();
1179 lockevent_inc(rwsem_sleep_writer);
1180 set_current_state(state);
1181trylock_again:
1182 raw_spin_lock_irq(&sem->wait_lock);
1183 }
1184 __set_current_state(TASK_RUNNING);
1185 raw_spin_unlock_irq(&sem->wait_lock);
1186 lockevent_inc(rwsem_wlock);
1187 trace_contention_end(sem, 0);
1188 return sem;
1189
1190out_nolock:
1191 __set_current_state(TASK_RUNNING);
1192 raw_spin_lock_irq(&sem->wait_lock);
1193 rwsem_del_wake_waiter(sem, &waiter, &wake_q);
1194 lockevent_inc(rwsem_wlock_fail);
1195 trace_contention_end(sem, -EINTR);
1196 return ERR_PTR(-EINTR);
1197}
1198
1199/*
1200 * handle waking up a waiter on the semaphore
1201 * - up_read/up_write has decremented the active part of count if we come here
1202 */
1203static struct rw_semaphore *rwsem_wake(struct rw_semaphore *sem)
1204{
1205 unsigned long flags;
1206 DEFINE_WAKE_Q(wake_q);
1207
1208 raw_spin_lock_irqsave(&sem->wait_lock, flags);
1209
1210 if (!list_empty(&sem->wait_list))
1211 rwsem_mark_wake(sem, RWSEM_WAKE_ANY, &wake_q);
1212
1213 raw_spin_unlock_irqrestore(&sem->wait_lock, flags);
1214 wake_up_q(&wake_q);
1215
1216 return sem;
1217}
1218
1219/*
1220 * downgrade a write lock into a read lock
1221 * - caller incremented waiting part of count and discovered it still negative
1222 * - just wake up any readers at the front of the queue
1223 */
1224static struct rw_semaphore *rwsem_downgrade_wake(struct rw_semaphore *sem)
1225{
1226 unsigned long flags;
1227 DEFINE_WAKE_Q(wake_q);
1228
1229 raw_spin_lock_irqsave(&sem->wait_lock, flags);
1230
1231 if (!list_empty(&sem->wait_list))
1232 rwsem_mark_wake(sem, RWSEM_WAKE_READ_OWNED, &wake_q);
1233
1234 raw_spin_unlock_irqrestore(&sem->wait_lock, flags);
1235 wake_up_q(&wake_q);
1236
1237 return sem;
1238}
1239
1240/*
1241 * lock for reading
1242 */
1243static __always_inline int __down_read_common(struct rw_semaphore *sem, int state)
1244{
1245 int ret = 0;
1246 long count;
1247
1248 preempt_disable();
1249 if (!rwsem_read_trylock(sem, &count)) {
1250 if (IS_ERR(rwsem_down_read_slowpath(sem, count, state))) {
1251 ret = -EINTR;
1252 goto out;
1253 }
1254 DEBUG_RWSEMS_WARN_ON(!is_rwsem_reader_owned(sem), sem);
1255 }
1256out:
1257 preempt_enable();
1258 return ret;
1259}
1260
1261static __always_inline void __down_read(struct rw_semaphore *sem)
1262{
1263 __down_read_common(sem, TASK_UNINTERRUPTIBLE);
1264}
1265
1266static __always_inline int __down_read_interruptible(struct rw_semaphore *sem)
1267{
1268 return __down_read_common(sem, TASK_INTERRUPTIBLE);
1269}
1270
1271static __always_inline int __down_read_killable(struct rw_semaphore *sem)
1272{
1273 return __down_read_common(sem, TASK_KILLABLE);
1274}
1275
1276static inline int __down_read_trylock(struct rw_semaphore *sem)
1277{
1278 int ret = 0;
1279 long tmp;
1280
1281 DEBUG_RWSEMS_WARN_ON(sem->magic != sem, sem);
1282
1283 preempt_disable();
1284 tmp = atomic_long_read(&sem->count);
1285 while (!(tmp & RWSEM_READ_FAILED_MASK)) {
1286 if (atomic_long_try_cmpxchg_acquire(&sem->count, &tmp,
1287 tmp + RWSEM_READER_BIAS)) {
1288 rwsem_set_reader_owned(sem);
1289 ret = 1;
1290 break;
1291 }
1292 }
1293 preempt_enable();
1294 return ret;
1295}
1296
1297/*
1298 * lock for writing
1299 */
1300static inline int __down_write_common(struct rw_semaphore *sem, int state)
1301{
1302 int ret = 0;
1303
1304 preempt_disable();
1305 if (unlikely(!rwsem_write_trylock(sem))) {
1306 if (IS_ERR(rwsem_down_write_slowpath(sem, state)))
1307 ret = -EINTR;
1308 }
1309 preempt_enable();
1310 return ret;
1311}
1312
1313static inline void __down_write(struct rw_semaphore *sem)
1314{
1315 __down_write_common(sem, TASK_UNINTERRUPTIBLE);
1316}
1317
1318static inline int __down_write_killable(struct rw_semaphore *sem)
1319{
1320 return __down_write_common(sem, TASK_KILLABLE);
1321}
1322
1323static inline int __down_write_trylock(struct rw_semaphore *sem)
1324{
1325 int ret;
1326
1327 preempt_disable();
1328 DEBUG_RWSEMS_WARN_ON(sem->magic != sem, sem);
1329 ret = rwsem_write_trylock(sem);
1330 preempt_enable();
1331
1332 return ret;
1333}
1334
1335/*
1336 * unlock after reading
1337 */
1338static inline void __up_read(struct rw_semaphore *sem)
1339{
1340 long tmp;
1341
1342 DEBUG_RWSEMS_WARN_ON(sem->magic != sem, sem);
1343 DEBUG_RWSEMS_WARN_ON(!is_rwsem_reader_owned(sem), sem);
1344
1345 preempt_disable();
1346 rwsem_clear_reader_owned(sem);
1347 tmp = atomic_long_add_return_release(-RWSEM_READER_BIAS, &sem->count);
1348 DEBUG_RWSEMS_WARN_ON(tmp < 0, sem);
1349 if (unlikely((tmp & (RWSEM_LOCK_MASK|RWSEM_FLAG_WAITERS)) ==
1350 RWSEM_FLAG_WAITERS)) {
1351 clear_nonspinnable(sem);
1352 rwsem_wake(sem);
1353 }
1354 preempt_enable();
1355}
1356
1357/*
1358 * unlock after writing
1359 */
1360static inline void __up_write(struct rw_semaphore *sem)
1361{
1362 long tmp;
1363
1364 DEBUG_RWSEMS_WARN_ON(sem->magic != sem, sem);
1365 /*
1366 * sem->owner may differ from current if the ownership is transferred
1367 * to an anonymous writer by setting the RWSEM_NONSPINNABLE bits.
1368 */
1369 DEBUG_RWSEMS_WARN_ON((rwsem_owner(sem) != current) &&
1370 !rwsem_test_oflags(sem, RWSEM_NONSPINNABLE), sem);
1371
1372 preempt_disable();
1373 rwsem_clear_owner(sem);
1374 tmp = atomic_long_fetch_add_release(-RWSEM_WRITER_LOCKED, &sem->count);
1375 if (unlikely(tmp & RWSEM_FLAG_WAITERS))
1376 rwsem_wake(sem);
1377 preempt_enable();
1378}
1379
1380/*
1381 * downgrade write lock to read lock
1382 */
1383static inline void __downgrade_write(struct rw_semaphore *sem)
1384{
1385 long tmp;
1386
1387 /*
1388 * When downgrading from exclusive to shared ownership,
1389 * anything inside the write-locked region cannot leak
1390 * into the read side. In contrast, anything in the
1391 * read-locked region is ok to be re-ordered into the
1392 * write side. As such, rely on RELEASE semantics.
1393 */
1394 DEBUG_RWSEMS_WARN_ON(rwsem_owner(sem) != current, sem);
1395 preempt_disable();
1396 tmp = atomic_long_fetch_add_release(
1397 -RWSEM_WRITER_LOCKED+RWSEM_READER_BIAS, &sem->count);
1398 rwsem_set_reader_owned(sem);
1399 if (tmp & RWSEM_FLAG_WAITERS)
1400 rwsem_downgrade_wake(sem);
1401 preempt_enable();
1402}
1403
1404#else /* !CONFIG_PREEMPT_RT */
1405
1406#define RT_MUTEX_BUILD_MUTEX
1407#include "rtmutex.c"
1408
1409#define rwbase_set_and_save_current_state(state) \
1410 set_current_state(state)
1411
1412#define rwbase_restore_current_state() \
1413 __set_current_state(TASK_RUNNING)
1414
1415#define rwbase_rtmutex_lock_state(rtm, state) \
1416 __rt_mutex_lock(rtm, state)
1417
1418#define rwbase_rtmutex_slowlock_locked(rtm, state) \
1419 __rt_mutex_slowlock_locked(rtm, NULL, state)
1420
1421#define rwbase_rtmutex_unlock(rtm) \
1422 __rt_mutex_unlock(rtm)
1423
1424#define rwbase_rtmutex_trylock(rtm) \
1425 __rt_mutex_trylock(rtm)
1426
1427#define rwbase_signal_pending_state(state, current) \
1428 signal_pending_state(state, current)
1429
1430#define rwbase_schedule() \
1431 schedule()
1432
1433#include "rwbase_rt.c"
1434
1435void __init_rwsem(struct rw_semaphore *sem, const char *name,
1436 struct lock_class_key *key)
1437{
1438 init_rwbase_rt(&(sem)->rwbase);
1439
1440#ifdef CONFIG_DEBUG_LOCK_ALLOC
1441 debug_check_no_locks_freed((void *)sem, sizeof(*sem));
1442 lockdep_init_map_wait(&sem->dep_map, name, key, 0, LD_WAIT_SLEEP);
1443#endif
1444}
1445EXPORT_SYMBOL(__init_rwsem);
1446
1447static inline void __down_read(struct rw_semaphore *sem)
1448{
1449 rwbase_read_lock(&sem->rwbase, TASK_UNINTERRUPTIBLE);
1450}
1451
1452static inline int __down_read_interruptible(struct rw_semaphore *sem)
1453{
1454 return rwbase_read_lock(&sem->rwbase, TASK_INTERRUPTIBLE);
1455}
1456
1457static inline int __down_read_killable(struct rw_semaphore *sem)
1458{
1459 return rwbase_read_lock(&sem->rwbase, TASK_KILLABLE);
1460}
1461
1462static inline int __down_read_trylock(struct rw_semaphore *sem)
1463{
1464 return rwbase_read_trylock(&sem->rwbase);
1465}
1466
1467static inline void __up_read(struct rw_semaphore *sem)
1468{
1469 rwbase_read_unlock(&sem->rwbase, TASK_NORMAL);
1470}
1471
1472static inline void __sched __down_write(struct rw_semaphore *sem)
1473{
1474 rwbase_write_lock(&sem->rwbase, TASK_UNINTERRUPTIBLE);
1475}
1476
1477static inline int __sched __down_write_killable(struct rw_semaphore *sem)
1478{
1479 return rwbase_write_lock(&sem->rwbase, TASK_KILLABLE);
1480}
1481
1482static inline int __down_write_trylock(struct rw_semaphore *sem)
1483{
1484 return rwbase_write_trylock(&sem->rwbase);
1485}
1486
1487static inline void __up_write(struct rw_semaphore *sem)
1488{
1489 rwbase_write_unlock(&sem->rwbase);
1490}
1491
1492static inline void __downgrade_write(struct rw_semaphore *sem)
1493{
1494 rwbase_write_downgrade(&sem->rwbase);
1495}
1496
1497/* Debug stubs for the common API */
1498#define DEBUG_RWSEMS_WARN_ON(c, sem)
1499
1500static inline void __rwsem_set_reader_owned(struct rw_semaphore *sem,
1501 struct task_struct *owner)
1502{
1503}
1504
1505static inline bool is_rwsem_reader_owned(struct rw_semaphore *sem)
1506{
1507 int count = atomic_read(&sem->rwbase.readers);
1508
1509 return count < 0 && count != READER_BIAS;
1510}
1511
1512#endif /* CONFIG_PREEMPT_RT */
1513
1514/*
1515 * lock for reading
1516 */
1517void __sched down_read(struct rw_semaphore *sem)
1518{
1519 might_sleep();
1520 rwsem_acquire_read(&sem->dep_map, 0, 0, _RET_IP_);
1521
1522 LOCK_CONTENDED(sem, __down_read_trylock, __down_read);
1523}
1524EXPORT_SYMBOL(down_read);
1525
1526int __sched down_read_interruptible(struct rw_semaphore *sem)
1527{
1528 might_sleep();
1529 rwsem_acquire_read(&sem->dep_map, 0, 0, _RET_IP_);
1530
1531 if (LOCK_CONTENDED_RETURN(sem, __down_read_trylock, __down_read_interruptible)) {
1532 rwsem_release(&sem->dep_map, _RET_IP_);
1533 return -EINTR;
1534 }
1535
1536 return 0;
1537}
1538EXPORT_SYMBOL(down_read_interruptible);
1539
1540int __sched down_read_killable(struct rw_semaphore *sem)
1541{
1542 might_sleep();
1543 rwsem_acquire_read(&sem->dep_map, 0, 0, _RET_IP_);
1544
1545 if (LOCK_CONTENDED_RETURN(sem, __down_read_trylock, __down_read_killable)) {
1546 rwsem_release(&sem->dep_map, _RET_IP_);
1547 return -EINTR;
1548 }
1549
1550 return 0;
1551}
1552EXPORT_SYMBOL(down_read_killable);
1553
1554/*
1555 * trylock for reading -- returns 1 if successful, 0 if contention
1556 */
1557int down_read_trylock(struct rw_semaphore *sem)
1558{
1559 int ret = __down_read_trylock(sem);
1560
1561 if (ret == 1)
1562 rwsem_acquire_read(&sem->dep_map, 0, 1, _RET_IP_);
1563 return ret;
1564}
1565EXPORT_SYMBOL(down_read_trylock);
1566
1567/*
1568 * lock for writing
1569 */
1570void __sched down_write(struct rw_semaphore *sem)
1571{
1572 might_sleep();
1573 rwsem_acquire(&sem->dep_map, 0, 0, _RET_IP_);
1574 LOCK_CONTENDED(sem, __down_write_trylock, __down_write);
1575}
1576EXPORT_SYMBOL(down_write);
1577
1578/*
1579 * lock for writing
1580 */
1581int __sched down_write_killable(struct rw_semaphore *sem)
1582{
1583 might_sleep();
1584 rwsem_acquire(&sem->dep_map, 0, 0, _RET_IP_);
1585
1586 if (LOCK_CONTENDED_RETURN(sem, __down_write_trylock,
1587 __down_write_killable)) {
1588 rwsem_release(&sem->dep_map, _RET_IP_);
1589 return -EINTR;
1590 }
1591
1592 return 0;
1593}
1594EXPORT_SYMBOL(down_write_killable);
1595
1596/*
1597 * trylock for writing -- returns 1 if successful, 0 if contention
1598 */
1599int down_write_trylock(struct rw_semaphore *sem)
1600{
1601 int ret = __down_write_trylock(sem);
1602
1603 if (ret == 1)
1604 rwsem_acquire(&sem->dep_map, 0, 1, _RET_IP_);
1605
1606 return ret;
1607}
1608EXPORT_SYMBOL(down_write_trylock);
1609
1610/*
1611 * release a read lock
1612 */
1613void up_read(struct rw_semaphore *sem)
1614{
1615 rwsem_release(&sem->dep_map, _RET_IP_);
1616 __up_read(sem);
1617}
1618EXPORT_SYMBOL(up_read);
1619
1620/*
1621 * release a write lock
1622 */
1623void up_write(struct rw_semaphore *sem)
1624{
1625 rwsem_release(&sem->dep_map, _RET_IP_);
1626 __up_write(sem);
1627}
1628EXPORT_SYMBOL(up_write);
1629
1630/*
1631 * downgrade write lock to read lock
1632 */
1633void downgrade_write(struct rw_semaphore *sem)
1634{
1635 lock_downgrade(&sem->dep_map, _RET_IP_);
1636 __downgrade_write(sem);
1637}
1638EXPORT_SYMBOL(downgrade_write);
1639
1640#ifdef CONFIG_DEBUG_LOCK_ALLOC
1641
1642void down_read_nested(struct rw_semaphore *sem, int subclass)
1643{
1644 might_sleep();
1645 rwsem_acquire_read(&sem->dep_map, subclass, 0, _RET_IP_);
1646 LOCK_CONTENDED(sem, __down_read_trylock, __down_read);
1647}
1648EXPORT_SYMBOL(down_read_nested);
1649
1650int down_read_killable_nested(struct rw_semaphore *sem, int subclass)
1651{
1652 might_sleep();
1653 rwsem_acquire_read(&sem->dep_map, subclass, 0, _RET_IP_);
1654
1655 if (LOCK_CONTENDED_RETURN(sem, __down_read_trylock, __down_read_killable)) {
1656 rwsem_release(&sem->dep_map, _RET_IP_);
1657 return -EINTR;
1658 }
1659
1660 return 0;
1661}
1662EXPORT_SYMBOL(down_read_killable_nested);
1663
1664void _down_write_nest_lock(struct rw_semaphore *sem, struct lockdep_map *nest)
1665{
1666 might_sleep();
1667 rwsem_acquire_nest(&sem->dep_map, 0, 0, nest, _RET_IP_);
1668 LOCK_CONTENDED(sem, __down_write_trylock, __down_write);
1669}
1670EXPORT_SYMBOL(_down_write_nest_lock);
1671
1672void down_read_non_owner(struct rw_semaphore *sem)
1673{
1674 might_sleep();
1675 __down_read(sem);
1676 /*
1677 * The owner value for a reader-owned lock is mostly for debugging
1678 * purpose only and is not critical to the correct functioning of
1679 * rwsem. So it is perfectly fine to set it in a preempt-enabled
1680 * context here.
1681 */
1682 __rwsem_set_reader_owned(sem, NULL);
1683}
1684EXPORT_SYMBOL(down_read_non_owner);
1685
1686void down_write_nested(struct rw_semaphore *sem, int subclass)
1687{
1688 might_sleep();
1689 rwsem_acquire(&sem->dep_map, subclass, 0, _RET_IP_);
1690 LOCK_CONTENDED(sem, __down_write_trylock, __down_write);
1691}
1692EXPORT_SYMBOL(down_write_nested);
1693
1694int __sched down_write_killable_nested(struct rw_semaphore *sem, int subclass)
1695{
1696 might_sleep();
1697 rwsem_acquire(&sem->dep_map, subclass, 0, _RET_IP_);
1698
1699 if (LOCK_CONTENDED_RETURN(sem, __down_write_trylock,
1700 __down_write_killable)) {
1701 rwsem_release(&sem->dep_map, _RET_IP_);
1702 return -EINTR;
1703 }
1704
1705 return 0;
1706}
1707EXPORT_SYMBOL(down_write_killable_nested);
1708
1709void up_read_non_owner(struct rw_semaphore *sem)
1710{
1711 DEBUG_RWSEMS_WARN_ON(!is_rwsem_reader_owned(sem), sem);
1712 __up_read(sem);
1713}
1714EXPORT_SYMBOL(up_read_non_owner);
1715
1716#endif