1 /*-
2 * Copyright (c) 2006 John Baldwin <jhb@FreeBSD.org>
3 * All rights reserved.
4 *
5 * Redistribution and use in source and binary forms, with or without
6 * modification, are permitted provided that the following conditions
7 * are met:
8 * 1. Redistributions of source code must retain the above copyright
9 * notice, this list of conditions and the following disclaimer.
10 * 2. Redistributions in binary form must reproduce the above copyright
11 * notice, this list of conditions and the following disclaimer in the
12 * documentation and/or other materials provided with the distribution.
13 * 3. Neither the name of the author nor the names of any co-contributors
14 * may be used to endorse or promote products derived from this software
15 * without specific prior written permission.
16 *
17 * THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
18 * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
19 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
20 * ARE DISCLAIMED. IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
21 * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
22 * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
23 * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
24 * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
25 * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
26 * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
27 * SUCH DAMAGE.
28 */
29
30 /*
31 * Machine independent bits of reader/writer lock implementation.
32 */
33
34 #include <sys/cdefs.h>
35 __FBSDID("$FreeBSD: releng/7.4/sys/kern/kern_rwlock.c 197984 2009-10-12 15:56:27Z attilio $");
36
37 #include "opt_ddb.h"
38 #include "opt_no_adaptive_rwlocks.h"
39
40 #include <sys/param.h>
41 #include <sys/ktr.h>
42 #include <sys/lock.h>
43 #include <sys/mutex.h>
44 #include <sys/proc.h>
45 #include <sys/rwlock.h>
46 #include <sys/systm.h>
47 #include <sys/turnstile.h>
48
49 #include <machine/cpu.h>
50
51 CTASSERT((RW_RECURSE & LO_CLASSFLAGS) == RW_RECURSE);
52
53 #if defined(SMP) && !defined(NO_ADAPTIVE_RWLOCKS)
54 #define ADAPTIVE_RWLOCKS
55 #endif
56
57 #ifdef DDB
58 #include <ddb/ddb.h>
59
60 static void db_show_rwlock(struct lock_object *lock);
61 #endif
62 static void lock_rw(struct lock_object *lock, int how);
63 static int unlock_rw(struct lock_object *lock);
64
65 struct lock_class lock_class_rw = {
66 .lc_name = "rw",
67 .lc_flags = LC_SLEEPLOCK | LC_RECURSABLE | LC_UPGRADABLE,
68 #ifdef DDB
69 .lc_ddb_show = db_show_rwlock,
70 #endif
71 .lc_lock = lock_rw,
72 .lc_unlock = unlock_rw,
73 };
74
75 /*
76 * Return a pointer to the owning thread if the lock is write-locked or
77 * NULL if the lock is unlocked or read-locked.
78 */
79 #define rw_wowner(rw) \
80 ((rw)->rw_lock & RW_LOCK_READ ? NULL : \
81 (struct thread *)RW_OWNER((rw)->rw_lock))
82
83 /*
84 * Returns if a write owner is recursed. Write ownership is not assured
85 * here and should be previously checked.
86 */
87 #define rw_recursed(rw) ((rw)->rw_recurse != 0)
88
89 /*
90 * Return true if curthread helds the lock.
91 */
92 #define rw_wlocked(rw) (rw_wowner((rw)) == curthread)
93
94 /*
95 * Return a pointer to the owning thread for this lock who should receive
96 * any priority lent by threads that block on this lock. Currently this
97 * is identical to rw_wowner().
98 */
99 #define rw_owner(rw) rw_wowner(rw)
100
101 #ifndef INVARIANTS
102 #define _rw_assert(rw, what, file, line)
103 #endif
104
105 void
106 lock_rw(struct lock_object *lock, int how)
107 {
108 struct rwlock *rw;
109
110 rw = (struct rwlock *)lock;
111 if (how)
112 rw_wlock(rw);
113 else
114 rw_rlock(rw);
115 }
116
117 int
118 unlock_rw(struct lock_object *lock)
119 {
120 struct rwlock *rw;
121
122 rw = (struct rwlock *)lock;
123 rw_assert(rw, RA_LOCKED | LA_NOTRECURSED);
124 if (rw->rw_lock & RW_LOCK_READ) {
125 rw_runlock(rw);
126 return (0);
127 } else {
128 rw_wunlock(rw);
129 return (1);
130 }
131 }
132
133 void
134 rw_init_flags(struct rwlock *rw, const char *name, int opts)
135 {
136 int flags;
137
138 MPASS((opts & ~(RW_DUPOK | RW_NOPROFILE | RW_NOWITNESS | RW_QUIET |
139 RW_RECURSE)) == 0);
140 ASSERT_ATOMIC_LOAD_PTR(rw->rw_lock,
141 ("%s: rw_lock not aligned for %s:%p", __func__, name,
142 &rw->rw_lock));
143
144 flags = LO_UPGRADABLE | LO_RECURSABLE;
145 if (opts & RW_DUPOK)
146 flags |= LO_DUPOK;
147 if (opts & RW_NOPROFILE)
148 flags |= LO_NOPROFILE;
149 if (!(opts & RW_NOWITNESS))
150 flags |= LO_WITNESS;
151 if (opts & RW_QUIET)
152 flags |= LO_QUIET;
153 flags |= opts & RW_RECURSE;
154
155 rw->rw_lock = RW_UNLOCKED;
156 rw->rw_recurse = 0;
157 lock_init(&rw->lock_object, &lock_class_rw, name, NULL, flags);
158 }
159
160 void
161 rw_destroy(struct rwlock *rw)
162 {
163
164 KASSERT(rw->rw_lock == RW_UNLOCKED, ("rw lock not unlocked"));
165 KASSERT(rw->rw_recurse == 0, ("rw lock still recursed"));
166 rw->rw_lock = RW_DESTROYED;
167 lock_destroy(&rw->lock_object);
168 }
169
170 void
171 rw_sysinit(void *arg)
172 {
173 struct rw_args *args = arg;
174
175 rw_init(args->ra_rw, args->ra_desc);
176 }
177
178 int
179 rw_wowned(struct rwlock *rw)
180 {
181
182 return (rw_wowner(rw) == curthread);
183 }
184
185 void
186 _rw_wlock(struct rwlock *rw, const char *file, int line)
187 {
188
189 MPASS(curthread != NULL);
190 KASSERT(rw->rw_lock != RW_DESTROYED,
191 ("rw_wlock() of destroyed rwlock @ %s:%d", file, line));
192 WITNESS_CHECKORDER(&rw->lock_object, LOP_NEWORDER | LOP_EXCLUSIVE, file,
193 line);
194 __rw_wlock(rw, curthread, file, line);
195 LOCK_LOG_LOCK("WLOCK", &rw->lock_object, 0, rw->rw_recurse, file, line);
196 WITNESS_LOCK(&rw->lock_object, LOP_EXCLUSIVE, file, line);
197 curthread->td_locks++;
198 }
199
200 int
201 _rw_try_wlock(struct rwlock *rw, const char *file, int line)
202 {
203 int rval;
204
205 KASSERT(rw->rw_lock != RW_DESTROYED,
206 ("rw_try_wlock() of destroyed rwlock @ %s:%d", file, line));
207
208 if (rw_wlocked(rw) && (rw->lock_object.lo_flags & RW_RECURSE) != 0) {
209 rw->rw_recurse++;
210 atomic_set_ptr(&rw->rw_lock, RW_LOCK_RECURSED);
211 rval = 1;
212 } else
213 rval = atomic_cmpset_acq_ptr(&rw->rw_lock, RW_UNLOCKED,
214 (uintptr_t)curthread);
215
216 LOCK_LOG_TRY("WLOCK", &rw->lock_object, 0, rval, file, line);
217 if (rval) {
218 WITNESS_LOCK(&rw->lock_object, LOP_EXCLUSIVE | LOP_TRYLOCK,
219 file, line);
220 curthread->td_locks++;
221 }
222 return (rval);
223 }
224
225 void
226 _rw_wunlock(struct rwlock *rw, const char *file, int line)
227 {
228
229 MPASS(curthread != NULL);
230 KASSERT(rw->rw_lock != RW_DESTROYED,
231 ("rw_wunlock() of destroyed rwlock @ %s:%d", file, line));
232 _rw_assert(rw, RA_WLOCKED, file, line);
233 curthread->td_locks--;
234 WITNESS_UNLOCK(&rw->lock_object, LOP_EXCLUSIVE, file, line);
235 LOCK_LOG_LOCK("WUNLOCK", &rw->lock_object, 0, rw->rw_recurse, file,
236 line);
237 if (!rw_recursed(rw))
238 lock_profile_release_lock(&rw->lock_object);
239 __rw_wunlock(rw, curthread, file, line);
240 }
241
242 void
243 _rw_rlock(struct rwlock *rw, const char *file, int line)
244 {
245 struct turnstile *ts;
246 #ifdef ADAPTIVE_RWLOCKS
247 volatile struct thread *owner;
248 #endif
249 #ifdef LOCK_PROFILING_SHARED
250 uint64_t waittime = 0;
251 int contested = 0;
252 #endif
253 uintptr_t x;
254
255 KASSERT(rw->rw_lock != RW_DESTROYED,
256 ("rw_rlock() of destroyed rwlock @ %s:%d", file, line));
257 KASSERT(rw_wowner(rw) != curthread,
258 ("%s (%s): wlock already held @ %s:%d", __func__,
259 rw->lock_object.lo_name, file, line));
260 WITNESS_CHECKORDER(&rw->lock_object, LOP_NEWORDER, file, line);
261
262 /*
263 * Note that we don't make any attempt to try to block read
264 * locks once a writer has blocked on the lock. The reason is
265 * that we currently allow for read locks to recurse and we
266 * don't keep track of all the holders of read locks. Thus, if
267 * we were to block readers once a writer blocked and a reader
268 * tried to recurse on their reader lock after a writer had
269 * blocked we would end up in a deadlock since the reader would
270 * be blocked on the writer, and the writer would be blocked
271 * waiting for the reader to release its original read lock.
272 */
273 for (;;) {
274 /*
275 * Handle the easy case. If no other thread has a write
276 * lock, then try to bump up the count of read locks. Note
277 * that we have to preserve the current state of the
278 * RW_LOCK_WRITE_WAITERS flag. If we fail to acquire a
279 * read lock, then rw_lock must have changed, so restart
280 * the loop. Note that this handles the case of a
281 * completely unlocked rwlock since such a lock is encoded
282 * as a read lock with no waiters.
283 */
284 x = rw->rw_lock;
285 if (x & RW_LOCK_READ) {
286
287 /*
288 * The RW_LOCK_READ_WAITERS flag should only be set
289 * if another thread currently holds a write lock,
290 * and in that case RW_LOCK_READ should be clear.
291 */
292 MPASS((x & RW_LOCK_READ_WAITERS) == 0);
293 if (atomic_cmpset_acq_ptr(&rw->rw_lock, x,
294 x + RW_ONE_READER)) {
295 #ifdef LOCK_PROFILING_SHARED
296 if (RW_READERS(x) == 0)
297 lock_profile_obtain_lock_success(
298 &rw->lock_object, contested,
299 waittime, file, line);
300 #endif
301 if (LOCK_LOG_TEST(&rw->lock_object, 0))
302 CTR4(KTR_LOCK,
303 "%s: %p succeed %p -> %p", __func__,
304 rw, (void *)x,
305 (void *)(x + RW_ONE_READER));
306 break;
307 }
308 cpu_spinwait();
309 continue;
310 }
311
312 #ifdef ADAPTIVE_RWLOCKS
313 /*
314 * If the owner is running on another CPU, spin until
315 * the owner stops running or the state of the lock
316 * changes.
317 */
318 owner = (struct thread *)RW_OWNER(x);
319 if (TD_IS_RUNNING(owner)) {
320 if (LOCK_LOG_TEST(&rw->lock_object, 0))
321 CTR3(KTR_LOCK, "%s: spinning on %p held by %p",
322 __func__, rw, owner);
323 #ifdef LOCK_PROFILING_SHARED
324 lock_profile_obtain_lock_failed(&rw->lock_object,
325 &contested, &waittime);
326 #endif
327 while ((struct thread*)RW_OWNER(rw->rw_lock) == owner &&
328 TD_IS_RUNNING(owner))
329 cpu_spinwait();
330 continue;
331 }
332 #endif
333
334 /*
335 * Okay, now it's the hard case. Some other thread already
336 * has a write lock, so acquire the turnstile lock so we can
337 * begin the process of blocking.
338 */
339 ts = turnstile_trywait(&rw->lock_object);
340
341 /*
342 * The lock might have been released while we spun, so
343 * recheck its state and restart the loop if there is no
344 * longer a write lock.
345 */
346 x = rw->rw_lock;
347 if (x & RW_LOCK_READ) {
348 turnstile_cancel(ts);
349 cpu_spinwait();
350 continue;
351 }
352
353 #ifdef ADAPTIVE_RWLOCKS
354 /*
355 * If the current owner of the lock is executing on another
356 * CPU quit the hard path and try to spin.
357 */
358 owner = (struct thread *)RW_OWNER(x);
359 if (TD_IS_RUNNING(owner)) {
360 turnstile_cancel(ts);
361 cpu_spinwait();
362 continue;
363 }
364 #endif
365
366 /*
367 * Ok, it's still a write lock. If the RW_LOCK_READ_WAITERS
368 * flag is already set, then we can go ahead and block. If
369 * it is not set then try to set it. If we fail to set it
370 * drop the turnstile lock and restart the loop.
371 */
372 if (!(x & RW_LOCK_READ_WAITERS)) {
373 if (!atomic_cmpset_ptr(&rw->rw_lock, x,
374 x | RW_LOCK_READ_WAITERS)) {
375 turnstile_cancel(ts);
376 cpu_spinwait();
377 continue;
378 }
379 if (LOCK_LOG_TEST(&rw->lock_object, 0))
380 CTR2(KTR_LOCK, "%s: %p set read waiters flag",
381 __func__, rw);
382 }
383
384 /*
385 * We were unable to acquire the lock and the read waiters
386 * flag is set, so we must block on the turnstile.
387 */
388 if (LOCK_LOG_TEST(&rw->lock_object, 0))
389 CTR2(KTR_LOCK, "%s: %p blocking on turnstile", __func__,
390 rw);
391 #ifdef LOCK_PROFILING_SHARED
392 lock_profile_obtain_lock_failed(&rw->lock_object, &contested,
393 &waittime);
394 #endif
395 turnstile_wait(ts, rw_owner(rw), TS_SHARED_QUEUE);
396 if (LOCK_LOG_TEST(&rw->lock_object, 0))
397 CTR2(KTR_LOCK, "%s: %p resuming from turnstile",
398 __func__, rw);
399 }
400
401 /*
402 * TODO: acquire "owner of record" here. Here be turnstile dragons
403 * however. turnstiles don't like owners changing between calls to
404 * turnstile_wait() currently.
405 */
406
407 LOCK_LOG_LOCK("RLOCK", &rw->lock_object, 0, 0, file, line);
408 WITNESS_LOCK(&rw->lock_object, 0, file, line);
409 curthread->td_locks++;
410 }
411
412 int
413 _rw_try_rlock(struct rwlock *rw, const char *file, int line)
414 {
415 uintptr_t x;
416
417 for (;;) {
418 x = rw->rw_lock;
419 KASSERT(rw->rw_lock != RW_DESTROYED,
420 ("rw_try_rlock() of destroyed rwlock @ %s:%d", file, line));
421 if (!(x & RW_LOCK_READ))
422 break;
423 if (atomic_cmpset_acq_ptr(&rw->rw_lock, x, x + RW_ONE_READER)) {
424 LOCK_LOG_TRY("RLOCK", &rw->lock_object, 0, 1, file,
425 line);
426 WITNESS_LOCK(&rw->lock_object, LOP_TRYLOCK, file, line);
427 curthread->td_locks++;
428 return (1);
429 }
430 }
431
432 LOCK_LOG_TRY("RLOCK", &rw->lock_object, 0, 0, file, line);
433 return (0);
434 }
435
436 void
437 _rw_runlock(struct rwlock *rw, const char *file, int line)
438 {
439 struct turnstile *ts;
440 uintptr_t x;
441
442 KASSERT(rw->rw_lock != RW_DESTROYED,
443 ("rw_runlock() of destroyed rwlock @ %s:%d", file, line));
444 _rw_assert(rw, RA_RLOCKED, file, line);
445 curthread->td_locks--;
446 WITNESS_UNLOCK(&rw->lock_object, 0, file, line);
447 LOCK_LOG_LOCK("RUNLOCK", &rw->lock_object, 0, 0, file, line);
448
449 /* TODO: drop "owner of record" here. */
450
451 for (;;) {
452 /*
453 * See if there is more than one read lock held. If so,
454 * just drop one and return.
455 */
456 x = rw->rw_lock;
457 if (RW_READERS(x) > 1) {
458 if (atomic_cmpset_rel_ptr(&rw->rw_lock, x,
459 x - RW_ONE_READER)) {
460 if (LOCK_LOG_TEST(&rw->lock_object, 0))
461 CTR4(KTR_LOCK,
462 "%s: %p succeeded %p -> %p",
463 __func__, rw, (void *)x,
464 (void *)(x - RW_ONE_READER));
465 break;
466 }
467 continue;
468 }
469
470
471 /*
472 * We should never have read waiters while at least one
473 * thread holds a read lock. (See note above)
474 */
475 KASSERT(!(x & RW_LOCK_READ_WAITERS),
476 ("%s: waiting readers", __func__));
477 #ifdef LOCK_PROFILING_SHARED
478 lock_profile_release_lock(&rw->lock_object);
479 #endif
480
481 /*
482 * If there aren't any waiters for a write lock, then try
483 * to drop it quickly.
484 */
485 if (!(x & RW_LOCK_WRITE_WAITERS)) {
486
487 /*
488 * There shouldn't be any flags set and we should
489 * be the only read lock. If we fail to release
490 * the single read lock, then another thread might
491 * have just acquired a read lock, so go back up
492 * to the multiple read locks case.
493 */
494 MPASS(x == RW_READERS_LOCK(1));
495 if (atomic_cmpset_rel_ptr(&rw->rw_lock,
496 RW_READERS_LOCK(1), RW_UNLOCKED)) {
497 if (LOCK_LOG_TEST(&rw->lock_object, 0))
498 CTR2(KTR_LOCK, "%s: %p last succeeded",
499 __func__, rw);
500 break;
501 }
502 continue;
503 }
504
505 /*
506 * There should just be one reader with one or more
507 * writers waiting.
508 */
509 MPASS(x == (RW_READERS_LOCK(1) | RW_LOCK_WRITE_WAITERS));
510
511 /*
512 * Ok, we know we have a waiting writer and we think we
513 * are the last reader, so grab the turnstile lock.
514 */
515 turnstile_chain_lock(&rw->lock_object);
516
517 /*
518 * Try to drop our lock leaving the lock in a unlocked
519 * state.
520 *
521 * If you wanted to do explicit lock handoff you'd have to
522 * do it here. You'd also want to use turnstile_signal()
523 * and you'd have to handle the race where a higher
524 * priority thread blocks on the write lock before the
525 * thread you wakeup actually runs and have the new thread
526 * "steal" the lock. For now it's a lot simpler to just
527 * wakeup all of the waiters.
528 *
529 * As above, if we fail, then another thread might have
530 * acquired a read lock, so drop the turnstile lock and
531 * restart.
532 */
533 if (!atomic_cmpset_rel_ptr(&rw->rw_lock,
534 RW_READERS_LOCK(1) | RW_LOCK_WRITE_WAITERS, RW_UNLOCKED)) {
535 turnstile_chain_unlock(&rw->lock_object);
536 continue;
537 }
538 if (LOCK_LOG_TEST(&rw->lock_object, 0))
539 CTR2(KTR_LOCK, "%s: %p last succeeded with waiters",
540 __func__, rw);
541
542 /*
543 * Ok. The lock is released and all that's left is to
544 * wake up the waiters. Note that the lock might not be
545 * free anymore, but in that case the writers will just
546 * block again if they run before the new lock holder(s)
547 * release the lock.
548 */
549 ts = turnstile_lookup(&rw->lock_object);
550 MPASS(ts != NULL);
551 turnstile_broadcast(ts, TS_EXCLUSIVE_QUEUE);
552 turnstile_unpend(ts, TS_SHARED_LOCK);
553 turnstile_chain_unlock(&rw->lock_object);
554 break;
555 }
556 }
557
558 /*
559 * This function is called when we are unable to obtain a write lock on the
560 * first try. This means that at least one other thread holds either a
561 * read or write lock.
562 */
563 void
564 _rw_wlock_hard(struct rwlock *rw, uintptr_t tid, const char *file, int line)
565 {
566 struct turnstile *ts;
567 #ifdef ADAPTIVE_RWLOCKS
568 volatile struct thread *owner;
569 #endif
570 uint64_t waittime = 0;
571 uintptr_t v;
572 int contested = 0;
573
574 if (rw_wlocked(rw)) {
575 KASSERT(rw->lock_object.lo_flags & RW_RECURSE,
576 ("%s: recursing but non-recursive rw %s @ %s:%d\n",
577 __func__, rw->lock_object.lo_name, file, line));
578 rw->rw_recurse++;
579 atomic_set_ptr(&rw->rw_lock, RW_LOCK_RECURSED);
580 if (LOCK_LOG_TEST(&rw->lock_object, 0))
581 CTR2(KTR_LOCK, "%s: %p recursing", __func__, rw);
582 return;
583 }
584
585 if (LOCK_LOG_TEST(&rw->lock_object, 0))
586 CTR5(KTR_LOCK, "%s: %s contested (lock=%p) at %s:%d", __func__,
587 rw->lock_object.lo_name, (void *)rw->rw_lock, file, line);
588
589 while (!_rw_write_lock(rw, tid)) {
590 #ifdef ADAPTIVE_RWLOCKS
591 /*
592 * If the lock is write locked and the owner is
593 * running on another CPU, spin until the owner stops
594 * running or the state of the lock changes.
595 */
596 v = rw->rw_lock;
597 owner = (struct thread *)RW_OWNER(v);
598 if (!(v & RW_LOCK_READ) && TD_IS_RUNNING(owner)) {
599 if (LOCK_LOG_TEST(&rw->lock_object, 0))
600 CTR3(KTR_LOCK, "%s: spinning on %p held by %p",
601 __func__, rw, owner);
602 lock_profile_obtain_lock_failed(&rw->lock_object,
603 &contested, &waittime);
604 while ((struct thread*)RW_OWNER(rw->rw_lock) == owner &&
605 TD_IS_RUNNING(owner))
606 cpu_spinwait();
607 continue;
608 }
609 #endif
610
611 ts = turnstile_trywait(&rw->lock_object);
612 v = rw->rw_lock;
613
614 /*
615 * If the lock was released while spinning on the
616 * turnstile chain lock, try again.
617 */
618 if (v == RW_UNLOCKED) {
619 turnstile_cancel(ts);
620 cpu_spinwait();
621 continue;
622 }
623
624 #ifdef ADAPTIVE_RWLOCKS
625 /*
626 * If the current owner of the lock is executing on another
627 * CPU quit the hard path and try to spin.
628 */
629 if (!(v & RW_LOCK_READ)) {
630 owner = (struct thread *)RW_OWNER(v);
631 if (TD_IS_RUNNING(owner)) {
632 turnstile_cancel(ts);
633 cpu_spinwait();
634 continue;
635 }
636 }
637 #endif
638
639 /*
640 * If the lock was released by a writer with both readers
641 * and writers waiting and a reader hasn't woken up and
642 * acquired the lock yet, rw_lock will be set to the
643 * value RW_UNLOCKED | RW_LOCK_WRITE_WAITERS. If we see
644 * that value, try to acquire it once. Note that we have
645 * to preserve the RW_LOCK_WRITE_WAITERS flag as there are
646 * other writers waiting still. If we fail, restart the
647 * loop.
648 */
649 if (v == (RW_UNLOCKED | RW_LOCK_WRITE_WAITERS)) {
650 if (atomic_cmpset_acq_ptr(&rw->rw_lock,
651 RW_UNLOCKED | RW_LOCK_WRITE_WAITERS,
652 tid | RW_LOCK_WRITE_WAITERS)) {
653 turnstile_claim(ts);
654 CTR2(KTR_LOCK, "%s: %p claimed by new writer",
655 __func__, rw);
656 break;
657 }
658 turnstile_cancel(ts);
659 cpu_spinwait();
660 continue;
661 }
662
663 /*
664 * If the RW_LOCK_WRITE_WAITERS flag isn't set, then try to
665 * set it. If we fail to set it, then loop back and try
666 * again.
667 */
668 if (!(v & RW_LOCK_WRITE_WAITERS)) {
669 if (!atomic_cmpset_ptr(&rw->rw_lock, v,
670 v | RW_LOCK_WRITE_WAITERS)) {
671 turnstile_cancel(ts);
672 cpu_spinwait();
673 continue;
674 }
675 if (LOCK_LOG_TEST(&rw->lock_object, 0))
676 CTR2(KTR_LOCK, "%s: %p set write waiters flag",
677 __func__, rw);
678 }
679
680 /*
681 * We were unable to acquire the lock and the write waiters
682 * flag is set, so we must block on the turnstile.
683 */
684 if (LOCK_LOG_TEST(&rw->lock_object, 0))
685 CTR2(KTR_LOCK, "%s: %p blocking on turnstile", __func__,
686 rw);
687 lock_profile_obtain_lock_failed(&rw->lock_object, &contested,
688 &waittime);
689 turnstile_wait(ts, rw_owner(rw), TS_EXCLUSIVE_QUEUE);
690 if (LOCK_LOG_TEST(&rw->lock_object, 0))
691 CTR2(KTR_LOCK, "%s: %p resuming from turnstile",
692 __func__, rw);
693 }
694 lock_profile_obtain_lock_success(&rw->lock_object, contested, waittime,
695 file, line);
696 }
697
698 /*
699 * This function is called if the first try at releasing a write lock failed.
700 * This means that one of the 2 waiter bits must be set indicating that at
701 * least one thread is waiting on this lock.
702 */
703 void
704 _rw_wunlock_hard(struct rwlock *rw, uintptr_t tid, const char *file, int line)
705 {
706 struct turnstile *ts;
707 uintptr_t v;
708 int queue;
709
710 if (rw_wlocked(rw) && rw_recursed(rw)) {
711 if ((--rw->rw_recurse) == 0)
712 atomic_clear_ptr(&rw->rw_lock, RW_LOCK_RECURSED);
713 if (LOCK_LOG_TEST(&rw->lock_object, 0))
714 CTR2(KTR_LOCK, "%s: %p unrecursing", __func__, rw);
715 return;
716 }
717
718 KASSERT(rw->rw_lock & (RW_LOCK_READ_WAITERS | RW_LOCK_WRITE_WAITERS),
719 ("%s: neither of the waiter flags are set", __func__));
720
721 if (LOCK_LOG_TEST(&rw->lock_object, 0))
722 CTR2(KTR_LOCK, "%s: %p contested", __func__, rw);
723
724 turnstile_chain_lock(&rw->lock_object);
725 ts = turnstile_lookup(&rw->lock_object);
726
727 MPASS(ts != NULL);
728
729 /*
730 * Use the same algo as sx locks for now. Prefer waking up shared
731 * waiters if we have any over writers. This is probably not ideal.
732 *
733 * 'v' is the value we are going to write back to rw_lock. If we
734 * have waiters on both queues, we need to preserve the state of
735 * the waiter flag for the queue we don't wake up. For now this is
736 * hardcoded for the algorithm mentioned above.
737 *
738 * In the case of both readers and writers waiting we wakeup the
739 * readers but leave the RW_LOCK_WRITE_WAITERS flag set. If a
740 * new writer comes in before a reader it will claim the lock up
741 * above. There is probably a potential priority inversion in
742 * there that could be worked around either by waking both queues
743 * of waiters or doing some complicated lock handoff gymnastics.
744 */
745 v = RW_UNLOCKED;
746 if (rw->rw_lock & RW_LOCK_READ_WAITERS) {
747 queue = TS_SHARED_QUEUE;
748 v |= (rw->rw_lock & RW_LOCK_WRITE_WAITERS);
749 } else
750 queue = TS_EXCLUSIVE_QUEUE;
751
752 /* Wake up all waiters for the specific queue. */
753 if (LOCK_LOG_TEST(&rw->lock_object, 0))
754 CTR3(KTR_LOCK, "%s: %p waking up %s waiters", __func__, rw,
755 queue == TS_SHARED_QUEUE ? "read" : "write");
756 turnstile_broadcast(ts, queue);
757 atomic_store_rel_ptr(&rw->rw_lock, v);
758 turnstile_unpend(ts, TS_EXCLUSIVE_LOCK);
759 turnstile_chain_unlock(&rw->lock_object);
760 }
761
762 /*
763 * Attempt to do a non-blocking upgrade from a read lock to a write
764 * lock. This will only succeed if this thread holds a single read
765 * lock. Returns true if the upgrade succeeded and false otherwise.
766 */
767 int
768 _rw_try_upgrade(struct rwlock *rw, const char *file, int line)
769 {
770 uintptr_t v, tid;
771 struct turnstile *ts;
772 int success;
773
774 KASSERT(rw->rw_lock != RW_DESTROYED,
775 ("rw_try_upgrade() of destroyed rwlock @ %s:%d", file, line));
776 _rw_assert(rw, RA_RLOCKED, file, line);
777
778 /*
779 * Attempt to switch from one reader to a writer. If there
780 * are any write waiters, then we will have to lock the
781 * turnstile first to prevent races with another writer
782 * calling turnstile_wait() before we have claimed this
783 * turnstile. So, do the simple case of no waiters first.
784 */
785 tid = (uintptr_t)curthread;
786 if (!(rw->rw_lock & RW_LOCK_WRITE_WAITERS)) {
787 success = atomic_cmpset_ptr(&rw->rw_lock, RW_READERS_LOCK(1),
788 tid);
789 goto out;
790 }
791
792 /*
793 * Ok, we think we have write waiters, so lock the
794 * turnstile.
795 */
796 ts = turnstile_trywait(&rw->lock_object);
797
798 /*
799 * Try to switch from one reader to a writer again. This time
800 * we honor the current state of the RW_LOCK_WRITE_WAITERS
801 * flag. If we obtain the lock with the flag set, then claim
802 * ownership of the turnstile.
803 */
804 v = rw->rw_lock & RW_LOCK_WRITE_WAITERS;
805 success = atomic_cmpset_ptr(&rw->rw_lock, RW_READERS_LOCK(1) | v,
806 tid | v);
807 if (success && v)
808 turnstile_claim(ts);
809 else
810 turnstile_cancel(ts);
811 out:
812 LOCK_LOG_TRY("WUPGRADE", &rw->lock_object, 0, success, file, line);
813 if (success)
814 WITNESS_UPGRADE(&rw->lock_object, LOP_EXCLUSIVE | LOP_TRYLOCK,
815 file, line);
816 return (success);
817 }
818
819 /*
820 * Downgrade a write lock into a single read lock.
821 */
822 void
823 _rw_downgrade(struct rwlock *rw, const char *file, int line)
824 {
825 struct turnstile *ts;
826 uintptr_t tid, v;
827
828 KASSERT(rw->rw_lock != RW_DESTROYED,
829 ("rw_downgrade() of destroyed rwlock @ %s:%d", file, line));
830 _rw_assert(rw, RA_WLOCKED | RA_NOTRECURSED, file, line);
831 #ifndef INVARIANTS
832 if (rw_recursed(rw))
833 panic("downgrade of a recursed lock");
834 #endif
835
836 WITNESS_DOWNGRADE(&rw->lock_object, 0, file, line);
837
838 /*
839 * Convert from a writer to a single reader. First we handle
840 * the easy case with no waiters. If there are any waiters, we
841 * lock the turnstile, "disown" the lock, and awaken any read
842 * waiters.
843 */
844 tid = (uintptr_t)curthread;
845 if (atomic_cmpset_rel_ptr(&rw->rw_lock, tid, RW_READERS_LOCK(1)))
846 goto out;
847
848 /*
849 * Ok, we think we have waiters, so lock the turnstile so we can
850 * read the waiter flags without any races.
851 */
852 turnstile_chain_lock(&rw->lock_object);
853 v = rw->rw_lock;
854 MPASS(v & (RW_LOCK_READ_WAITERS | RW_LOCK_WRITE_WAITERS));
855
856 /*
857 * Downgrade from a write lock while preserving
858 * RW_LOCK_WRITE_WAITERS and give up ownership of the
859 * turnstile. If there are any read waiters, wake them up.
860 */
861 ts = turnstile_lookup(&rw->lock_object);
862 MPASS(ts != NULL);
863 if (v & RW_LOCK_READ_WAITERS)
864 turnstile_broadcast(ts, TS_SHARED_QUEUE);
865 atomic_store_rel_ptr(&rw->rw_lock, RW_READERS_LOCK(1) |
866 (v & RW_LOCK_WRITE_WAITERS));
867 if (v & RW_LOCK_READ_WAITERS)
868 turnstile_unpend(ts, TS_EXCLUSIVE_LOCK);
869 else
870 turnstile_disown(ts);
871 turnstile_chain_unlock(&rw->lock_object);
872 out:
873 LOCK_LOG_LOCK("WDOWNGRADE", &rw->lock_object, 0, 0, file, line);
874 }
875
876 #ifdef INVARIANT_SUPPORT
877 #ifndef INVARIANTS
878 #undef _rw_assert
879 #endif
880
881 /*
882 * In the non-WITNESS case, rw_assert() can only detect that at least
883 * *some* thread owns an rlock, but it cannot guarantee that *this*
884 * thread owns an rlock.
885 */
886 void
887 _rw_assert(struct rwlock *rw, int what, const char *file, int line)
888 {
889
890 if (panicstr != NULL)
891 return;
892 switch (what) {
893 case RA_LOCKED:
894 case RA_LOCKED | RA_RECURSED:
895 case RA_LOCKED | RA_NOTRECURSED:
896 case RA_RLOCKED:
897 #ifdef WITNESS
898 witness_assert(&rw->lock_object, what, file, line);
899 #else
900 /*
901 * If some other thread has a write lock or we have one
902 * and are asserting a read lock, fail. Also, if no one
903 * has a lock at all, fail.
904 */
905 if (rw->rw_lock == RW_UNLOCKED ||
906 (!(rw->rw_lock & RW_LOCK_READ) && (what == RA_RLOCKED ||
907 rw_wowner(rw) != curthread)))
908 panic("Lock %s not %slocked @ %s:%d\n",
909 rw->lock_object.lo_name, (what == RA_RLOCKED) ?
910 "read " : "", file, line);
911
912 if (!(rw->rw_lock & RW_LOCK_READ)) {
913 if (rw_recursed(rw)) {
914 if (what & RA_NOTRECURSED)
915 panic("Lock %s recursed @ %s:%d\n",
916 rw->lock_object.lo_name, file,
917 line);
918 } else if (what & RA_RECURSED)
919 panic("Lock %s not recursed @ %s:%d\n",
920 rw->lock_object.lo_name, file, line);
921 }
922 #endif
923 break;
924 case RA_WLOCKED:
925 case RA_WLOCKED | RA_RECURSED:
926 case RA_WLOCKED | RA_NOTRECURSED:
927 if (rw_wowner(rw) != curthread)
928 panic("Lock %s not exclusively locked @ %s:%d\n",
929 rw->lock_object.lo_name, file, line);
930 if (rw_recursed(rw)) {
931 if (what & RA_NOTRECURSED)
932 panic("Lock %s recursed @ %s:%d\n",
933 rw->lock_object.lo_name, file, line);
934 } else if (what & RA_RECURSED)
935 panic("Lock %s not recursed @ %s:%d\n",
936 rw->lock_object.lo_name, file, line);
937 break;
938 case RA_UNLOCKED:
939 #ifdef WITNESS
940 witness_assert(&rw->lock_object, what, file, line);
941 #else
942 /*
943 * If we hold a write lock fail. We can't reliably check
944 * to see if we hold a read lock or not.
945 */
946 if (rw_wowner(rw) == curthread)
947 panic("Lock %s exclusively locked @ %s:%d\n",
948 rw->lock_object.lo_name, file, line);
949 #endif
950 break;
951 default:
952 panic("Unknown rw lock assertion: %d @ %s:%d", what, file,
953 line);
954 }
955 }
956 #endif /* INVARIANT_SUPPORT */
957
958 #ifdef DDB
959 void
960 db_show_rwlock(struct lock_object *lock)
961 {
962 struct rwlock *rw;
963 struct thread *td;
964
965 rw = (struct rwlock *)lock;
966
967 db_printf(" state: ");
968 if (rw->rw_lock == RW_UNLOCKED)
969 db_printf("UNLOCKED\n");
970 else if (rw->rw_lock == RW_DESTROYED) {
971 db_printf("DESTROYED\n");
972 return;
973 } else if (rw->rw_lock & RW_LOCK_READ)
974 db_printf("RLOCK: %ju locks\n",
975 (uintmax_t)(RW_READERS(rw->rw_lock)));
976 else {
977 td = rw_wowner(rw);
978 db_printf("WLOCK: %p (tid %d, pid %d, \"%s\")\n", td,
979 td->td_tid, td->td_proc->p_pid, td->td_proc->p_comm);
980 if (rw_recursed(rw))
981 db_printf(" recursed: %u\n", rw->rw_recurse);
982 }
983 db_printf(" waiters: ");
984 switch (rw->rw_lock & (RW_LOCK_READ_WAITERS | RW_LOCK_WRITE_WAITERS)) {
985 case RW_LOCK_READ_WAITERS:
986 db_printf("readers\n");
987 break;
988 case RW_LOCK_WRITE_WAITERS:
989 db_printf("writers\n");
990 break;
991 case RW_LOCK_READ_WAITERS | RW_LOCK_WRITE_WAITERS:
992 db_printf("readers and writers\n");
993 break;
994 default:
995 db_printf("none\n");
996 break;
997 }
998 }
999
1000 #endif
Cache object: 9ea6a67d0996c980e3b7664ecf107982
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