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/6.4/sys/kern/kern_rwlock.c 172355 2007-09-27 20:24:56Z jhb $");
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
63 struct lock_class lock_class_rw = {
64 .lc_name = "rw",
65 .lc_flags = LC_SLEEPLOCK | LC_RECURSABLE | LC_UPGRADABLE,
66 #ifdef DDB
67 .lc_ddb_show = db_show_rwlock,
68 #endif
69 };
70
71 /*
72 * Return a pointer to the owning thread if the lock is write-locked or
73 * NULL if the lock is unlocked or read-locked.
74 */
75 #define rw_wowner(rw) \
76 ((rw)->rw_lock & RW_LOCK_READ ? NULL : \
77 (struct thread *)RW_OWNER((rw)->rw_lock))
78
79 /*
80 * Returns if a write owner is recursed. Write ownership is not assured
81 * here and should be previously checked.
82 */
83 #define rw_recursed(rw) ((rw)->rw_recurse != 0)
84
85 /*
86 * Return true if curthread helds the lock.
87 */
88 #define rw_wlocked(rw) (rw_wowner((rw)) == curthread)
89
90 /*
91 * Return a pointer to the owning thread for this lock who should receive
92 * any priority lent by threads that block on this lock. Currently this
93 * is identical to rw_wowner().
94 */
95 #define rw_owner(rw) rw_wowner(rw)
96
97 #ifndef INVARIANTS
98 #define _rw_assert(rw, what, file, line)
99 #endif
100
101 void
102 rw_init_flags(struct rwlock *rw, const char *name, int opts)
103 {
104 int flags;
105
106 MPASS((opts & ~(RW_DUPOK | RW_NOPROFILE | RW_NOWITNESS | RW_QUIET |
107 RW_RECURSE)) == 0);
108
109 flags = LO_UPGRADABLE | LO_RECURSABLE;
110 if (opts & RW_DUPOK)
111 flags |= LO_DUPOK;
112 if (!(opts & RW_NOWITNESS))
113 flags |= LO_WITNESS;
114 if (opts & RW_QUIET)
115 flags |= LO_QUIET;
116 flags |= opts & RW_RECURSE;
117
118 rw->rw_lock = RW_UNLOCKED;
119 rw->rw_recurse = 0;
120 lock_init(&rw->lock_object, &lock_class_rw, name, NULL, flags);
121 }
122
123 void
124 rw_destroy(struct rwlock *rw)
125 {
126
127 KASSERT(rw->rw_lock == RW_UNLOCKED, ("rw lock not unlocked"));
128 KASSERT(rw->rw_recurse == 0, ("rw lock still recursed"));
129 rw->rw_lock = RW_DESTROYED;
130 lock_destroy(&rw->lock_object);
131 }
132
133 void
134 rw_sysinit(void *arg)
135 {
136 struct rw_args *args = arg;
137
138 rw_init(args->ra_rw, args->ra_desc);
139 }
140
141 int
142 rw_wowned(struct rwlock *rw)
143 {
144
145 return (rw_wowner(rw) == curthread);
146 }
147
148 void
149 _rw_wlock(struct rwlock *rw, const char *file, int line)
150 {
151
152 MPASS(curthread != NULL);
153 KASSERT(rw->rw_lock != RW_DESTROYED,
154 ("rw_wlock() of destroyed rwlock @ %s:%d", file, line));
155 KASSERT(rw_wowner(rw) != curthread,
156 ("%s (%s): wlock already held @ %s:%d", __func__,
157 rw->lock_object.lo_name, file, line));
158 WITNESS_CHECKORDER(&rw->lock_object, LOP_NEWORDER | LOP_EXCLUSIVE, file,
159 line);
160 __rw_wlock(rw, curthread, file, line);
161 LOCK_LOG_LOCK("WLOCK", &rw->lock_object, 0, rw->rw_recurse, file, line);
162 WITNESS_LOCK(&rw->lock_object, LOP_EXCLUSIVE, file, line);
163 curthread->td_locks++;
164 }
165
166 void
167 _rw_wunlock(struct rwlock *rw, const char *file, int line)
168 {
169
170 MPASS(curthread != NULL);
171 KASSERT(rw->rw_lock != RW_DESTROYED,
172 ("rw_wunlock() of destroyed rwlock @ %s:%d", file, line));
173 _rw_assert(rw, RA_WLOCKED, file, line);
174 curthread->td_locks--;
175 WITNESS_UNLOCK(&rw->lock_object, LOP_EXCLUSIVE, file, line);
176 LOCK_LOG_LOCK("WUNLOCK", &rw->lock_object, 0, rw->rw_recurse, file,
177 line);
178 if (!rw_recursed(rw))
179 lock_profile_release_lock(&rw->lock_object);
180 __rw_wunlock(rw, curthread, file, line);
181 }
182
183 void
184 _rw_rlock(struct rwlock *rw, const char *file, int line)
185 {
186 #ifdef ADAPTIVE_RWLOCKS
187 volatile struct thread *owner;
188 #endif
189 //uint64_t waittime = 0; /* XXX: notsup */
190 //int contested = 0; /* XXX: notsup */
191 uintptr_t x;
192
193 KASSERT(rw->rw_lock != RW_DESTROYED,
194 ("rw_rlock() of destroyed rwlock @ %s:%d", file, line));
195 KASSERT(rw_wowner(rw) != curthread,
196 ("%s (%s): wlock already held @ %s:%d", __func__,
197 rw->lock_object.lo_name, file, line));
198 WITNESS_CHECKORDER(&rw->lock_object, LOP_NEWORDER, file, line);
199
200 /*
201 * Note that we don't make any attempt to try to block read
202 * locks once a writer has blocked on the lock. The reason is
203 * that we currently allow for read locks to recurse and we
204 * don't keep track of all the holders of read locks. Thus, if
205 * we were to block readers once a writer blocked and a reader
206 * tried to recurse on their reader lock after a writer had
207 * blocked we would end up in a deadlock since the reader would
208 * be blocked on the writer, and the writer would be blocked
209 * waiting for the reader to release its original read lock.
210 */
211 for (;;) {
212 /*
213 * Handle the easy case. If no other thread has a write
214 * lock, then try to bump up the count of read locks. Note
215 * that we have to preserve the current state of the
216 * RW_LOCK_WRITE_WAITERS flag. If we fail to acquire a
217 * read lock, then rw_lock must have changed, so restart
218 * the loop. Note that this handles the case of a
219 * completely unlocked rwlock since such a lock is encoded
220 * as a read lock with no waiters.
221 */
222 x = rw->rw_lock;
223 if (x & RW_LOCK_READ) {
224
225 /*
226 * The RW_LOCK_READ_WAITERS flag should only be set
227 * if another thread currently holds a write lock,
228 * and in that case RW_LOCK_READ should be clear.
229 */
230 MPASS((x & RW_LOCK_READ_WAITERS) == 0);
231 if (atomic_cmpset_acq_ptr(&rw->rw_lock, x,
232 x + RW_ONE_READER)) {
233 if (LOCK_LOG_TEST(&rw->lock_object, 0))
234 CTR4(KTR_LOCK,
235 "%s: %p succeed %p -> %p", __func__,
236 rw, (void *)x,
237 (void *)(x + RW_ONE_READER));
238 if (RW_READERS(x) == 0)
239 lock_profile_obtain_lock_success(
240 &rw->lock_object, contested, waittime,
241 file, line);
242 break;
243 }
244 cpu_spinwait();
245 continue;
246 }
247 lock_profile_obtain_lock_failed(&rw->lock_object, &contested,
248 &waittime);
249
250 /*
251 * Okay, now it's the hard case. Some other thread already
252 * has a write lock, so acquire the turnstile lock so we can
253 * begin the process of blocking.
254 */
255 turnstile_lock(&rw->lock_object);
256
257 /*
258 * The lock might have been released while we spun, so
259 * recheck its state and restart the loop if there is no
260 * longer a write lock.
261 */
262 x = rw->rw_lock;
263 if (x & RW_LOCK_READ) {
264 turnstile_release(&rw->lock_object);
265 cpu_spinwait();
266 continue;
267 }
268
269 /*
270 * Ok, it's still a write lock. If the RW_LOCK_READ_WAITERS
271 * flag is already set, then we can go ahead and block. If
272 * it is not set then try to set it. If we fail to set it
273 * drop the turnstile lock and restart the loop.
274 */
275 if (!(x & RW_LOCK_READ_WAITERS)) {
276 if (!atomic_cmpset_ptr(&rw->rw_lock, x,
277 x | RW_LOCK_READ_WAITERS)) {
278 turnstile_release(&rw->lock_object);
279 cpu_spinwait();
280 continue;
281 }
282 if (LOCK_LOG_TEST(&rw->lock_object, 0))
283 CTR2(KTR_LOCK, "%s: %p set read waiters flag",
284 __func__, rw);
285 }
286
287 #ifdef ADAPTIVE_RWLOCKS
288 /*
289 * If the owner is running on another CPU, spin until
290 * the owner stops running or the state of the lock
291 * changes.
292 */
293 owner = (struct thread *)RW_OWNER(x);
294 if (TD_IS_RUNNING(owner)) {
295 turnstile_release(&rw->lock_object);
296 if (LOCK_LOG_TEST(&rw->lock_object, 0))
297 CTR3(KTR_LOCK, "%s: spinning on %p held by %p",
298 __func__, rw, owner);
299 while ((struct thread*)RW_OWNER(rw->rw_lock)== owner &&
300 TD_IS_RUNNING(owner))
301 cpu_spinwait();
302 continue;
303 }
304 #endif
305
306 /*
307 * We were unable to acquire the lock and the read waiters
308 * flag is set, so we must block on the turnstile.
309 */
310 if (LOCK_LOG_TEST(&rw->lock_object, 0))
311 CTR2(KTR_LOCK, "%s: %p blocking on turnstile", __func__,
312 rw);
313 turnstile_wait(&rw->lock_object, rw_owner(rw),
314 TS_SHARED_QUEUE);
315 if (LOCK_LOG_TEST(&rw->lock_object, 0))
316 CTR2(KTR_LOCK, "%s: %p resuming from turnstile",
317 __func__, rw);
318 }
319
320 /*
321 * TODO: acquire "owner of record" here. Here be turnstile dragons
322 * however. turnstiles don't like owners changing between calls to
323 * turnstile_wait() currently.
324 */
325
326 LOCK_LOG_LOCK("RLOCK", &rw->lock_object, 0, 0, file, line);
327 WITNESS_LOCK(&rw->lock_object, 0, file, line);
328 curthread->td_locks++;
329 }
330
331 void
332 _rw_runlock(struct rwlock *rw, const char *file, int line)
333 {
334 struct turnstile *ts;
335 uintptr_t x;
336
337 KASSERT(rw->rw_lock != RW_DESTROYED,
338 ("rw_runlock() of destroyed rwlock @ %s:%d", file, line));
339 _rw_assert(rw, RA_RLOCKED, file, line);
340 curthread->td_locks--;
341 WITNESS_UNLOCK(&rw->lock_object, 0, file, line);
342 LOCK_LOG_LOCK("RUNLOCK", &rw->lock_object, 0, 0, file, line);
343
344 /* TODO: drop "owner of record" here. */
345
346 for (;;) {
347 /*
348 * See if there is more than one read lock held. If so,
349 * just drop one and return.
350 */
351 x = rw->rw_lock;
352 if (RW_READERS(x) > 1) {
353 if (atomic_cmpset_ptr(&rw->rw_lock, x,
354 x - RW_ONE_READER)) {
355 if (LOCK_LOG_TEST(&rw->lock_object, 0))
356 CTR4(KTR_LOCK,
357 "%s: %p succeeded %p -> %p",
358 __func__, rw, (void *)x,
359 (void *)(x - RW_ONE_READER));
360 break;
361 }
362 continue;
363 }
364
365
366 /*
367 * We should never have read waiters while at least one
368 * thread holds a read lock. (See note above)
369 */
370 KASSERT(!(x & RW_LOCK_READ_WAITERS),
371 ("%s: waiting readers", __func__));
372
373 /*
374 * If there aren't any waiters for a write lock, then try
375 * to drop it quickly.
376 */
377 if (!(x & RW_LOCK_WRITE_WAITERS)) {
378
379 /*
380 * There shouldn't be any flags set and we should
381 * be the only read lock. If we fail to release
382 * the single read lock, then another thread might
383 * have just acquired a read lock, so go back up
384 * to the multiple read locks case.
385 */
386 MPASS(x == RW_READERS_LOCK(1));
387 if (atomic_cmpset_ptr(&rw->rw_lock, RW_READERS_LOCK(1),
388 RW_UNLOCKED)) {
389 if (LOCK_LOG_TEST(&rw->lock_object, 0))
390 CTR2(KTR_LOCK, "%s: %p last succeeded",
391 __func__, rw);
392 break;
393 }
394 continue;
395 }
396
397 /*
398 * There should just be one reader with one or more
399 * writers waiting.
400 */
401 MPASS(x == (RW_READERS_LOCK(1) | RW_LOCK_WRITE_WAITERS));
402
403 /*
404 * Ok, we know we have a waiting writer and we think we
405 * are the last reader, so grab the turnstile lock.
406 */
407 turnstile_lock(&rw->lock_object);
408
409 /*
410 * Try to drop our lock leaving the lock in a unlocked
411 * state.
412 *
413 * If you wanted to do explicit lock handoff you'd have to
414 * do it here. You'd also want to use turnstile_signal()
415 * and you'd have to handle the race where a higher
416 * priority thread blocks on the write lock before the
417 * thread you wakeup actually runs and have the new thread
418 * "steal" the lock. For now it's a lot simpler to just
419 * wakeup all of the waiters.
420 *
421 * As above, if we fail, then another thread might have
422 * acquired a read lock, so drop the turnstile lock and
423 * restart.
424 */
425 if (!atomic_cmpset_ptr(&rw->rw_lock,
426 RW_READERS_LOCK(1) | RW_LOCK_WRITE_WAITERS, RW_UNLOCKED)) {
427 turnstile_release(&rw->lock_object);
428 continue;
429 }
430 if (LOCK_LOG_TEST(&rw->lock_object, 0))
431 CTR2(KTR_LOCK, "%s: %p last succeeded with waiters",
432 __func__, rw);
433
434 /*
435 * Ok. The lock is released and all that's left is to
436 * wake up the waiters. Note that the lock might not be
437 * free anymore, but in that case the writers will just
438 * block again if they run before the new lock holder(s)
439 * release the lock.
440 */
441 ts = turnstile_lookup(&rw->lock_object);
442 MPASS(ts != NULL);
443 turnstile_broadcast(ts, TS_EXCLUSIVE_QUEUE);
444 turnstile_unpend(ts, TS_SHARED_LOCK);
445 break;
446 }
447 lock_profile_release_lock(&rw->lock_object);
448 }
449
450 /*
451 * This function is called when we are unable to obtain a write lock on the
452 * first try. This means that at least one other thread holds either a
453 * read or write lock.
454 */
455 void
456 _rw_wlock_hard(struct rwlock *rw, uintptr_t tid, const char *file, int line)
457 {
458 #ifdef ADAPTIVE_RWLOCKS
459 volatile struct thread *owner;
460 #endif
461 uintptr_t v;
462
463 if (rw_wlocked(rw)) {
464 KASSERT(rw->lock_object.lo_flags & RW_RECURSE,
465 ("%s: recursing but non-recursive rw %s @ %s:%d\n",
466 __func__, rw->lock_object.lo_name, file, line));
467 rw->rw_recurse++;
468 atomic_set_ptr(&rw->rw_lock, RW_LOCK_RECURSED);
469 if (LOCK_LOG_TEST(&rw->lock_object, 0))
470 CTR2(KTR_LOCK, "%s: %p recursing", __func__, rw);
471 return;
472 }
473
474 if (LOCK_LOG_TEST(&rw->lock_object, 0))
475 CTR5(KTR_LOCK, "%s: %s contested (lock=%p) at %s:%d", __func__,
476 rw->lock_object.lo_name, (void *)rw->rw_lock, file, line);
477
478 while (!_rw_write_lock(rw, tid)) {
479 turnstile_lock(&rw->lock_object);
480 v = rw->rw_lock;
481
482 /*
483 * If the lock was released while spinning on the
484 * turnstile chain lock, try again.
485 */
486 if (v == RW_UNLOCKED) {
487 turnstile_release(&rw->lock_object);
488 cpu_spinwait();
489 continue;
490 }
491
492 /*
493 * If the lock was released by a writer with both readers
494 * and writers waiting and a reader hasn't woken up and
495 * acquired the lock yet, rw_lock will be set to the
496 * value RW_UNLOCKED | RW_LOCK_WRITE_WAITERS. If we see
497 * that value, try to acquire it once. Note that we have
498 * to preserve the RW_LOCK_WRITE_WAITERS flag as there are
499 * other writers waiting still. If we fail, restart the
500 * loop.
501 */
502 if (v == (RW_UNLOCKED | RW_LOCK_WRITE_WAITERS)) {
503 if (atomic_cmpset_acq_ptr(&rw->rw_lock,
504 RW_UNLOCKED | RW_LOCK_WRITE_WAITERS,
505 tid | RW_LOCK_WRITE_WAITERS)) {
506 turnstile_claim(&rw->lock_object);
507 CTR2(KTR_LOCK, "%s: %p claimed by new writer",
508 __func__, rw);
509 break;
510 }
511 turnstile_release(&rw->lock_object);
512 cpu_spinwait();
513 continue;
514 }
515
516 /*
517 * If the RW_LOCK_WRITE_WAITERS flag isn't set, then try to
518 * set it. If we fail to set it, then loop back and try
519 * again.
520 */
521 if (!(v & RW_LOCK_WRITE_WAITERS)) {
522 if (!atomic_cmpset_ptr(&rw->rw_lock, v,
523 v | RW_LOCK_WRITE_WAITERS)) {
524 turnstile_release(&rw->lock_object);
525 cpu_spinwait();
526 continue;
527 }
528 if (LOCK_LOG_TEST(&rw->lock_object, 0))
529 CTR2(KTR_LOCK, "%s: %p set write waiters flag",
530 __func__, rw);
531 }
532
533 #ifdef ADAPTIVE_RWLOCKS
534 /*
535 * If the lock is write locked and the owner is
536 * running on another CPU, spin until the owner stops
537 * running or the state of the lock changes.
538 */
539 owner = (struct thread *)RW_OWNER(v);
540 if (!(v & RW_LOCK_READ) && TD_IS_RUNNING(owner)) {
541 turnstile_release(&rw->lock_object);
542 if (LOCK_LOG_TEST(&rw->lock_object, 0))
543 CTR3(KTR_LOCK, "%s: spinning on %p held by %p",
544 __func__, rw, owner);
545 while ((struct thread*)RW_OWNER(rw->rw_lock)== owner &&
546 TD_IS_RUNNING(owner))
547 cpu_spinwait();
548 continue;
549 }
550 #endif
551
552 /*
553 * We were unable to acquire the lock and the write waiters
554 * flag is set, so we must block on the turnstile.
555 */
556 if (LOCK_LOG_TEST(&rw->lock_object, 0))
557 CTR2(KTR_LOCK, "%s: %p blocking on turnstile", __func__,
558 rw);
559 turnstile_wait(&rw->lock_object, rw_owner(rw),
560 TS_EXCLUSIVE_QUEUE);
561 if (LOCK_LOG_TEST(&rw->lock_object, 0))
562 CTR2(KTR_LOCK, "%s: %p resuming from turnstile",
563 __func__, rw);
564 }
565 }
566
567 /*
568 * This function is called if the first try at releasing a write lock failed.
569 * This means that one of the 2 waiter bits must be set indicating that at
570 * least one thread is waiting on this lock.
571 */
572 void
573 _rw_wunlock_hard(struct rwlock *rw, uintptr_t tid, const char *file, int line)
574 {
575 struct turnstile *ts;
576 uintptr_t v;
577 int queue;
578
579 if (rw_wlocked(rw) && rw_recursed(rw)) {
580 if ((--rw->rw_recurse) == 0)
581 atomic_clear_ptr(&rw->rw_lock, RW_LOCK_RECURSED);
582 if (LOCK_LOG_TEST(&rw->lock_object, 0))
583 CTR2(KTR_LOCK, "%s: %p unrecursing", __func__, rw);
584 return;
585 }
586
587 KASSERT(rw->rw_lock & (RW_LOCK_READ_WAITERS | RW_LOCK_WRITE_WAITERS),
588 ("%s: neither of the waiter flags are set", __func__));
589
590 if (LOCK_LOG_TEST(&rw->lock_object, 0))
591 CTR2(KTR_LOCK, "%s: %p contested", __func__, rw);
592
593 turnstile_lock(&rw->lock_object);
594 ts = turnstile_lookup(&rw->lock_object);
595
596 #ifdef ADAPTIVE_RWLOCKS
597 /*
598 * There might not be a turnstile for this lock if all of
599 * the waiters are adaptively spinning. In that case, just
600 * reset the lock to the unlocked state and return.
601 */
602 if (ts == NULL) {
603 atomic_store_rel_ptr(&rw->rw_lock, RW_UNLOCKED);
604 if (LOCK_LOG_TEST(&rw->lock_object, 0))
605 CTR2(KTR_LOCK, "%s: %p no sleepers", __func__, rw);
606 turnstile_release(&rw->lock_object);
607 return;
608 }
609 #else
610 MPASS(ts != NULL);
611 #endif
612
613 /*
614 * Use the same algo as sx locks for now. Prefer waking up shared
615 * waiters if we have any over writers. This is probably not ideal.
616 *
617 * 'v' is the value we are going to write back to rw_lock. If we
618 * have waiters on both queues, we need to preserve the state of
619 * the waiter flag for the queue we don't wake up. For now this is
620 * hardcoded for the algorithm mentioned above.
621 *
622 * In the case of both readers and writers waiting we wakeup the
623 * readers but leave the RW_LOCK_WRITE_WAITERS flag set. If a
624 * new writer comes in before a reader it will claim the lock up
625 * above. There is probably a potential priority inversion in
626 * there that could be worked around either by waking both queues
627 * of waiters or doing some complicated lock handoff gymnastics.
628 *
629 * Note that in the ADAPTIVE_RWLOCKS case, if both flags are
630 * set, there might not be any actual writers on the turnstile
631 * as they might all be spinning. In that case, we don't want
632 * to preserve the RW_LOCK_WRITE_WAITERS flag as the turnstile
633 * is going to go away once we wakeup all the readers.
634 */
635 v = RW_UNLOCKED;
636 if (rw->rw_lock & RW_LOCK_READ_WAITERS) {
637 queue = TS_SHARED_QUEUE;
638 #ifdef ADAPTIVE_RWLOCKS
639 if (rw->rw_lock & RW_LOCK_WRITE_WAITERS &&
640 !turnstile_empty(ts, TS_EXCLUSIVE_QUEUE))
641 v |= RW_LOCK_WRITE_WAITERS;
642 #else
643 v |= (rw->rw_lock & RW_LOCK_WRITE_WAITERS);
644 #endif
645 } else
646 queue = TS_EXCLUSIVE_QUEUE;
647
648 #ifdef ADAPTIVE_RWLOCKS
649 /*
650 * We have to make sure that we actually have waiters to
651 * wakeup. If they are all spinning, then we just need to
652 * disown the turnstile and return.
653 */
654 if (turnstile_empty(ts, queue)) {
655 if (LOCK_LOG_TEST(&rw->lock_object, 0))
656 CTR2(KTR_LOCK, "%s: %p no sleepers 2", __func__, rw);
657 atomic_store_rel_ptr(&rw->rw_lock, v);
658 turnstile_disown(ts);
659 turnstile_release(&rw->lock_object);
660 return;
661 }
662 #endif
663
664 /* Wake up all waiters for the specific queue. */
665 if (LOCK_LOG_TEST(&rw->lock_object, 0))
666 CTR3(KTR_LOCK, "%s: %p waking up %s waiters", __func__, rw,
667 queue == TS_SHARED_QUEUE ? "read" : "write");
668 turnstile_broadcast(ts, queue);
669 atomic_store_rel_ptr(&rw->rw_lock, v);
670 turnstile_unpend(ts, TS_EXCLUSIVE_LOCK);
671 }
672
673 /*
674 * Attempt to do a non-blocking upgrade from a read lock to a write
675 * lock. This will only succeed if this thread holds a single read
676 * lock. Returns true if the upgrade succeeded and false otherwise.
677 */
678 int
679 _rw_try_upgrade(struct rwlock *rw, const char *file, int line)
680 {
681 uintptr_t v, tid;
682 int success;
683
684 KASSERT(rw->rw_lock != RW_DESTROYED,
685 ("rw_try_upgrade() of destroyed rwlock @ %s:%d", file, line));
686 _rw_assert(rw, RA_RLOCKED, file, line);
687
688 /*
689 * Attempt to switch from one reader to a writer. If there
690 * are any write waiters, then we will have to lock the
691 * turnstile first to prevent races with another writer
692 * calling turnstile_wait() before we have claimed this
693 * turnstile. So, do the simple case of no waiters first.
694 */
695 tid = (uintptr_t)curthread;
696 if (!(rw->rw_lock & RW_LOCK_WRITE_WAITERS)) {
697 success = atomic_cmpset_ptr(&rw->rw_lock, RW_READERS_LOCK(1),
698 tid);
699 goto out;
700 }
701
702 /*
703 * Ok, we think we have write waiters, so lock the
704 * turnstile.
705 */
706 turnstile_lock(&rw->lock_object);
707
708 /*
709 * Try to switch from one reader to a writer again. This time
710 * we honor the current state of the RW_LOCK_WRITE_WAITERS
711 * flag. If we obtain the lock with the flag set, then claim
712 * ownership of the turnstile. In the ADAPTIVE_RWLOCKS case
713 * it is possible for there to not be an associated turnstile
714 * even though there are waiters if all of the waiters are
715 * spinning.
716 */
717 v = rw->rw_lock & RW_LOCK_WRITE_WAITERS;
718 success = atomic_cmpset_ptr(&rw->rw_lock, RW_READERS_LOCK(1) | v,
719 tid | v);
720 #ifdef ADAPTIVE_RWLOCKS
721 if (success && v && turnstile_lookup(&rw->lock_object) != NULL)
722 #else
723 if (success && v)
724 #endif
725 turnstile_claim(&rw->lock_object);
726 else
727 turnstile_release(&rw->lock_object);
728 out:
729 LOCK_LOG_TRY("WUPGRADE", &rw->lock_object, 0, success, file, line);
730 if (success)
731 WITNESS_UPGRADE(&rw->lock_object, LOP_EXCLUSIVE | LOP_TRYLOCK,
732 file, line);
733 return (success);
734 }
735
736 /*
737 * Downgrade a write lock into a single read lock.
738 */
739 void
740 _rw_downgrade(struct rwlock *rw, const char *file, int line)
741 {
742 struct turnstile *ts;
743 uintptr_t tid, v;
744
745 KASSERT(rw->rw_lock != RW_DESTROYED,
746 ("rw_downgrade() of destroyed rwlock @ %s:%d", file, line));
747 _rw_assert(rw, RA_WLOCKED | RA_NOTRECURSED, file, line);
748 #ifndef INVARIANTS
749 if (rw_recursed(rw))
750 panic("downgrade of a recursed lock");
751 #endif
752
753 WITNESS_DOWNGRADE(&rw->lock_object, 0, file, line);
754
755 /*
756 * Convert from a writer to a single reader. First we handle
757 * the easy case with no waiters. If there are any waiters, we
758 * lock the turnstile, "disown" the lock, and awaken any read
759 * waiters.
760 */
761 tid = (uintptr_t)curthread;
762 if (atomic_cmpset_rel_ptr(&rw->rw_lock, tid, RW_READERS_LOCK(1)))
763 goto out;
764
765 /*
766 * Ok, we think we have waiters, so lock the turnstile so we can
767 * read the waiter flags without any races.
768 */
769 turnstile_lock(&rw->lock_object);
770 v = rw->rw_lock;
771 MPASS(v & (RW_LOCK_READ_WAITERS | RW_LOCK_WRITE_WAITERS));
772
773 /*
774 * Downgrade from a write lock while preserving
775 * RW_LOCK_WRITE_WAITERS and give up ownership of the
776 * turnstile. If there are any read waiters, wake them up.
777 *
778 * For ADAPTIVE_RWLOCKS, we have to allow for the fact that
779 * all of the read waiters might be spinning. In that case,
780 * act as if RW_LOCK_READ_WAITERS is not set. Also, only
781 * preserve the RW_LOCK_WRITE_WAITERS flag if at least one
782 * writer is blocked on the turnstile.
783 */
784 ts = turnstile_lookup(&rw->lock_object);
785 #ifdef ADAPTIVE_RWLOCKS
786 if (ts == NULL)
787 v &= ~(RW_LOCK_READ_WAITERS | RW_LOCK_WRITE_WAITERS);
788 else if (v & RW_LOCK_READ_WAITERS &&
789 turnstile_empty(ts, TS_SHARED_QUEUE))
790 v &= ~RW_LOCK_READ_WAITERS;
791 else if (v & RW_LOCK_WRITE_WAITERS &&
792 turnstile_empty(ts, TS_EXCLUSIVE_QUEUE))
793 v &= ~RW_LOCK_WRITE_WAITERS;
794 #else
795 MPASS(ts != NULL);
796 #endif
797 if (v & RW_LOCK_READ_WAITERS)
798 turnstile_broadcast(ts, TS_SHARED_QUEUE);
799 atomic_store_rel_ptr(&rw->rw_lock, RW_READERS_LOCK(1) |
800 (v & RW_LOCK_WRITE_WAITERS));
801 if (v & RW_LOCK_READ_WAITERS) {
802 turnstile_unpend(ts, TS_EXCLUSIVE_LOCK);
803 } else if (ts) {
804 turnstile_disown(ts);
805 turnstile_release(&rw->lock_object);
806 }
807 out:
808 LOCK_LOG_LOCK("WDOWNGRADE", &rw->lock_object, 0, 0, file, line);
809 }
810
811 #ifdef INVARIANT_SUPPORT
812 #ifndef INVARIANTS
813 #undef _rw_assert
814 #endif
815
816 /*
817 * In the non-WITNESS case, rw_assert() can only detect that at least
818 * *some* thread owns an rlock, but it cannot guarantee that *this*
819 * thread owns an rlock.
820 */
821 void
822 _rw_assert(struct rwlock *rw, int what, const char *file, int line)
823 {
824
825 if (panicstr != NULL)
826 return;
827 switch (what) {
828 case RA_LOCKED:
829 case RA_LOCKED | RA_RECURSED:
830 case RA_LOCKED | RA_NOTRECURSED:
831 case RA_RLOCKED:
832 #ifdef WITNESS
833 witness_assert(&rw->lock_object, what, file, line);
834 #else
835 /*
836 * If some other thread has a write lock or we have one
837 * and are asserting a read lock, fail. Also, if no one
838 * has a lock at all, fail.
839 */
840 if (rw->rw_lock == RW_UNLOCKED ||
841 (!(rw->rw_lock & RW_LOCK_READ) && (what == RA_RLOCKED ||
842 rw_wowner(rw) != curthread)))
843 panic("Lock %s not %slocked @ %s:%d\n",
844 rw->lock_object.lo_name, (what == RA_RLOCKED) ?
845 "read " : "", file, line);
846
847 if (!(rw->rw_lock & RW_LOCK_READ)) {
848 if (rw_recursed(rw)) {
849 if (what & RA_NOTRECURSED)
850 panic("Lock %s recursed @ %s:%d\n",
851 rw->lock_object.lo_name, file,
852 line);
853 } else if (what & RA_RECURSED)
854 panic("Lock %s not recursed @ %s:%d\n",
855 rw->lock_object.lo_name, file, line);
856 }
857 #endif
858 break;
859 case RA_WLOCKED:
860 case RA_WLOCKED | RA_RECURSED:
861 case RA_WLOCKED | RA_NOTRECURSED:
862 if (rw_wowner(rw) != curthread)
863 panic("Lock %s not exclusively locked @ %s:%d\n",
864 rw->lock_object.lo_name, file, line);
865 if (rw_recursed(rw)) {
866 if (what & RA_NOTRECURSED)
867 panic("Lock %s recursed @ %s:%d\n",
868 rw->lock_object.lo_name, file, line);
869 } else if (what & RA_RECURSED)
870 panic("Lock %s not recursed @ %s:%d\n",
871 rw->lock_object.lo_name, file, line);
872 break;
873 case RA_UNLOCKED:
874 #ifdef WITNESS
875 witness_assert(&rw->lock_object, what, file, line);
876 #else
877 /*
878 * If we hold a write lock fail. We can't reliably check
879 * to see if we hold a read lock or not.
880 */
881 if (rw_wowner(rw) == curthread)
882 panic("Lock %s exclusively locked @ %s:%d\n",
883 rw->lock_object.lo_name, file, line);
884 #endif
885 break;
886 default:
887 panic("Unknown rw lock assertion: %d @ %s:%d", what, file,
888 line);
889 }
890 }
891 #endif /* INVARIANT_SUPPORT */
892
893 #ifdef DDB
894 void
895 db_show_rwlock(struct lock_object *lock)
896 {
897 struct rwlock *rw;
898 struct thread *td;
899
900 rw = (struct rwlock *)lock;
901
902 db_printf(" state: ");
903 if (rw->rw_lock == RW_UNLOCKED)
904 db_printf("UNLOCKED\n");
905 else if (rw->rw_lock == RW_DESTROYED) {
906 db_printf("DESTROYED\n");
907 return;
908 } else if (rw->rw_lock & RW_LOCK_READ)
909 db_printf("RLOCK: %ju locks\n",
910 (uintmax_t)(RW_READERS(rw->rw_lock)));
911 else {
912 td = rw_wowner(rw);
913 db_printf("WLOCK: %p (tid %d, pid %d, \"%s\")\n", td,
914 td->td_tid, td->td_proc->p_pid, td->td_proc->p_comm);
915 if (rw_recursed(rw))
916 db_printf(" recursed: %u\n", rw->rw_recurse);
917 }
918 db_printf(" waiters: ");
919 switch (rw->rw_lock & (RW_LOCK_READ_WAITERS | RW_LOCK_WRITE_WAITERS)) {
920 case RW_LOCK_READ_WAITERS:
921 db_printf("readers\n");
922 break;
923 case RW_LOCK_WRITE_WAITERS:
924 db_printf("writers\n");
925 break;
926 case RW_LOCK_READ_WAITERS | RW_LOCK_WRITE_WAITERS:
927 db_printf("readers and writers\n");
928 break;
929 default:
930 db_printf("none\n");
931 break;
932 }
933 }
934
935 #endif
Cache object: 90014e14551f427b974d84e67ab8c684
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