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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 - rwsem may be owned by readers (just a hint)
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#ifdef CONFIG_DEBUG_RWSEMS
185/*
186 * Return just the real task structure pointer of the owner
187 */
188static inline struct task_struct *rwsem_owner(struct rw_semaphore *sem)
189{
190 return (struct task_struct *)
191 (atomic_long_read(&sem->owner) & ~RWSEM_OWNER_FLAGS_MASK);
192}
193
194/*
195 * Return true if the rwsem is owned by a reader.
196 */
197static inline bool is_rwsem_reader_owned(struct rw_semaphore *sem)
198{
199 /*
200 * Check the count to see if it is write-locked.
201 */
202 long count = atomic_long_read(&sem->count);
203
204 if (count & RWSEM_WRITER_MASK)
205 return false;
206 return rwsem_test_oflags(sem, RWSEM_READER_OWNED);
207}
208
209/*
210 * With CONFIG_DEBUG_RWSEMS configured, it will make sure that if there
211 * is a task pointer in owner of a reader-owned rwsem, it will be the
212 * real owner or one of the real owners. The only exception is when the
213 * unlock is done by up_read_non_owner().
214 */
215static inline void rwsem_clear_reader_owned(struct rw_semaphore *sem)
216{
217 unsigned long val = atomic_long_read(&sem->owner);
218
219 while ((val & ~RWSEM_OWNER_FLAGS_MASK) == (unsigned long)current) {
220 if (atomic_long_try_cmpxchg(&sem->owner, &val,
221 val & RWSEM_OWNER_FLAGS_MASK))
222 return;
223 }
224}
225#else
226static inline void rwsem_clear_reader_owned(struct rw_semaphore *sem)
227{
228}
229#endif
230
231/*
232 * Set the RWSEM_NONSPINNABLE bits if the RWSEM_READER_OWNED flag
233 * remains set. Otherwise, the operation will be aborted.
234 */
235static inline void rwsem_set_nonspinnable(struct rw_semaphore *sem)
236{
237 unsigned long owner = atomic_long_read(&sem->owner);
238
239 do {
240 if (!(owner & RWSEM_READER_OWNED))
241 break;
242 if (owner & RWSEM_NONSPINNABLE)
243 break;
244 } while (!atomic_long_try_cmpxchg(&sem->owner, &owner,
245 owner | RWSEM_NONSPINNABLE));
246}
247
248static inline bool rwsem_read_trylock(struct rw_semaphore *sem, long *cntp)
249{
250 *cntp = atomic_long_add_return_acquire(RWSEM_READER_BIAS, &sem->count);
251
252 if (WARN_ON_ONCE(*cntp < 0))
253 rwsem_set_nonspinnable(sem);
254
255 if (!(*cntp & RWSEM_READ_FAILED_MASK)) {
256 rwsem_set_reader_owned(sem);
257 return true;
258 }
259
260 return false;
261}
262
263static inline bool rwsem_write_trylock(struct rw_semaphore *sem)
264{
265 long tmp = RWSEM_UNLOCKED_VALUE;
266
267 if (atomic_long_try_cmpxchg_acquire(&sem->count, &tmp, RWSEM_WRITER_LOCKED)) {
268 rwsem_set_owner(sem);
269 return true;
270 }
271
272 return false;
273}
274
275/*
276 * Return the real task structure pointer of the owner and the embedded
277 * flags in the owner. pflags must be non-NULL.
278 */
279static inline struct task_struct *
280rwsem_owner_flags(struct rw_semaphore *sem, unsigned long *pflags)
281{
282 unsigned long owner = atomic_long_read(&sem->owner);
283
284 *pflags = owner & RWSEM_OWNER_FLAGS_MASK;
285 return (struct task_struct *)(owner & ~RWSEM_OWNER_FLAGS_MASK);
286}
287
288/*
289 * Guide to the rw_semaphore's count field.
290 *
291 * When the RWSEM_WRITER_LOCKED bit in count is set, the lock is owned
292 * by a writer.
293 *
294 * The lock is owned by readers when
295 * (1) the RWSEM_WRITER_LOCKED isn't set in count,
296 * (2) some of the reader bits are set in count, and
297 * (3) the owner field has RWSEM_READ_OWNED bit set.
298 *
299 * Having some reader bits set is not enough to guarantee a readers owned
300 * lock as the readers may be in the process of backing out from the count
301 * and a writer has just released the lock. So another writer may steal
302 * the lock immediately after that.
303 */
304
305/*
306 * Initialize an rwsem:
307 */
308void __init_rwsem(struct rw_semaphore *sem, const char *name,
309 struct lock_class_key *key)
310{
311#ifdef CONFIG_DEBUG_LOCK_ALLOC
312 /*
313 * Make sure we are not reinitializing a held semaphore:
314 */
315 debug_check_no_locks_freed((void *)sem, sizeof(*sem));
316 lockdep_init_map_wait(&sem->dep_map, name, key, 0, LD_WAIT_SLEEP);
317#endif
318#ifdef CONFIG_DEBUG_RWSEMS
319 sem->magic = sem;
320#endif
321 atomic_long_set(&sem->count, RWSEM_UNLOCKED_VALUE);
322 raw_spin_lock_init(&sem->wait_lock);
323 INIT_LIST_HEAD(&sem->wait_list);
324 atomic_long_set(&sem->owner, 0L);
325#ifdef CONFIG_RWSEM_SPIN_ON_OWNER
326 osq_lock_init(&sem->osq);
327#endif
328}
329EXPORT_SYMBOL(__init_rwsem);
330
331enum rwsem_waiter_type {
332 RWSEM_WAITING_FOR_WRITE,
333 RWSEM_WAITING_FOR_READ
334};
335
336struct rwsem_waiter {
337 struct list_head list;
338 struct task_struct *task;
339 enum rwsem_waiter_type type;
340 unsigned long timeout;
341 bool handoff_set;
342};
343#define rwsem_first_waiter(sem) \
344 list_first_entry(&sem->wait_list, struct rwsem_waiter, list)
345
346enum rwsem_wake_type {
347 RWSEM_WAKE_ANY, /* Wake whatever's at head of wait list */
348 RWSEM_WAKE_READERS, /* Wake readers only */
349 RWSEM_WAKE_READ_OWNED /* Waker thread holds the read lock */
350};
351
352/*
353 * The typical HZ value is either 250 or 1000. So set the minimum waiting
354 * time to at least 4ms or 1 jiffy (if it is higher than 4ms) in the wait
355 * queue before initiating the handoff protocol.
356 */
357#define RWSEM_WAIT_TIMEOUT DIV_ROUND_UP(HZ, 250)
358
359/*
360 * Magic number to batch-wakeup waiting readers, even when writers are
361 * also present in the queue. This both limits the amount of work the
362 * waking thread must do and also prevents any potential counter overflow,
363 * however unlikely.
364 */
365#define MAX_READERS_WAKEUP 0x100
366
367static inline void
368rwsem_add_waiter(struct rw_semaphore *sem, struct rwsem_waiter *waiter)
369{
370 lockdep_assert_held(&sem->wait_lock);
371 list_add_tail(&waiter->list, &sem->wait_list);
372 /* caller will set RWSEM_FLAG_WAITERS */
373}
374
375/*
376 * Remove a waiter from the wait_list and clear flags.
377 *
378 * Both rwsem_mark_wake() and rwsem_try_write_lock() contain a full 'copy' of
379 * this function. Modify with care.
380 *
381 * Return: true if wait_list isn't empty and false otherwise
382 */
383static inline bool
384rwsem_del_waiter(struct rw_semaphore *sem, struct rwsem_waiter *waiter)
385{
386 lockdep_assert_held(&sem->wait_lock);
387 list_del(&waiter->list);
388 if (likely(!list_empty(&sem->wait_list)))
389 return true;
390
391 atomic_long_andnot(RWSEM_FLAG_HANDOFF | RWSEM_FLAG_WAITERS, &sem->count);
392 return false;
393}
394
395/*
396 * handle the lock release when processes blocked on it that can now run
397 * - if we come here from up_xxxx(), then the RWSEM_FLAG_WAITERS bit must
398 * have been set.
399 * - there must be someone on the queue
400 * - the wait_lock must be held by the caller
401 * - tasks are marked for wakeup, the caller must later invoke wake_up_q()
402 * to actually wakeup the blocked task(s) and drop the reference count,
403 * preferably when the wait_lock is released
404 * - woken process blocks are discarded from the list after having task zeroed
405 * - writers are only marked woken if downgrading is false
406 *
407 * Implies rwsem_del_waiter() for all woken readers.
408 */
409static void rwsem_mark_wake(struct rw_semaphore *sem,
410 enum rwsem_wake_type wake_type,
411 struct wake_q_head *wake_q)
412{
413 struct rwsem_waiter *waiter, *tmp;
414 long oldcount, woken = 0, adjustment = 0;
415 struct list_head wlist;
416
417 lockdep_assert_held(&sem->wait_lock);
418
419 /*
420 * Take a peek at the queue head waiter such that we can determine
421 * the wakeup(s) to perform.
422 */
423 waiter = rwsem_first_waiter(sem);
424
425 if (waiter->type == RWSEM_WAITING_FOR_WRITE) {
426 if (wake_type == RWSEM_WAKE_ANY) {
427 /*
428 * Mark writer at the front of the queue for wakeup.
429 * Until the task is actually later awoken later by
430 * the caller, other writers are able to steal it.
431 * Readers, on the other hand, will block as they
432 * will notice the queued writer.
433 */
434 wake_q_add(wake_q, waiter->task);
435 lockevent_inc(rwsem_wake_writer);
436 }
437
438 return;
439 }
440
441 /*
442 * No reader wakeup if there are too many of them already.
443 */
444 if (unlikely(atomic_long_read(&sem->count) < 0))
445 return;
446
447 /*
448 * Writers might steal the lock before we grant it to the next reader.
449 * We prefer to do the first reader grant before counting readers
450 * so we can bail out early if a writer stole the lock.
451 */
452 if (wake_type != RWSEM_WAKE_READ_OWNED) {
453 struct task_struct *owner;
454
455 adjustment = RWSEM_READER_BIAS;
456 oldcount = atomic_long_fetch_add(adjustment, &sem->count);
457 if (unlikely(oldcount & RWSEM_WRITER_MASK)) {
458 /*
459 * When we've been waiting "too" long (for writers
460 * to give up the lock), request a HANDOFF to
461 * force the issue.
462 */
463 if (time_after(jiffies, waiter->timeout)) {
464 if (!(oldcount & RWSEM_FLAG_HANDOFF)) {
465 adjustment -= RWSEM_FLAG_HANDOFF;
466 lockevent_inc(rwsem_rlock_handoff);
467 }
468 waiter->handoff_set = true;
469 }
470
471 atomic_long_add(-adjustment, &sem->count);
472 return;
473 }
474 /*
475 * Set it to reader-owned to give spinners an early
476 * indication that readers now have the lock.
477 * The reader nonspinnable bit seen at slowpath entry of
478 * the reader is copied over.
479 */
480 owner = waiter->task;
481 __rwsem_set_reader_owned(sem, owner);
482 }
483
484 /*
485 * Grant up to MAX_READERS_WAKEUP read locks to all the readers in the
486 * queue. We know that the woken will be at least 1 as we accounted
487 * for above. Note we increment the 'active part' of the count by the
488 * number of readers before waking any processes up.
489 *
490 * This is an adaptation of the phase-fair R/W locks where at the
491 * reader phase (first waiter is a reader), all readers are eligible
492 * to acquire the lock at the same time irrespective of their order
493 * in the queue. The writers acquire the lock according to their
494 * order in the queue.
495 *
496 * We have to do wakeup in 2 passes to prevent the possibility that
497 * the reader count may be decremented before it is incremented. It
498 * is because the to-be-woken waiter may not have slept yet. So it
499 * may see waiter->task got cleared, finish its critical section and
500 * do an unlock before the reader count increment.
501 *
502 * 1) Collect the read-waiters in a separate list, count them and
503 * fully increment the reader count in rwsem.
504 * 2) For each waiters in the new list, clear waiter->task and
505 * put them into wake_q to be woken up later.
506 */
507 INIT_LIST_HEAD(&wlist);
508 list_for_each_entry_safe(waiter, tmp, &sem->wait_list, list) {
509 if (waiter->type == RWSEM_WAITING_FOR_WRITE)
510 continue;
511
512 woken++;
513 list_move_tail(&waiter->list, &wlist);
514
515 /*
516 * Limit # of readers that can be woken up per wakeup call.
517 */
518 if (unlikely(woken >= MAX_READERS_WAKEUP))
519 break;
520 }
521
522 adjustment = woken * RWSEM_READER_BIAS - adjustment;
523 lockevent_cond_inc(rwsem_wake_reader, woken);
524
525 oldcount = atomic_long_read(&sem->count);
526 if (list_empty(&sem->wait_list)) {
527 /*
528 * Combined with list_move_tail() above, this implies
529 * rwsem_del_waiter().
530 */
531 adjustment -= RWSEM_FLAG_WAITERS;
532 if (oldcount & RWSEM_FLAG_HANDOFF)
533 adjustment -= RWSEM_FLAG_HANDOFF;
534 } else if (woken) {
535 /*
536 * When we've woken a reader, we no longer need to force
537 * writers to give up the lock and we can clear HANDOFF.
538 */
539 if (oldcount & RWSEM_FLAG_HANDOFF)
540 adjustment -= RWSEM_FLAG_HANDOFF;
541 }
542
543 if (adjustment)
544 atomic_long_add(adjustment, &sem->count);
545
546 /* 2nd pass */
547 list_for_each_entry_safe(waiter, tmp, &wlist, list) {
548 struct task_struct *tsk;
549
550 tsk = waiter->task;
551 get_task_struct(tsk);
552
553 /*
554 * Ensure calling get_task_struct() before setting the reader
555 * waiter to nil such that rwsem_down_read_slowpath() cannot
556 * race with do_exit() by always holding a reference count
557 * to the task to wakeup.
558 */
559 smp_store_release(&waiter->task, NULL);
560 /*
561 * Ensure issuing the wakeup (either by us or someone else)
562 * after setting the reader waiter to nil.
563 */
564 wake_q_add_safe(wake_q, tsk);
565 }
566}
567
568/*
569 * Remove a waiter and try to wake up other waiters in the wait queue
570 * This function is called from the out_nolock path of both the reader and
571 * writer slowpaths with wait_lock held. It releases the wait_lock and
572 * optionally wake up waiters before it returns.
573 */
574static inline void
575rwsem_del_wake_waiter(struct rw_semaphore *sem, struct rwsem_waiter *waiter,
576 struct wake_q_head *wake_q)
577 __releases(&sem->wait_lock)
578{
579 bool first = rwsem_first_waiter(sem) == waiter;
580
581 wake_q_init(wake_q);
582
583 /*
584 * If the wait_list isn't empty and the waiter to be deleted is
585 * the first waiter, we wake up the remaining waiters as they may
586 * be eligible to acquire or spin on the lock.
587 */
588 if (rwsem_del_waiter(sem, waiter) && first)
589 rwsem_mark_wake(sem, RWSEM_WAKE_ANY, wake_q);
590 raw_spin_unlock_irq(&sem->wait_lock);
591 if (!wake_q_empty(wake_q))
592 wake_up_q(wake_q);
593}
594
595/*
596 * This function must be called with the sem->wait_lock held to prevent
597 * race conditions between checking the rwsem wait list and setting the
598 * sem->count accordingly.
599 *
600 * Implies rwsem_del_waiter() on success.
601 */
602static inline bool rwsem_try_write_lock(struct rw_semaphore *sem,
603 struct rwsem_waiter *waiter)
604{
605 struct rwsem_waiter *first = rwsem_first_waiter(sem);
606 long count, new;
607
608 lockdep_assert_held(&sem->wait_lock);
609
610 count = atomic_long_read(&sem->count);
611 do {
612 bool has_handoff = !!(count & RWSEM_FLAG_HANDOFF);
613
614 if (has_handoff) {
615 /*
616 * Honor handoff bit and yield only when the first
617 * waiter is the one that set it. Otherwisee, we
618 * still try to acquire the rwsem.
619 */
620 if (first->handoff_set && (waiter != first))
621 return false;
622 }
623
624 new = count;
625
626 if (count & RWSEM_LOCK_MASK) {
627 /*
628 * A waiter (first or not) can set the handoff bit
629 * if it is an RT task or wait in the wait queue
630 * for too long.
631 */
632 if (has_handoff || (!rt_or_dl_task(waiter->task) &&
633 !time_after(jiffies, waiter->timeout)))
634 return false;
635
636 new |= RWSEM_FLAG_HANDOFF;
637 } else {
638 new |= RWSEM_WRITER_LOCKED;
639 new &= ~RWSEM_FLAG_HANDOFF;
640
641 if (list_is_singular(&sem->wait_list))
642 new &= ~RWSEM_FLAG_WAITERS;
643 }
644 } while (!atomic_long_try_cmpxchg_acquire(&sem->count, &count, new));
645
646 /*
647 * We have either acquired the lock with handoff bit cleared or set
648 * the handoff bit. Only the first waiter can have its handoff_set
649 * set here to enable optimistic spinning in slowpath loop.
650 */
651 if (new & RWSEM_FLAG_HANDOFF) {
652 first->handoff_set = true;
653 lockevent_inc(rwsem_wlock_handoff);
654 return false;
655 }
656
657 /*
658 * Have rwsem_try_write_lock() fully imply rwsem_del_waiter() on
659 * success.
660 */
661 list_del(&waiter->list);
662 rwsem_set_owner(sem);
663 return true;
664}
665
666/*
667 * The rwsem_spin_on_owner() function returns the following 4 values
668 * depending on the lock owner state.
669 * OWNER_NULL : owner is currently NULL
670 * OWNER_WRITER: when owner changes and is a writer
671 * OWNER_READER: when owner changes and the new owner may be a reader.
672 * OWNER_NONSPINNABLE:
673 * when optimistic spinning has to stop because either the
674 * owner stops running, is unknown, or its timeslice has
675 * been used up.
676 */
677enum owner_state {
678 OWNER_NULL = 1 << 0,
679 OWNER_WRITER = 1 << 1,
680 OWNER_READER = 1 << 2,
681 OWNER_NONSPINNABLE = 1 << 3,
682};
683
684#ifdef CONFIG_RWSEM_SPIN_ON_OWNER
685/*
686 * Try to acquire write lock before the writer has been put on wait queue.
687 */
688static inline bool rwsem_try_write_lock_unqueued(struct rw_semaphore *sem)
689{
690 long count = atomic_long_read(&sem->count);
691
692 while (!(count & (RWSEM_LOCK_MASK|RWSEM_FLAG_HANDOFF))) {
693 if (atomic_long_try_cmpxchg_acquire(&sem->count, &count,
694 count | RWSEM_WRITER_LOCKED)) {
695 rwsem_set_owner(sem);
696 lockevent_inc(rwsem_opt_lock);
697 return true;
698 }
699 }
700 return false;
701}
702
703static inline bool rwsem_can_spin_on_owner(struct rw_semaphore *sem)
704{
705 struct task_struct *owner;
706 unsigned long flags;
707 bool ret = true;
708
709 if (need_resched()) {
710 lockevent_inc(rwsem_opt_fail);
711 return false;
712 }
713
714 /*
715 * Disable preemption is equal to the RCU read-side crital section,
716 * thus the task_strcut structure won't go away.
717 */
718 owner = rwsem_owner_flags(sem, &flags);
719 /*
720 * Don't check the read-owner as the entry may be stale.
721 */
722 if ((flags & RWSEM_NONSPINNABLE) ||
723 (owner && !(flags & RWSEM_READER_OWNED) && !owner_on_cpu(owner)))
724 ret = false;
725
726 lockevent_cond_inc(rwsem_opt_fail, !ret);
727 return ret;
728}
729
730#define OWNER_SPINNABLE (OWNER_NULL | OWNER_WRITER | OWNER_READER)
731
732static inline enum owner_state
733rwsem_owner_state(struct task_struct *owner, unsigned long flags)
734{
735 if (flags & RWSEM_NONSPINNABLE)
736 return OWNER_NONSPINNABLE;
737
738 if (flags & RWSEM_READER_OWNED)
739 return OWNER_READER;
740
741 return owner ? OWNER_WRITER : OWNER_NULL;
742}
743
744static noinline enum owner_state
745rwsem_spin_on_owner(struct rw_semaphore *sem)
746{
747 struct task_struct *new, *owner;
748 unsigned long flags, new_flags;
749 enum owner_state state;
750
751 lockdep_assert_preemption_disabled();
752
753 owner = rwsem_owner_flags(sem, &flags);
754 state = rwsem_owner_state(owner, flags);
755 if (state != OWNER_WRITER)
756 return state;
757
758 for (;;) {
759 /*
760 * When a waiting writer set the handoff flag, it may spin
761 * on the owner as well. Once that writer acquires the lock,
762 * we can spin on it. So we don't need to quit even when the
763 * handoff bit is set.
764 */
765 new = rwsem_owner_flags(sem, &new_flags);
766 if ((new != owner) || (new_flags != flags)) {
767 state = rwsem_owner_state(new, new_flags);
768 break;
769 }
770
771 /*
772 * Ensure we emit the owner->on_cpu, dereference _after_
773 * checking sem->owner still matches owner, if that fails,
774 * owner might point to free()d memory, if it still matches,
775 * our spinning context already disabled preemption which is
776 * equal to RCU read-side crital section ensures the memory
777 * stays valid.
778 */
779 barrier();
780
781 if (need_resched() || !owner_on_cpu(owner)) {
782 state = OWNER_NONSPINNABLE;
783 break;
784 }
785
786 cpu_relax();
787 }
788
789 return state;
790}
791
792/*
793 * Calculate reader-owned rwsem spinning threshold for writer
794 *
795 * The more readers own the rwsem, the longer it will take for them to
796 * wind down and free the rwsem. So the empirical formula used to
797 * determine the actual spinning time limit here is:
798 *
799 * Spinning threshold = (10 + nr_readers/2)us
800 *
801 * The limit is capped to a maximum of 25us (30 readers). This is just
802 * a heuristic and is subjected to change in the future.
803 */
804static inline u64 rwsem_rspin_threshold(struct rw_semaphore *sem)
805{
806 long count = atomic_long_read(&sem->count);
807 int readers = count >> RWSEM_READER_SHIFT;
808 u64 delta;
809
810 if (readers > 30)
811 readers = 30;
812 delta = (20 + readers) * NSEC_PER_USEC / 2;
813
814 return sched_clock() + delta;
815}
816
817static bool rwsem_optimistic_spin(struct rw_semaphore *sem)
818{
819 bool taken = false;
820 int prev_owner_state = OWNER_NULL;
821 int loop = 0;
822 u64 rspin_threshold = 0;
823
824 /* sem->wait_lock should not be held when doing optimistic spinning */
825 if (!osq_lock(&sem->osq))
826 goto done;
827
828 /*
829 * Optimistically spin on the owner field and attempt to acquire the
830 * lock whenever the owner changes. Spinning will be stopped when:
831 * 1) the owning writer isn't running; or
832 * 2) readers own the lock and spinning time has exceeded limit.
833 */
834 for (;;) {
835 enum owner_state owner_state;
836
837 owner_state = rwsem_spin_on_owner(sem);
838 if (!(owner_state & OWNER_SPINNABLE))
839 break;
840
841 /*
842 * Try to acquire the lock
843 */
844 taken = rwsem_try_write_lock_unqueued(sem);
845
846 if (taken)
847 break;
848
849 /*
850 * Time-based reader-owned rwsem optimistic spinning
851 */
852 if (owner_state == OWNER_READER) {
853 /*
854 * Re-initialize rspin_threshold every time when
855 * the owner state changes from non-reader to reader.
856 * This allows a writer to steal the lock in between
857 * 2 reader phases and have the threshold reset at
858 * the beginning of the 2nd reader phase.
859 */
860 if (prev_owner_state != OWNER_READER) {
861 if (rwsem_test_oflags(sem, RWSEM_NONSPINNABLE))
862 break;
863 rspin_threshold = rwsem_rspin_threshold(sem);
864 loop = 0;
865 }
866
867 /*
868 * Check time threshold once every 16 iterations to
869 * avoid calling sched_clock() too frequently so
870 * as to reduce the average latency between the times
871 * when the lock becomes free and when the spinner
872 * is ready to do a trylock.
873 */
874 else if (!(++loop & 0xf) && (sched_clock() > rspin_threshold)) {
875 rwsem_set_nonspinnable(sem);
876 lockevent_inc(rwsem_opt_nospin);
877 break;
878 }
879 }
880
881 /*
882 * An RT task cannot do optimistic spinning if it cannot
883 * be sure the lock holder is running or live-lock may
884 * happen if the current task and the lock holder happen
885 * to run in the same CPU. However, aborting optimistic
886 * spinning while a NULL owner is detected may miss some
887 * opportunity where spinning can continue without causing
888 * problem.
889 *
890 * There are 2 possible cases where an RT task may be able
891 * to continue spinning.
892 *
893 * 1) The lock owner is in the process of releasing the
894 * lock, sem->owner is cleared but the lock has not
895 * been released yet.
896 * 2) The lock was free and owner cleared, but another
897 * task just comes in and acquire the lock before
898 * we try to get it. The new owner may be a spinnable
899 * writer.
900 *
901 * To take advantage of two scenarios listed above, the RT
902 * task is made to retry one more time to see if it can
903 * acquire the lock or continue spinning on the new owning
904 * writer. Of course, if the time lag is long enough or the
905 * new owner is not a writer or spinnable, the RT task will
906 * quit spinning.
907 *
908 * If the owner is a writer, the need_resched() check is
909 * done inside rwsem_spin_on_owner(). If the owner is not
910 * a writer, need_resched() check needs to be done here.
911 */
912 if (owner_state != OWNER_WRITER) {
913 if (need_resched())
914 break;
915 if (rt_or_dl_task(current) &&
916 (prev_owner_state != OWNER_WRITER))
917 break;
918 }
919 prev_owner_state = owner_state;
920
921 /*
922 * The cpu_relax() call is a compiler barrier which forces
923 * everything in this loop to be re-loaded. We don't need
924 * memory barriers as we'll eventually observe the right
925 * values at the cost of a few extra spins.
926 */
927 cpu_relax();
928 }
929 osq_unlock(&sem->osq);
930done:
931 lockevent_cond_inc(rwsem_opt_fail, !taken);
932 return taken;
933}
934
935/*
936 * Clear the owner's RWSEM_NONSPINNABLE bit if it is set. This should
937 * only be called when the reader count reaches 0.
938 */
939static inline void clear_nonspinnable(struct rw_semaphore *sem)
940{
941 if (unlikely(rwsem_test_oflags(sem, RWSEM_NONSPINNABLE)))
942 atomic_long_andnot(RWSEM_NONSPINNABLE, &sem->owner);
943}
944
945#else
946static inline bool rwsem_can_spin_on_owner(struct rw_semaphore *sem)
947{
948 return false;
949}
950
951static inline bool rwsem_optimistic_spin(struct rw_semaphore *sem)
952{
953 return false;
954}
955
956static inline void clear_nonspinnable(struct rw_semaphore *sem) { }
957
958static inline enum owner_state
959rwsem_spin_on_owner(struct rw_semaphore *sem)
960{
961 return OWNER_NONSPINNABLE;
962}
963#endif
964
965/*
966 * Prepare to wake up waiter(s) in the wait queue by putting them into the
967 * given wake_q if the rwsem lock owner isn't a writer. If rwsem is likely
968 * reader-owned, wake up read lock waiters in queue front or wake up any
969 * front waiter otherwise.
970
971 * This is being called from both reader and writer slow paths.
972 */
973static inline void rwsem_cond_wake_waiter(struct rw_semaphore *sem, long count,
974 struct wake_q_head *wake_q)
975{
976 enum rwsem_wake_type wake_type;
977
978 if (count & RWSEM_WRITER_MASK)
979 return;
980
981 if (count & RWSEM_READER_MASK) {
982 wake_type = RWSEM_WAKE_READERS;
983 } else {
984 wake_type = RWSEM_WAKE_ANY;
985 clear_nonspinnable(sem);
986 }
987 rwsem_mark_wake(sem, wake_type, wake_q);
988}
989
990/*
991 * Wait for the read lock to be granted
992 */
993static struct rw_semaphore __sched *
994rwsem_down_read_slowpath(struct rw_semaphore *sem, long count, unsigned int state)
995{
996 long adjustment = -RWSEM_READER_BIAS;
997 long rcnt = (count >> RWSEM_READER_SHIFT);
998 struct rwsem_waiter waiter;
999 DEFINE_WAKE_Q(wake_q);
1000
1001 /*
1002 * To prevent a constant stream of readers from starving a sleeping
1003 * writer, don't attempt optimistic lock stealing if the lock is
1004 * very likely owned by readers.
1005 */
1006 if ((atomic_long_read(&sem->owner) & RWSEM_READER_OWNED) &&
1007 (rcnt > 1) && !(count & RWSEM_WRITER_LOCKED))
1008 goto queue;
1009
1010 /*
1011 * Reader optimistic lock stealing.
1012 */
1013 if (!(count & (RWSEM_WRITER_LOCKED | RWSEM_FLAG_HANDOFF))) {
1014 rwsem_set_reader_owned(sem);
1015 lockevent_inc(rwsem_rlock_steal);
1016
1017 /*
1018 * Wake up other readers in the wait queue if it is
1019 * the first reader.
1020 */
1021 if ((rcnt == 1) && (count & RWSEM_FLAG_WAITERS)) {
1022 raw_spin_lock_irq(&sem->wait_lock);
1023 if (!list_empty(&sem->wait_list))
1024 rwsem_mark_wake(sem, RWSEM_WAKE_READ_OWNED,
1025 &wake_q);
1026 raw_spin_unlock_irq(&sem->wait_lock);
1027 wake_up_q(&wake_q);
1028 }
1029 return sem;
1030 }
1031
1032queue:
1033 waiter.task = current;
1034 waiter.type = RWSEM_WAITING_FOR_READ;
1035 waiter.timeout = jiffies + RWSEM_WAIT_TIMEOUT;
1036 waiter.handoff_set = false;
1037
1038 raw_spin_lock_irq(&sem->wait_lock);
1039 if (list_empty(&sem->wait_list)) {
1040 /*
1041 * In case the wait queue is empty and the lock isn't owned
1042 * by a writer, this reader can exit the slowpath and return
1043 * immediately as its RWSEM_READER_BIAS has already been set
1044 * in the count.
1045 */
1046 if (!(atomic_long_read(&sem->count) & RWSEM_WRITER_MASK)) {
1047 /* Provide lock ACQUIRE */
1048 smp_acquire__after_ctrl_dep();
1049 raw_spin_unlock_irq(&sem->wait_lock);
1050 rwsem_set_reader_owned(sem);
1051 lockevent_inc(rwsem_rlock_fast);
1052 return sem;
1053 }
1054 adjustment += RWSEM_FLAG_WAITERS;
1055 }
1056 rwsem_add_waiter(sem, &waiter);
1057
1058 /* we're now waiting on the lock, but no longer actively locking */
1059 count = atomic_long_add_return(adjustment, &sem->count);
1060
1061 rwsem_cond_wake_waiter(sem, count, &wake_q);
1062 raw_spin_unlock_irq(&sem->wait_lock);
1063
1064 if (!wake_q_empty(&wake_q))
1065 wake_up_q(&wake_q);
1066
1067 trace_contention_begin(sem, LCB_F_READ);
1068
1069 /* wait to be given the lock */
1070 for (;;) {
1071 set_current_state(state);
1072 if (!smp_load_acquire(&waiter.task)) {
1073 /* Matches rwsem_mark_wake()'s smp_store_release(). */
1074 break;
1075 }
1076 if (signal_pending_state(state, current)) {
1077 raw_spin_lock_irq(&sem->wait_lock);
1078 if (waiter.task)
1079 goto out_nolock;
1080 raw_spin_unlock_irq(&sem->wait_lock);
1081 /* Ordered by sem->wait_lock against rwsem_mark_wake(). */
1082 break;
1083 }
1084 schedule_preempt_disabled();
1085 lockevent_inc(rwsem_sleep_reader);
1086 }
1087
1088 __set_current_state(TASK_RUNNING);
1089 lockevent_inc(rwsem_rlock);
1090 trace_contention_end(sem, 0);
1091 return sem;
1092
1093out_nolock:
1094 rwsem_del_wake_waiter(sem, &waiter, &wake_q);
1095 __set_current_state(TASK_RUNNING);
1096 lockevent_inc(rwsem_rlock_fail);
1097 trace_contention_end(sem, -EINTR);
1098 return ERR_PTR(-EINTR);
1099}
1100
1101/*
1102 * Wait until we successfully acquire the write lock
1103 */
1104static struct rw_semaphore __sched *
1105rwsem_down_write_slowpath(struct rw_semaphore *sem, int state)
1106{
1107 struct rwsem_waiter waiter;
1108 DEFINE_WAKE_Q(wake_q);
1109
1110 /* do optimistic spinning and steal lock if possible */
1111 if (rwsem_can_spin_on_owner(sem) && rwsem_optimistic_spin(sem)) {
1112 /* rwsem_optimistic_spin() implies ACQUIRE on success */
1113 return sem;
1114 }
1115
1116 /*
1117 * Optimistic spinning failed, proceed to the slowpath
1118 * and block until we can acquire the sem.
1119 */
1120 waiter.task = current;
1121 waiter.type = RWSEM_WAITING_FOR_WRITE;
1122 waiter.timeout = jiffies + RWSEM_WAIT_TIMEOUT;
1123 waiter.handoff_set = false;
1124
1125 raw_spin_lock_irq(&sem->wait_lock);
1126 rwsem_add_waiter(sem, &waiter);
1127
1128 /* we're now waiting on the lock */
1129 if (rwsem_first_waiter(sem) != &waiter) {
1130 rwsem_cond_wake_waiter(sem, atomic_long_read(&sem->count),
1131 &wake_q);
1132 if (!wake_q_empty(&wake_q)) {
1133 /*
1134 * We want to minimize wait_lock hold time especially
1135 * when a large number of readers are to be woken up.
1136 */
1137 raw_spin_unlock_irq(&sem->wait_lock);
1138 wake_up_q(&wake_q);
1139 raw_spin_lock_irq(&sem->wait_lock);
1140 }
1141 } else {
1142 atomic_long_or(RWSEM_FLAG_WAITERS, &sem->count);
1143 }
1144
1145 /* wait until we successfully acquire the lock */
1146 set_current_state(state);
1147 trace_contention_begin(sem, LCB_F_WRITE);
1148
1149 for (;;) {
1150 if (rwsem_try_write_lock(sem, &waiter)) {
1151 /* rwsem_try_write_lock() implies ACQUIRE on success */
1152 break;
1153 }
1154
1155 raw_spin_unlock_irq(&sem->wait_lock);
1156
1157 if (signal_pending_state(state, current))
1158 goto out_nolock;
1159
1160 /*
1161 * After setting the handoff bit and failing to acquire
1162 * the lock, attempt to spin on owner to accelerate lock
1163 * transfer. If the previous owner is a on-cpu writer and it
1164 * has just released the lock, OWNER_NULL will be returned.
1165 * In this case, we attempt to acquire the lock again
1166 * without sleeping.
1167 */
1168 if (waiter.handoff_set) {
1169 enum owner_state owner_state;
1170
1171 owner_state = rwsem_spin_on_owner(sem);
1172 if (owner_state == OWNER_NULL)
1173 goto trylock_again;
1174 }
1175
1176 schedule_preempt_disabled();
1177 lockevent_inc(rwsem_sleep_writer);
1178 set_current_state(state);
1179trylock_again:
1180 raw_spin_lock_irq(&sem->wait_lock);
1181 }
1182 __set_current_state(TASK_RUNNING);
1183 raw_spin_unlock_irq(&sem->wait_lock);
1184 lockevent_inc(rwsem_wlock);
1185 trace_contention_end(sem, 0);
1186 return sem;
1187
1188out_nolock:
1189 __set_current_state(TASK_RUNNING);
1190 raw_spin_lock_irq(&sem->wait_lock);
1191 rwsem_del_wake_waiter(sem, &waiter, &wake_q);
1192 lockevent_inc(rwsem_wlock_fail);
1193 trace_contention_end(sem, -EINTR);
1194 return ERR_PTR(-EINTR);
1195}
1196
1197/*
1198 * handle waking up a waiter on the semaphore
1199 * - up_read/up_write has decremented the active part of count if we come here
1200 */
1201static struct rw_semaphore *rwsem_wake(struct rw_semaphore *sem)
1202{
1203 unsigned long flags;
1204 DEFINE_WAKE_Q(wake_q);
1205
1206 raw_spin_lock_irqsave(&sem->wait_lock, flags);
1207
1208 if (!list_empty(&sem->wait_list))
1209 rwsem_mark_wake(sem, RWSEM_WAKE_ANY, &wake_q);
1210
1211 raw_spin_unlock_irqrestore(&sem->wait_lock, flags);
1212 wake_up_q(&wake_q);
1213
1214 return sem;
1215}
1216
1217/*
1218 * downgrade a write lock into a read lock
1219 * - caller incremented waiting part of count and discovered it still negative
1220 * - just wake up any readers at the front of the queue
1221 */
1222static struct rw_semaphore *rwsem_downgrade_wake(struct rw_semaphore *sem)
1223{
1224 unsigned long flags;
1225 DEFINE_WAKE_Q(wake_q);
1226
1227 raw_spin_lock_irqsave(&sem->wait_lock, flags);
1228
1229 if (!list_empty(&sem->wait_list))
1230 rwsem_mark_wake(sem, RWSEM_WAKE_READ_OWNED, &wake_q);
1231
1232 raw_spin_unlock_irqrestore(&sem->wait_lock, flags);
1233 wake_up_q(&wake_q);
1234
1235 return sem;
1236}
1237
1238/*
1239 * lock for reading
1240 */
1241static __always_inline int __down_read_common(struct rw_semaphore *sem, int state)
1242{
1243 int ret = 0;
1244 long count;
1245
1246 preempt_disable();
1247 if (!rwsem_read_trylock(sem, &count)) {
1248 if (IS_ERR(rwsem_down_read_slowpath(sem, count, state))) {
1249 ret = -EINTR;
1250 goto out;
1251 }
1252 DEBUG_RWSEMS_WARN_ON(!is_rwsem_reader_owned(sem), sem);
1253 }
1254out:
1255 preempt_enable();
1256 return ret;
1257}
1258
1259static __always_inline void __down_read(struct rw_semaphore *sem)
1260{
1261 __down_read_common(sem, TASK_UNINTERRUPTIBLE);
1262}
1263
1264static __always_inline int __down_read_interruptible(struct rw_semaphore *sem)
1265{
1266 return __down_read_common(sem, TASK_INTERRUPTIBLE);
1267}
1268
1269static __always_inline int __down_read_killable(struct rw_semaphore *sem)
1270{
1271 return __down_read_common(sem, TASK_KILLABLE);
1272}
1273
1274static inline int __down_read_trylock(struct rw_semaphore *sem)
1275{
1276 int ret = 0;
1277 long tmp;
1278
1279 DEBUG_RWSEMS_WARN_ON(sem->magic != sem, sem);
1280
1281 preempt_disable();
1282 tmp = atomic_long_read(&sem->count);
1283 while (!(tmp & RWSEM_READ_FAILED_MASK)) {
1284 if (atomic_long_try_cmpxchg_acquire(&sem->count, &tmp,
1285 tmp + RWSEM_READER_BIAS)) {
1286 rwsem_set_reader_owned(sem);
1287 ret = 1;
1288 break;
1289 }
1290 }
1291 preempt_enable();
1292 return ret;
1293}
1294
1295/*
1296 * lock for writing
1297 */
1298static __always_inline int __down_write_common(struct rw_semaphore *sem, int state)
1299{
1300 int ret = 0;
1301
1302 preempt_disable();
1303 if (unlikely(!rwsem_write_trylock(sem))) {
1304 if (IS_ERR(rwsem_down_write_slowpath(sem, state)))
1305 ret = -EINTR;
1306 }
1307 preempt_enable();
1308 return ret;
1309}
1310
1311static __always_inline void __down_write(struct rw_semaphore *sem)
1312{
1313 __down_write_common(sem, TASK_UNINTERRUPTIBLE);
1314}
1315
1316static __always_inline int __down_write_killable(struct rw_semaphore *sem)
1317{
1318 return __down_write_common(sem, TASK_KILLABLE);
1319}
1320
1321static inline int __down_write_trylock(struct rw_semaphore *sem)
1322{
1323 int ret;
1324
1325 preempt_disable();
1326 DEBUG_RWSEMS_WARN_ON(sem->magic != sem, sem);
1327 ret = rwsem_write_trylock(sem);
1328 preempt_enable();
1329
1330 return ret;
1331}
1332
1333/*
1334 * unlock after reading
1335 */
1336static inline void __up_read(struct rw_semaphore *sem)
1337{
1338 long tmp;
1339
1340 DEBUG_RWSEMS_WARN_ON(sem->magic != sem, sem);
1341 DEBUG_RWSEMS_WARN_ON(!is_rwsem_reader_owned(sem), sem);
1342
1343 preempt_disable();
1344 rwsem_clear_reader_owned(sem);
1345 tmp = atomic_long_add_return_release(-RWSEM_READER_BIAS, &sem->count);
1346 DEBUG_RWSEMS_WARN_ON(tmp < 0, sem);
1347 if (unlikely((tmp & (RWSEM_LOCK_MASK|RWSEM_FLAG_WAITERS)) ==
1348 RWSEM_FLAG_WAITERS)) {
1349 clear_nonspinnable(sem);
1350 rwsem_wake(sem);
1351 }
1352 preempt_enable();
1353}
1354
1355/*
1356 * unlock after writing
1357 */
1358static inline void __up_write(struct rw_semaphore *sem)
1359{
1360 long tmp;
1361
1362 DEBUG_RWSEMS_WARN_ON(sem->magic != sem, sem);
1363 /*
1364 * sem->owner may differ from current if the ownership is transferred
1365 * to an anonymous writer by setting the RWSEM_NONSPINNABLE bits.
1366 */
1367 DEBUG_RWSEMS_WARN_ON((rwsem_owner(sem) != current) &&
1368 !rwsem_test_oflags(sem, RWSEM_NONSPINNABLE), sem);
1369
1370 preempt_disable();
1371 rwsem_clear_owner(sem);
1372 tmp = atomic_long_fetch_add_release(-RWSEM_WRITER_LOCKED, &sem->count);
1373 if (unlikely(tmp & RWSEM_FLAG_WAITERS))
1374 rwsem_wake(sem);
1375 preempt_enable();
1376}
1377
1378/*
1379 * downgrade write lock to read lock
1380 */
1381static inline void __downgrade_write(struct rw_semaphore *sem)
1382{
1383 long tmp;
1384
1385 /*
1386 * When downgrading from exclusive to shared ownership,
1387 * anything inside the write-locked region cannot leak
1388 * into the read side. In contrast, anything in the
1389 * read-locked region is ok to be re-ordered into the
1390 * write side. As such, rely on RELEASE semantics.
1391 */
1392 DEBUG_RWSEMS_WARN_ON(rwsem_owner(sem) != current, sem);
1393 preempt_disable();
1394 tmp = atomic_long_fetch_add_release(
1395 -RWSEM_WRITER_LOCKED+RWSEM_READER_BIAS, &sem->count);
1396 rwsem_set_reader_owned(sem);
1397 if (tmp & RWSEM_FLAG_WAITERS)
1398 rwsem_downgrade_wake(sem);
1399 preempt_enable();
1400}
1401
1402#else /* !CONFIG_PREEMPT_RT */
1403
1404#define RT_MUTEX_BUILD_MUTEX
1405#include "rtmutex.c"
1406
1407#define rwbase_set_and_save_current_state(state) \
1408 set_current_state(state)
1409
1410#define rwbase_restore_current_state() \
1411 __set_current_state(TASK_RUNNING)
1412
1413#define rwbase_rtmutex_lock_state(rtm, state) \
1414 __rt_mutex_lock(rtm, state)
1415
1416#define rwbase_rtmutex_slowlock_locked(rtm, state, wq) \
1417 __rt_mutex_slowlock_locked(rtm, NULL, state, wq)
1418
1419#define rwbase_rtmutex_unlock(rtm) \
1420 __rt_mutex_unlock(rtm)
1421
1422#define rwbase_rtmutex_trylock(rtm) \
1423 __rt_mutex_trylock(rtm)
1424
1425#define rwbase_signal_pending_state(state, current) \
1426 signal_pending_state(state, current)
1427
1428#define rwbase_pre_schedule() \
1429 rt_mutex_pre_schedule()
1430
1431#define rwbase_schedule() \
1432 rt_mutex_schedule()
1433
1434#define rwbase_post_schedule() \
1435 rt_mutex_post_schedule()
1436
1437#include "rwbase_rt.c"
1438
1439void __init_rwsem(struct rw_semaphore *sem, const char *name,
1440 struct lock_class_key *key)
1441{
1442 init_rwbase_rt(&(sem)->rwbase);
1443
1444#ifdef CONFIG_DEBUG_LOCK_ALLOC
1445 debug_check_no_locks_freed((void *)sem, sizeof(*sem));
1446 lockdep_init_map_wait(&sem->dep_map, name, key, 0, LD_WAIT_SLEEP);
1447#endif
1448}
1449EXPORT_SYMBOL(__init_rwsem);
1450
1451static inline void __down_read(struct rw_semaphore *sem)
1452{
1453 rwbase_read_lock(&sem->rwbase, TASK_UNINTERRUPTIBLE);
1454}
1455
1456static inline int __down_read_interruptible(struct rw_semaphore *sem)
1457{
1458 return rwbase_read_lock(&sem->rwbase, TASK_INTERRUPTIBLE);
1459}
1460
1461static inline int __down_read_killable(struct rw_semaphore *sem)
1462{
1463 return rwbase_read_lock(&sem->rwbase, TASK_KILLABLE);
1464}
1465
1466static inline int __down_read_trylock(struct rw_semaphore *sem)
1467{
1468 return rwbase_read_trylock(&sem->rwbase);
1469}
1470
1471static inline void __up_read(struct rw_semaphore *sem)
1472{
1473 rwbase_read_unlock(&sem->rwbase, TASK_NORMAL);
1474}
1475
1476static inline void __sched __down_write(struct rw_semaphore *sem)
1477{
1478 rwbase_write_lock(&sem->rwbase, TASK_UNINTERRUPTIBLE);
1479}
1480
1481static inline int __sched __down_write_killable(struct rw_semaphore *sem)
1482{
1483 return rwbase_write_lock(&sem->rwbase, TASK_KILLABLE);
1484}
1485
1486static inline int __down_write_trylock(struct rw_semaphore *sem)
1487{
1488 return rwbase_write_trylock(&sem->rwbase);
1489}
1490
1491static inline void __up_write(struct rw_semaphore *sem)
1492{
1493 rwbase_write_unlock(&sem->rwbase);
1494}
1495
1496static inline void __downgrade_write(struct rw_semaphore *sem)
1497{
1498 rwbase_write_downgrade(&sem->rwbase);
1499}
1500
1501/* Debug stubs for the common API */
1502#define DEBUG_RWSEMS_WARN_ON(c, sem)
1503
1504static inline void __rwsem_set_reader_owned(struct rw_semaphore *sem,
1505 struct task_struct *owner)
1506{
1507}
1508
1509static inline bool is_rwsem_reader_owned(struct rw_semaphore *sem)
1510{
1511 int count = atomic_read(&sem->rwbase.readers);
1512
1513 return count < 0 && count != READER_BIAS;
1514}
1515
1516#endif /* CONFIG_PREEMPT_RT */
1517
1518/*
1519 * lock for reading
1520 */
1521void __sched down_read(struct rw_semaphore *sem)
1522{
1523 might_sleep();
1524 rwsem_acquire_read(&sem->dep_map, 0, 0, _RET_IP_);
1525
1526 LOCK_CONTENDED(sem, __down_read_trylock, __down_read);
1527}
1528EXPORT_SYMBOL(down_read);
1529
1530int __sched down_read_interruptible(struct rw_semaphore *sem)
1531{
1532 might_sleep();
1533 rwsem_acquire_read(&sem->dep_map, 0, 0, _RET_IP_);
1534
1535 if (LOCK_CONTENDED_RETURN(sem, __down_read_trylock, __down_read_interruptible)) {
1536 rwsem_release(&sem->dep_map, _RET_IP_);
1537 return -EINTR;
1538 }
1539
1540 return 0;
1541}
1542EXPORT_SYMBOL(down_read_interruptible);
1543
1544int __sched down_read_killable(struct rw_semaphore *sem)
1545{
1546 might_sleep();
1547 rwsem_acquire_read(&sem->dep_map, 0, 0, _RET_IP_);
1548
1549 if (LOCK_CONTENDED_RETURN(sem, __down_read_trylock, __down_read_killable)) {
1550 rwsem_release(&sem->dep_map, _RET_IP_);
1551 return -EINTR;
1552 }
1553
1554 return 0;
1555}
1556EXPORT_SYMBOL(down_read_killable);
1557
1558/*
1559 * trylock for reading -- returns 1 if successful, 0 if contention
1560 */
1561int down_read_trylock(struct rw_semaphore *sem)
1562{
1563 int ret = __down_read_trylock(sem);
1564
1565 if (ret == 1)
1566 rwsem_acquire_read(&sem->dep_map, 0, 1, _RET_IP_);
1567 return ret;
1568}
1569EXPORT_SYMBOL(down_read_trylock);
1570
1571/*
1572 * lock for writing
1573 */
1574void __sched down_write(struct rw_semaphore *sem)
1575{
1576 might_sleep();
1577 rwsem_acquire(&sem->dep_map, 0, 0, _RET_IP_);
1578 LOCK_CONTENDED(sem, __down_write_trylock, __down_write);
1579}
1580EXPORT_SYMBOL(down_write);
1581
1582/*
1583 * lock for writing
1584 */
1585int __sched down_write_killable(struct rw_semaphore *sem)
1586{
1587 might_sleep();
1588 rwsem_acquire(&sem->dep_map, 0, 0, _RET_IP_);
1589
1590 if (LOCK_CONTENDED_RETURN(sem, __down_write_trylock,
1591 __down_write_killable)) {
1592 rwsem_release(&sem->dep_map, _RET_IP_);
1593 return -EINTR;
1594 }
1595
1596 return 0;
1597}
1598EXPORT_SYMBOL(down_write_killable);
1599
1600/*
1601 * trylock for writing -- returns 1 if successful, 0 if contention
1602 */
1603int down_write_trylock(struct rw_semaphore *sem)
1604{
1605 int ret = __down_write_trylock(sem);
1606
1607 if (ret == 1)
1608 rwsem_acquire(&sem->dep_map, 0, 1, _RET_IP_);
1609
1610 return ret;
1611}
1612EXPORT_SYMBOL(down_write_trylock);
1613
1614/*
1615 * release a read lock
1616 */
1617void up_read(struct rw_semaphore *sem)
1618{
1619 rwsem_release(&sem->dep_map, _RET_IP_);
1620 __up_read(sem);
1621}
1622EXPORT_SYMBOL(up_read);
1623
1624/*
1625 * release a write lock
1626 */
1627void up_write(struct rw_semaphore *sem)
1628{
1629 rwsem_release(&sem->dep_map, _RET_IP_);
1630 __up_write(sem);
1631}
1632EXPORT_SYMBOL(up_write);
1633
1634/*
1635 * downgrade write lock to read lock
1636 */
1637void downgrade_write(struct rw_semaphore *sem)
1638{
1639 lock_downgrade(&sem->dep_map, _RET_IP_);
1640 __downgrade_write(sem);
1641}
1642EXPORT_SYMBOL(downgrade_write);
1643
1644#ifdef CONFIG_DEBUG_LOCK_ALLOC
1645
1646void down_read_nested(struct rw_semaphore *sem, int subclass)
1647{
1648 might_sleep();
1649 rwsem_acquire_read(&sem->dep_map, subclass, 0, _RET_IP_);
1650 LOCK_CONTENDED(sem, __down_read_trylock, __down_read);
1651}
1652EXPORT_SYMBOL(down_read_nested);
1653
1654int down_read_killable_nested(struct rw_semaphore *sem, int subclass)
1655{
1656 might_sleep();
1657 rwsem_acquire_read(&sem->dep_map, subclass, 0, _RET_IP_);
1658
1659 if (LOCK_CONTENDED_RETURN(sem, __down_read_trylock, __down_read_killable)) {
1660 rwsem_release(&sem->dep_map, _RET_IP_);
1661 return -EINTR;
1662 }
1663
1664 return 0;
1665}
1666EXPORT_SYMBOL(down_read_killable_nested);
1667
1668void _down_write_nest_lock(struct rw_semaphore *sem, struct lockdep_map *nest)
1669{
1670 might_sleep();
1671 rwsem_acquire_nest(&sem->dep_map, 0, 0, nest, _RET_IP_);
1672 LOCK_CONTENDED(sem, __down_write_trylock, __down_write);
1673}
1674EXPORT_SYMBOL(_down_write_nest_lock);
1675
1676void down_read_non_owner(struct rw_semaphore *sem)
1677{
1678 might_sleep();
1679 __down_read(sem);
1680 /*
1681 * The owner value for a reader-owned lock is mostly for debugging
1682 * purpose only and is not critical to the correct functioning of
1683 * rwsem. So it is perfectly fine to set it in a preempt-enabled
1684 * context here.
1685 */
1686 __rwsem_set_reader_owned(sem, NULL);
1687}
1688EXPORT_SYMBOL(down_read_non_owner);
1689
1690void down_write_nested(struct rw_semaphore *sem, int subclass)
1691{
1692 might_sleep();
1693 rwsem_acquire(&sem->dep_map, subclass, 0, _RET_IP_);
1694 LOCK_CONTENDED(sem, __down_write_trylock, __down_write);
1695}
1696EXPORT_SYMBOL(down_write_nested);
1697
1698int __sched down_write_killable_nested(struct rw_semaphore *sem, int subclass)
1699{
1700 might_sleep();
1701 rwsem_acquire(&sem->dep_map, subclass, 0, _RET_IP_);
1702
1703 if (LOCK_CONTENDED_RETURN(sem, __down_write_trylock,
1704 __down_write_killable)) {
1705 rwsem_release(&sem->dep_map, _RET_IP_);
1706 return -EINTR;
1707 }
1708
1709 return 0;
1710}
1711EXPORT_SYMBOL(down_write_killable_nested);
1712
1713void up_read_non_owner(struct rw_semaphore *sem)
1714{
1715 DEBUG_RWSEMS_WARN_ON(!is_rwsem_reader_owned(sem), sem);
1716 __up_read(sem);
1717}
1718EXPORT_SYMBOL(up_read_non_owner);
1719
1720#endif