Line data Source code
1 : // SPDX-License-Identifier: GPL-2.0
2 : /* kernel/rwsem.c: R/W semaphores, public implementation
3 : *
4 : * Written by David Howells (dhowells@redhat.com).
5 : * Derived from asm-i386/semaphore.h
6 : *
7 : * Writer lock-stealing by Alex Shi <alex.shi@intel.com>
8 : * and Michel Lespinasse <walken@google.com>
9 : *
10 : * Optimistic spinning by Tim Chen <tim.c.chen@intel.com>
11 : * and Davidlohr Bueso <davidlohr@hp.com>. Based on mutexes.
12 : *
13 : * Rwsem count bit fields re-definition and rwsem rearchitecture by
14 : * Waiman Long <longman@redhat.com> and
15 : * Peter Zijlstra <peterz@infradead.org>.
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 : */
140 : static inline void rwsem_set_owner(struct rw_semaphore *sem)
141 : {
142 : lockdep_assert_preemption_disabled();
143 51942 : atomic_long_set(&sem->owner, (long)current);
144 : }
145 :
146 : static inline void rwsem_clear_owner(struct rw_semaphore *sem)
147 : {
148 : lockdep_assert_preemption_disabled();
149 51942 : atomic_long_set(&sem->owner, 0);
150 : }
151 :
152 : /*
153 : * Test the flags in the owner field.
154 : */
155 : static 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 : */
170 : static inline void __rwsem_set_reader_owned(struct rw_semaphore *sem,
171 : struct task_struct *owner)
172 : {
173 607 : unsigned long val = (unsigned long)owner | RWSEM_READER_OWNED |
174 1214 : (atomic_long_read(&sem->owner) & RWSEM_NONSPINNABLE);
175 :
176 1214 : atomic_long_set(&sem->owner, val);
177 : }
178 :
179 : static inline void rwsem_set_reader_owned(struct rw_semaphore *sem)
180 : {
181 1214 : __rwsem_set_reader_owned(sem, current);
182 : }
183 :
184 : /*
185 : * Return true if the rwsem is owned by a reader.
186 : */
187 : static 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 : */
208 : static 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
219 : static 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 : */
228 : static inline void rwsem_set_nonspinnable(struct rw_semaphore *sem)
229 : {
230 0 : unsigned long owner = atomic_long_read(&sem->owner);
231 :
232 : do {
233 0 : if (!(owner & RWSEM_READER_OWNED))
234 : break;
235 0 : if (owner & RWSEM_NONSPINNABLE)
236 : break;
237 0 : } while (!atomic_long_try_cmpxchg(&sem->owner, &owner,
238 0 : owner | RWSEM_NONSPINNABLE));
239 : }
240 :
241 607 : static inline bool rwsem_read_trylock(struct rw_semaphore *sem, long *cntp)
242 : {
243 1214 : *cntp = atomic_long_add_return_acquire(RWSEM_READER_BIAS, &sem->count);
244 :
245 607 : if (WARN_ON_ONCE(*cntp < 0))
246 : rwsem_set_nonspinnable(sem);
247 :
248 607 : if (!(*cntp & RWSEM_READ_FAILED_MASK)) {
249 607 : rwsem_set_reader_owned(sem);
250 607 : return true;
251 : }
252 :
253 : return false;
254 : }
255 :
256 : static inline bool rwsem_write_trylock(struct rw_semaphore *sem)
257 : {
258 25971 : long tmp = RWSEM_UNLOCKED_VALUE;
259 :
260 51942 : if (atomic_long_try_cmpxchg_acquire(&sem->count, &tmp, RWSEM_WRITER_LOCKED)) {
261 25971 : 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 : */
271 : static 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 : */
281 : static inline struct task_struct *
282 : rwsem_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 : */
310 338 : void __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
323 676 : atomic_long_set(&sem->count, RWSEM_UNLOCKED_VALUE);
324 : raw_spin_lock_init(&sem->wait_lock);
325 676 : INIT_LIST_HEAD(&sem->wait_list);
326 676 : atomic_long_set(&sem->owner, 0L);
327 : #ifdef CONFIG_RWSEM_SPIN_ON_OWNER
328 : osq_lock_init(&sem->osq);
329 : #endif
330 338 : }
331 : EXPORT_SYMBOL(__init_rwsem);
332 :
333 : enum rwsem_waiter_type {
334 : RWSEM_WAITING_FOR_WRITE,
335 : RWSEM_WAITING_FOR_READ
336 : };
337 :
338 : struct 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 :
348 : enum 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 :
369 : static inline void
370 : rwsem_add_waiter(struct rw_semaphore *sem, struct rwsem_waiter *waiter)
371 : {
372 : lockdep_assert_held(&sem->wait_lock);
373 0 : 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 : */
385 : static inline bool
386 : rwsem_del_waiter(struct rw_semaphore *sem, struct rwsem_waiter *waiter)
387 : {
388 : lockdep_assert_held(&sem->wait_lock);
389 0 : list_del(&waiter->list);
390 0 : if (likely(!list_empty(&sem->wait_list)))
391 : return true;
392 :
393 0 : 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 : */
411 0 : static 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 0 : 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 0 : waiter = rwsem_first_waiter(sem);
426 :
427 0 : if (waiter->type == RWSEM_WAITING_FOR_WRITE) {
428 0 : 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 0 : wake_q_add(wake_q, waiter->task);
437 : lockevent_inc(rwsem_wake_writer);
438 : }
439 :
440 0 : return;
441 : }
442 :
443 : /*
444 : * No reader wakeup if there are too many of them already.
445 : */
446 0 : 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 0 : if (wake_type != RWSEM_WAKE_READ_OWNED) {
455 : struct task_struct *owner;
456 :
457 0 : adjustment = RWSEM_READER_BIAS;
458 0 : oldcount = atomic_long_fetch_add(adjustment, &sem->count);
459 0 : 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 0 : if (time_after(jiffies, waiter->timeout)) {
466 0 : if (!(oldcount & RWSEM_FLAG_HANDOFF)) {
467 0 : adjustment -= RWSEM_FLAG_HANDOFF;
468 : lockevent_inc(rwsem_rlock_handoff);
469 : }
470 0 : waiter->handoff_set = true;
471 : }
472 :
473 0 : 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 0 : 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 0 : INIT_LIST_HEAD(&wlist);
510 0 : list_for_each_entry_safe(waiter, tmp, &sem->wait_list, list) {
511 0 : if (waiter->type == RWSEM_WAITING_FOR_WRITE)
512 0 : continue;
513 :
514 0 : woken++;
515 0 : list_move_tail(&waiter->list, &wlist);
516 :
517 : /*
518 : * Limit # of readers that can be woken up per wakeup call.
519 : */
520 0 : if (unlikely(woken >= MAX_READERS_WAKEUP))
521 : break;
522 : }
523 :
524 0 : adjustment = woken * RWSEM_READER_BIAS - adjustment;
525 : lockevent_cond_inc(rwsem_wake_reader, woken);
526 :
527 0 : oldcount = atomic_long_read(&sem->count);
528 0 : if (list_empty(&sem->wait_list)) {
529 : /*
530 : * Combined with list_move_tail() above, this implies
531 : * rwsem_del_waiter().
532 : */
533 0 : adjustment -= RWSEM_FLAG_WAITERS;
534 0 : if (oldcount & RWSEM_FLAG_HANDOFF)
535 0 : adjustment -= RWSEM_FLAG_HANDOFF;
536 0 : } 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 0 : if (oldcount & RWSEM_FLAG_HANDOFF)
542 0 : adjustment -= RWSEM_FLAG_HANDOFF;
543 : }
544 :
545 0 : if (adjustment)
546 0 : atomic_long_add(adjustment, &sem->count);
547 :
548 : /* 2nd pass */
549 0 : list_for_each_entry_safe(waiter, tmp, &wlist, list) {
550 : struct task_struct *tsk;
551 :
552 0 : tsk = waiter->task;
553 0 : 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 0 : 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 0 : 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 : */
576 : static inline void
577 0 : rwsem_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 0 : bool first = rwsem_first_waiter(sem) == waiter;
582 :
583 0 : 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 0 : if (rwsem_del_waiter(sem, waiter) && first)
591 0 : rwsem_mark_wake(sem, RWSEM_WAKE_ANY, wake_q);
592 0 : raw_spin_unlock_irq(&sem->wait_lock);
593 0 : if (!wake_q_empty(wake_q))
594 0 : wake_up_q(wake_q);
595 0 : }
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 : */
604 0 : static inline bool rwsem_try_write_lock(struct rw_semaphore *sem,
605 : struct rwsem_waiter *waiter)
606 : {
607 0 : struct rwsem_waiter *first = rwsem_first_waiter(sem);
608 : long count, new;
609 :
610 : lockdep_assert_held(&sem->wait_lock);
611 :
612 0 : count = atomic_long_read(&sem->count);
613 : do {
614 0 : bool has_handoff = !!(count & RWSEM_FLAG_HANDOFF);
615 :
616 0 : 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 0 : if (first->handoff_set && (waiter != first))
623 : return false;
624 : }
625 :
626 0 : new = count;
627 :
628 0 : 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 0 : if (has_handoff || (!rt_task(waiter->task) &&
635 0 : !time_after(jiffies, waiter->timeout)))
636 : return false;
637 :
638 0 : new |= RWSEM_FLAG_HANDOFF;
639 : } else {
640 0 : new |= RWSEM_WRITER_LOCKED;
641 0 : new &= ~RWSEM_FLAG_HANDOFF;
642 :
643 0 : if (list_is_singular(&sem->wait_list))
644 0 : new &= ~RWSEM_FLAG_WAITERS;
645 : }
646 0 : } 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 0 : if (new & RWSEM_FLAG_HANDOFF) {
654 0 : first->handoff_set = true;
655 : lockevent_inc(rwsem_wlock_handoff);
656 0 : return false;
657 : }
658 :
659 : /*
660 : * Have rwsem_try_write_lock() fully imply rwsem_del_waiter() on
661 : * success.
662 : */
663 0 : list_del(&waiter->list);
664 0 : rwsem_set_owner(sem);
665 0 : 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 : */
679 : enum 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 : */
690 : static 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 :
705 : static 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 :
734 : static inline enum owner_state
735 : rwsem_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 :
746 : static noinline enum owner_state
747 : rwsem_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 : */
806 : static 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 :
819 : static 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);
932 : done:
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 : */
941 : static 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
948 : static inline bool rwsem_can_spin_on_owner(struct rw_semaphore *sem)
949 : {
950 : return false;
951 : }
952 :
953 : static inline bool rwsem_optimistic_spin(struct rw_semaphore *sem)
954 : {
955 : return false;
956 : }
957 :
958 : static inline void clear_nonspinnable(struct rw_semaphore *sem) { }
959 :
960 : static inline enum owner_state
961 : rwsem_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 : */
975 : static 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 0 : if (count & RWSEM_WRITER_MASK)
981 : return;
982 :
983 0 : if (count & RWSEM_READER_MASK) {
984 : wake_type = RWSEM_WAKE_READERS;
985 : } else {
986 0 : wake_type = RWSEM_WAKE_ANY;
987 0 : clear_nonspinnable(sem);
988 : }
989 0 : rwsem_mark_wake(sem, wake_type, wake_q);
990 : }
991 :
992 : /*
993 : * Wait for the read lock to be granted
994 : */
995 : static struct rw_semaphore __sched *
996 0 : rwsem_down_read_slowpath(struct rw_semaphore *sem, long count, unsigned int state)
997 : {
998 0 : long adjustment = -RWSEM_READER_BIAS;
999 0 : long rcnt = (count >> RWSEM_READER_SHIFT);
1000 : struct rwsem_waiter waiter;
1001 0 : 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 0 : if ((atomic_long_read(&sem->owner) & RWSEM_READER_OWNED) &&
1009 0 : (rcnt > 1) && !(count & RWSEM_WRITER_LOCKED))
1010 : goto queue;
1011 :
1012 : /*
1013 : * Reader optimistic lock stealing.
1014 : */
1015 0 : if (!(count & (RWSEM_WRITER_LOCKED | RWSEM_FLAG_HANDOFF))) {
1016 0 : 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 0 : if ((rcnt == 1) && (count & RWSEM_FLAG_WAITERS)) {
1024 0 : raw_spin_lock_irq(&sem->wait_lock);
1025 0 : if (!list_empty(&sem->wait_list))
1026 0 : rwsem_mark_wake(sem, RWSEM_WAKE_READ_OWNED,
1027 : &wake_q);
1028 0 : raw_spin_unlock_irq(&sem->wait_lock);
1029 0 : wake_up_q(&wake_q);
1030 : }
1031 : return sem;
1032 : }
1033 :
1034 : queue:
1035 0 : waiter.task = current;
1036 0 : waiter.type = RWSEM_WAITING_FOR_READ;
1037 0 : waiter.timeout = jiffies + RWSEM_WAIT_TIMEOUT;
1038 0 : waiter.handoff_set = false;
1039 :
1040 0 : raw_spin_lock_irq(&sem->wait_lock);
1041 0 : 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 0 : if (!(atomic_long_read(&sem->count) & RWSEM_WRITER_MASK)) {
1049 : /* Provide lock ACQUIRE */
1050 0 : smp_acquire__after_ctrl_dep();
1051 0 : raw_spin_unlock_irq(&sem->wait_lock);
1052 0 : rwsem_set_reader_owned(sem);
1053 : lockevent_inc(rwsem_rlock_fast);
1054 0 : return sem;
1055 : }
1056 : adjustment += RWSEM_FLAG_WAITERS;
1057 : }
1058 0 : rwsem_add_waiter(sem, &waiter);
1059 :
1060 : /* we're now waiting on the lock, but no longer actively locking */
1061 0 : count = atomic_long_add_return(adjustment, &sem->count);
1062 :
1063 0 : rwsem_cond_wake_waiter(sem, count, &wake_q);
1064 0 : raw_spin_unlock_irq(&sem->wait_lock);
1065 :
1066 0 : if (!wake_q_empty(&wake_q))
1067 0 : 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 0 : set_current_state(state);
1074 0 : if (!smp_load_acquire(&waiter.task)) {
1075 : /* Matches rwsem_mark_wake()'s smp_store_release(). */
1076 : break;
1077 : }
1078 0 : if (signal_pending_state(state, current)) {
1079 0 : raw_spin_lock_irq(&sem->wait_lock);
1080 0 : if (waiter.task)
1081 : goto out_nolock;
1082 0 : raw_spin_unlock_irq(&sem->wait_lock);
1083 : /* Ordered by sem->wait_lock against rwsem_mark_wake(). */
1084 0 : break;
1085 : }
1086 0 : schedule_preempt_disabled();
1087 : lockevent_inc(rwsem_sleep_reader);
1088 : }
1089 :
1090 0 : __set_current_state(TASK_RUNNING);
1091 : lockevent_inc(rwsem_rlock);
1092 0 : trace_contention_end(sem, 0);
1093 0 : return sem;
1094 :
1095 : out_nolock:
1096 0 : rwsem_del_wake_waiter(sem, &waiter, &wake_q);
1097 0 : __set_current_state(TASK_RUNNING);
1098 : lockevent_inc(rwsem_rlock_fail);
1099 0 : trace_contention_end(sem, -EINTR);
1100 0 : return ERR_PTR(-EINTR);
1101 : }
1102 :
1103 : /*
1104 : * Wait until we successfully acquire the write lock
1105 : */
1106 : static struct rw_semaphore __sched *
1107 0 : rwsem_down_write_slowpath(struct rw_semaphore *sem, int state)
1108 : {
1109 : struct rwsem_waiter waiter;
1110 0 : DEFINE_WAKE_Q(wake_q);
1111 :
1112 : /* do optimistic spinning and steal lock if possible */
1113 0 : 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 0 : waiter.task = current;
1123 0 : waiter.type = RWSEM_WAITING_FOR_WRITE;
1124 0 : waiter.timeout = jiffies + RWSEM_WAIT_TIMEOUT;
1125 0 : waiter.handoff_set = false;
1126 :
1127 0 : raw_spin_lock_irq(&sem->wait_lock);
1128 0 : rwsem_add_waiter(sem, &waiter);
1129 :
1130 : /* we're now waiting on the lock */
1131 0 : if (rwsem_first_waiter(sem) != &waiter) {
1132 0 : rwsem_cond_wake_waiter(sem, atomic_long_read(&sem->count),
1133 : &wake_q);
1134 0 : 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 0 : raw_spin_unlock_irq(&sem->wait_lock);
1140 0 : wake_up_q(&wake_q);
1141 0 : raw_spin_lock_irq(&sem->wait_lock);
1142 : }
1143 : } else {
1144 0 : atomic_long_or(RWSEM_FLAG_WAITERS, &sem->count);
1145 : }
1146 :
1147 : /* wait until we successfully acquire the lock */
1148 0 : set_current_state(state);
1149 0 : trace_contention_begin(sem, LCB_F_WRITE);
1150 :
1151 : for (;;) {
1152 0 : if (rwsem_try_write_lock(sem, &waiter)) {
1153 : /* rwsem_try_write_lock() implies ACQUIRE on success */
1154 : break;
1155 : }
1156 :
1157 0 : raw_spin_unlock_irq(&sem->wait_lock);
1158 :
1159 0 : 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 0 : schedule_preempt_disabled();
1179 : lockevent_inc(rwsem_sleep_writer);
1180 0 : set_current_state(state);
1181 : trylock_again:
1182 0 : raw_spin_lock_irq(&sem->wait_lock);
1183 : }
1184 0 : __set_current_state(TASK_RUNNING);
1185 0 : raw_spin_unlock_irq(&sem->wait_lock);
1186 : lockevent_inc(rwsem_wlock);
1187 0 : trace_contention_end(sem, 0);
1188 0 : return sem;
1189 :
1190 : out_nolock:
1191 0 : __set_current_state(TASK_RUNNING);
1192 0 : raw_spin_lock_irq(&sem->wait_lock);
1193 0 : rwsem_del_wake_waiter(sem, &waiter, &wake_q);
1194 : lockevent_inc(rwsem_wlock_fail);
1195 0 : trace_contention_end(sem, -EINTR);
1196 0 : 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 : */
1203 0 : static struct rw_semaphore *rwsem_wake(struct rw_semaphore *sem)
1204 : {
1205 : unsigned long flags;
1206 0 : DEFINE_WAKE_Q(wake_q);
1207 :
1208 0 : raw_spin_lock_irqsave(&sem->wait_lock, flags);
1209 :
1210 0 : if (!list_empty(&sem->wait_list))
1211 0 : rwsem_mark_wake(sem, RWSEM_WAKE_ANY, &wake_q);
1212 :
1213 0 : raw_spin_unlock_irqrestore(&sem->wait_lock, flags);
1214 0 : wake_up_q(&wake_q);
1215 :
1216 0 : 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 : */
1224 0 : static struct rw_semaphore *rwsem_downgrade_wake(struct rw_semaphore *sem)
1225 : {
1226 : unsigned long flags;
1227 0 : DEFINE_WAKE_Q(wake_q);
1228 :
1229 0 : raw_spin_lock_irqsave(&sem->wait_lock, flags);
1230 :
1231 0 : if (!list_empty(&sem->wait_list))
1232 0 : rwsem_mark_wake(sem, RWSEM_WAKE_READ_OWNED, &wake_q);
1233 :
1234 0 : raw_spin_unlock_irqrestore(&sem->wait_lock, flags);
1235 0 : wake_up_q(&wake_q);
1236 :
1237 0 : return sem;
1238 : }
1239 :
1240 : /*
1241 : * lock for reading
1242 : */
1243 : static __always_inline int __down_read_common(struct rw_semaphore *sem, int state)
1244 : {
1245 607 : int ret = 0;
1246 : long count;
1247 :
1248 607 : preempt_disable();
1249 607 : if (!rwsem_read_trylock(sem, &count)) {
1250 0 : if (IS_ERR(rwsem_down_read_slowpath(sem, count, state))) {
1251 0 : ret = -EINTR;
1252 : goto out;
1253 : }
1254 : DEBUG_RWSEMS_WARN_ON(!is_rwsem_reader_owned(sem), sem);
1255 : }
1256 : out:
1257 607 : preempt_enable();
1258 : return ret;
1259 : }
1260 :
1261 : static __always_inline void __down_read(struct rw_semaphore *sem)
1262 : {
1263 607 : __down_read_common(sem, TASK_UNINTERRUPTIBLE);
1264 : }
1265 :
1266 : static __always_inline int __down_read_interruptible(struct rw_semaphore *sem)
1267 : {
1268 0 : return __down_read_common(sem, TASK_INTERRUPTIBLE);
1269 : }
1270 :
1271 : static __always_inline int __down_read_killable(struct rw_semaphore *sem)
1272 : {
1273 0 : return __down_read_common(sem, TASK_KILLABLE);
1274 : }
1275 :
1276 5 : static inline int __down_read_trylock(struct rw_semaphore *sem)
1277 : {
1278 5 : int ret = 0;
1279 : long tmp;
1280 :
1281 : DEBUG_RWSEMS_WARN_ON(sem->magic != sem, sem);
1282 :
1283 5 : preempt_disable();
1284 10 : tmp = atomic_long_read(&sem->count);
1285 10 : while (!(tmp & RWSEM_READ_FAILED_MASK)) {
1286 0 : if (atomic_long_try_cmpxchg_acquire(&sem->count, &tmp,
1287 0 : tmp + RWSEM_READER_BIAS)) {
1288 0 : rwsem_set_reader_owned(sem);
1289 0 : ret = 1;
1290 0 : break;
1291 : }
1292 : }
1293 5 : preempt_enable();
1294 5 : return ret;
1295 : }
1296 :
1297 : /*
1298 : * lock for writing
1299 : */
1300 25971 : static inline int __down_write_common(struct rw_semaphore *sem, int state)
1301 : {
1302 25971 : int ret = 0;
1303 :
1304 25971 : preempt_disable();
1305 25971 : if (unlikely(!rwsem_write_trylock(sem))) {
1306 0 : if (IS_ERR(rwsem_down_write_slowpath(sem, state)))
1307 0 : ret = -EINTR;
1308 : }
1309 25971 : preempt_enable();
1310 25971 : return ret;
1311 : }
1312 :
1313 : static inline void __down_write(struct rw_semaphore *sem)
1314 : {
1315 25971 : __down_write_common(sem, TASK_UNINTERRUPTIBLE);
1316 : }
1317 :
1318 : static inline int __down_write_killable(struct rw_semaphore *sem)
1319 : {
1320 0 : return __down_write_common(sem, TASK_KILLABLE);
1321 : }
1322 :
1323 : static inline int __down_write_trylock(struct rw_semaphore *sem)
1324 : {
1325 : int ret;
1326 :
1327 0 : preempt_disable();
1328 : DEBUG_RWSEMS_WARN_ON(sem->magic != sem, sem);
1329 0 : ret = rwsem_write_trylock(sem);
1330 0 : preempt_enable();
1331 :
1332 : return ret;
1333 : }
1334 :
1335 : /*
1336 : * unlock after reading
1337 : */
1338 : static 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 607 : preempt_disable();
1346 607 : rwsem_clear_reader_owned(sem);
1347 1214 : tmp = atomic_long_add_return_release(-RWSEM_READER_BIAS, &sem->count);
1348 : DEBUG_RWSEMS_WARN_ON(tmp < 0, sem);
1349 607 : if (unlikely((tmp & (RWSEM_LOCK_MASK|RWSEM_FLAG_WAITERS)) ==
1350 : RWSEM_FLAG_WAITERS)) {
1351 0 : clear_nonspinnable(sem);
1352 0 : rwsem_wake(sem);
1353 : }
1354 607 : preempt_enable();
1355 : }
1356 :
1357 : /*
1358 : * unlock after writing
1359 : */
1360 : static 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 25971 : preempt_disable();
1373 25971 : rwsem_clear_owner(sem);
1374 51942 : tmp = atomic_long_fetch_add_release(-RWSEM_WRITER_LOCKED, &sem->count);
1375 25971 : if (unlikely(tmp & RWSEM_FLAG_WAITERS))
1376 0 : rwsem_wake(sem);
1377 25971 : preempt_enable();
1378 : }
1379 :
1380 : /*
1381 : * downgrade write lock to read lock
1382 : */
1383 0 : static 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 0 : preempt_disable();
1396 0 : tmp = atomic_long_fetch_add_release(
1397 : -RWSEM_WRITER_LOCKED+RWSEM_READER_BIAS, &sem->count);
1398 0 : rwsem_set_reader_owned(sem);
1399 0 : if (tmp & RWSEM_FLAG_WAITERS)
1400 0 : rwsem_downgrade_wake(sem);
1401 0 : preempt_enable();
1402 0 : }
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 :
1435 : void __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 : }
1445 : EXPORT_SYMBOL(__init_rwsem);
1446 :
1447 : static inline void __down_read(struct rw_semaphore *sem)
1448 : {
1449 : rwbase_read_lock(&sem->rwbase, TASK_UNINTERRUPTIBLE);
1450 : }
1451 :
1452 : static inline int __down_read_interruptible(struct rw_semaphore *sem)
1453 : {
1454 : return rwbase_read_lock(&sem->rwbase, TASK_INTERRUPTIBLE);
1455 : }
1456 :
1457 : static inline int __down_read_killable(struct rw_semaphore *sem)
1458 : {
1459 : return rwbase_read_lock(&sem->rwbase, TASK_KILLABLE);
1460 : }
1461 :
1462 : static inline int __down_read_trylock(struct rw_semaphore *sem)
1463 : {
1464 : return rwbase_read_trylock(&sem->rwbase);
1465 : }
1466 :
1467 : static inline void __up_read(struct rw_semaphore *sem)
1468 : {
1469 : rwbase_read_unlock(&sem->rwbase, TASK_NORMAL);
1470 : }
1471 :
1472 : static inline void __sched __down_write(struct rw_semaphore *sem)
1473 : {
1474 : rwbase_write_lock(&sem->rwbase, TASK_UNINTERRUPTIBLE);
1475 : }
1476 :
1477 : static inline int __sched __down_write_killable(struct rw_semaphore *sem)
1478 : {
1479 : return rwbase_write_lock(&sem->rwbase, TASK_KILLABLE);
1480 : }
1481 :
1482 : static inline int __down_write_trylock(struct rw_semaphore *sem)
1483 : {
1484 : return rwbase_write_trylock(&sem->rwbase);
1485 : }
1486 :
1487 : static inline void __up_write(struct rw_semaphore *sem)
1488 : {
1489 : rwbase_write_unlock(&sem->rwbase);
1490 : }
1491 :
1492 : static 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 :
1500 : static inline void __rwsem_set_reader_owned(struct rw_semaphore *sem,
1501 : struct task_struct *owner)
1502 : {
1503 : }
1504 :
1505 : static 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 : */
1517 607 : void __sched down_read(struct rw_semaphore *sem)
1518 : {
1519 : might_sleep();
1520 : rwsem_acquire_read(&sem->dep_map, 0, 0, _RET_IP_);
1521 :
1522 607 : LOCK_CONTENDED(sem, __down_read_trylock, __down_read);
1523 607 : }
1524 : EXPORT_SYMBOL(down_read);
1525 :
1526 0 : int __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 0 : 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 0 : return 0;
1537 : }
1538 : EXPORT_SYMBOL(down_read_interruptible);
1539 :
1540 0 : int __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 0 : 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 0 : return 0;
1551 : }
1552 : EXPORT_SYMBOL(down_read_killable);
1553 :
1554 : /*
1555 : * trylock for reading -- returns 1 if successful, 0 if contention
1556 : */
1557 5 : int down_read_trylock(struct rw_semaphore *sem)
1558 : {
1559 5 : int ret = __down_read_trylock(sem);
1560 :
1561 : if (ret == 1)
1562 : rwsem_acquire_read(&sem->dep_map, 0, 1, _RET_IP_);
1563 5 : return ret;
1564 : }
1565 : EXPORT_SYMBOL(down_read_trylock);
1566 :
1567 : /*
1568 : * lock for writing
1569 : */
1570 25971 : void __sched down_write(struct rw_semaphore *sem)
1571 : {
1572 : might_sleep();
1573 : rwsem_acquire(&sem->dep_map, 0, 0, _RET_IP_);
1574 25971 : LOCK_CONTENDED(sem, __down_write_trylock, __down_write);
1575 25971 : }
1576 : EXPORT_SYMBOL(down_write);
1577 :
1578 : /*
1579 : * lock for writing
1580 : */
1581 0 : int __sched down_write_killable(struct rw_semaphore *sem)
1582 : {
1583 : might_sleep();
1584 : rwsem_acquire(&sem->dep_map, 0, 0, _RET_IP_);
1585 :
1586 0 : 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 0 : return 0;
1593 : }
1594 : EXPORT_SYMBOL(down_write_killable);
1595 :
1596 : /*
1597 : * trylock for writing -- returns 1 if successful, 0 if contention
1598 : */
1599 0 : int down_write_trylock(struct rw_semaphore *sem)
1600 : {
1601 0 : int ret = __down_write_trylock(sem);
1602 :
1603 : if (ret == 1)
1604 : rwsem_acquire(&sem->dep_map, 0, 1, _RET_IP_);
1605 :
1606 0 : return ret;
1607 : }
1608 : EXPORT_SYMBOL(down_write_trylock);
1609 :
1610 : /*
1611 : * release a read lock
1612 : */
1613 607 : void up_read(struct rw_semaphore *sem)
1614 : {
1615 : rwsem_release(&sem->dep_map, _RET_IP_);
1616 607 : __up_read(sem);
1617 607 : }
1618 : EXPORT_SYMBOL(up_read);
1619 :
1620 : /*
1621 : * release a write lock
1622 : */
1623 25971 : void up_write(struct rw_semaphore *sem)
1624 : {
1625 : rwsem_release(&sem->dep_map, _RET_IP_);
1626 25971 : __up_write(sem);
1627 25971 : }
1628 : EXPORT_SYMBOL(up_write);
1629 :
1630 : /*
1631 : * downgrade write lock to read lock
1632 : */
1633 0 : void downgrade_write(struct rw_semaphore *sem)
1634 : {
1635 : lock_downgrade(&sem->dep_map, _RET_IP_);
1636 0 : __downgrade_write(sem);
1637 0 : }
1638 : EXPORT_SYMBOL(downgrade_write);
1639 :
1640 : #ifdef CONFIG_DEBUG_LOCK_ALLOC
1641 :
1642 : void 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 : }
1648 : EXPORT_SYMBOL(down_read_nested);
1649 :
1650 : int 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 : }
1662 : EXPORT_SYMBOL(down_read_killable_nested);
1663 :
1664 : void _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 : }
1670 : EXPORT_SYMBOL(_down_write_nest_lock);
1671 :
1672 : void 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 : }
1684 : EXPORT_SYMBOL(down_read_non_owner);
1685 :
1686 : void 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 : }
1692 : EXPORT_SYMBOL(down_write_nested);
1693 :
1694 : int __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 : }
1707 : EXPORT_SYMBOL(down_write_killable_nested);
1708 :
1709 : void 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 : }
1714 : EXPORT_SYMBOL(up_read_non_owner);
1715 :
1716 : #endif
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