FreeBSD/Linux Kernel Cross Reference
sys/kern/kern_sx.c
1 /*-
2 * Copyright (c) 2007 Attilio Rao <attilio@freebsd.org>
3 * Copyright (c) 2001 Jason Evans <jasone@freebsd.org>
4 * All rights reserved.
5 *
6 * Redistribution and use in source and binary forms, with or without
7 * modification, are permitted provided that the following conditions
8 * are met:
9 * 1. Redistributions of source code must retain the above copyright
10 * notice(s), this list of conditions and the following disclaimer as
11 * the first lines of this file unmodified other than the possible
12 * addition of one or more copyright notices.
13 * 2. Redistributions in binary form must reproduce the above copyright
14 * notice(s), this list of conditions and the following disclaimer in the
15 * documentation and/or other materials provided with the distribution.
16 *
17 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDER(S) ``AS IS'' AND ANY
18 * EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED
19 * WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
20 * DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER(S) BE LIABLE FOR ANY
21 * DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES
22 * (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
23 * SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
24 * 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 SUCH
27 * DAMAGE.
28 */
29
30 /*
31 * Shared/exclusive locks. This implementation attempts to ensure
32 * deterministic lock granting behavior, so that slocks and xlocks are
33 * interleaved.
34 *
35 * Priority propagation will not generally raise the priority of lock holders,
36 * so should not be relied upon in combination with sx locks.
37 */
38
39 #include "opt_ddb.h"
40 #include "opt_hwpmc_hooks.h"
41 #include "opt_kdtrace.h"
42 #include "opt_no_adaptive_sx.h"
43
44 #include <sys/cdefs.h>
45 __FBSDID("$FreeBSD$");
46
47 #include <sys/param.h>
48 #include <sys/systm.h>
49 #include <sys/kdb.h>
50 #include <sys/ktr.h>
51 #include <sys/lock.h>
52 #include <sys/mutex.h>
53 #include <sys/proc.h>
54 #include <sys/sleepqueue.h>
55 #include <sys/sx.h>
56 #include <sys/sysctl.h>
57
58 #if defined(SMP) && !defined(NO_ADAPTIVE_SX)
59 #include <machine/cpu.h>
60 #endif
61
62 #ifdef DDB
63 #include <ddb/ddb.h>
64 #endif
65
66 #if defined(SMP) && !defined(NO_ADAPTIVE_SX)
67 #define ADAPTIVE_SX
68 #endif
69
70 CTASSERT((SX_NOADAPTIVE & LO_CLASSFLAGS) == SX_NOADAPTIVE);
71
72 #ifdef HWPMC_HOOKS
73 #include <sys/pmckern.h>
74 PMC_SOFT_DECLARE( , , lock, failed);
75 #endif
76
77 /* Handy macros for sleep queues. */
78 #define SQ_EXCLUSIVE_QUEUE 0
79 #define SQ_SHARED_QUEUE 1
80
81 #ifdef ADAPTIVE_SX
82 #define ASX_RETRIES 10
83 #define ASX_LOOPS 10000
84 #endif
85
86 /*
87 * Variations on DROP_GIANT()/PICKUP_GIANT() for use in this file. We
88 * drop Giant anytime we have to sleep or if we adaptively spin.
89 */
90 #define GIANT_DECLARE \
91 int _giantcnt = 0; \
92 WITNESS_SAVE_DECL(Giant) \
93
94 #define GIANT_SAVE() do { \
95 if (mtx_owned(&Giant)) { \
96 WITNESS_SAVE(&Giant.lock_object, Giant); \
97 while (mtx_owned(&Giant)) { \
98 _giantcnt++; \
99 mtx_unlock(&Giant); \
100 } \
101 } \
102 } while (0)
103
104 #define GIANT_RESTORE() do { \
105 if (_giantcnt > 0) { \
106 mtx_assert(&Giant, MA_NOTOWNED); \
107 while (_giantcnt--) \
108 mtx_lock(&Giant); \
109 WITNESS_RESTORE(&Giant.lock_object, Giant); \
110 } \
111 } while (0)
112
113 /*
114 * Returns true if an exclusive lock is recursed. It assumes
115 * curthread currently has an exclusive lock.
116 */
117 #define sx_recurse lock_object.lo_data
118 #define sx_recursed(sx) ((sx)->sx_recurse != 0)
119
120 static void assert_sx(struct lock_object *lock, int what);
121 #ifdef DDB
122 static void db_show_sx(struct lock_object *lock);
123 #endif
124 static void lock_sx(struct lock_object *lock, int how);
125 #ifdef KDTRACE_HOOKS
126 static int owner_sx(struct lock_object *lock, struct thread **owner);
127 #endif
128 static int unlock_sx(struct lock_object *lock);
129
130 struct lock_class lock_class_sx = {
131 .lc_name = "sx",
132 .lc_flags = LC_SLEEPLOCK | LC_SLEEPABLE | LC_RECURSABLE | LC_UPGRADABLE,
133 .lc_assert = assert_sx,
134 #ifdef DDB
135 .lc_ddb_show = db_show_sx,
136 #endif
137 .lc_lock = lock_sx,
138 .lc_unlock = unlock_sx,
139 #ifdef KDTRACE_HOOKS
140 .lc_owner = owner_sx,
141 #endif
142 };
143
144 #ifndef INVARIANTS
145 #define _sx_assert(sx, what, file, line)
146 #endif
147
148 void
149 assert_sx(struct lock_object *lock, int what)
150 {
151
152 sx_assert((struct sx *)lock, what);
153 }
154
155 void
156 lock_sx(struct lock_object *lock, int how)
157 {
158 struct sx *sx;
159
160 sx = (struct sx *)lock;
161 if (how)
162 sx_xlock(sx);
163 else
164 sx_slock(sx);
165 }
166
167 int
168 unlock_sx(struct lock_object *lock)
169 {
170 struct sx *sx;
171
172 sx = (struct sx *)lock;
173 sx_assert(sx, SA_LOCKED | SA_NOTRECURSED);
174 if (sx_xlocked(sx)) {
175 sx_xunlock(sx);
176 return (1);
177 } else {
178 sx_sunlock(sx);
179 return (0);
180 }
181 }
182
183 #ifdef KDTRACE_HOOKS
184 int
185 owner_sx(struct lock_object *lock, struct thread **owner)
186 {
187 struct sx *sx = (struct sx *)lock;
188 uintptr_t x = sx->sx_lock;
189
190 *owner = (struct thread *)SX_OWNER(x);
191 return ((x & SX_LOCK_SHARED) != 0 ? (SX_SHARERS(x) != 0) :
192 (*owner != NULL));
193 }
194 #endif
195
196 void
197 sx_sysinit(void *arg)
198 {
199 struct sx_args *sargs = arg;
200
201 sx_init_flags(sargs->sa_sx, sargs->sa_desc, sargs->sa_flags);
202 }
203
204 void
205 sx_init_flags(struct sx *sx, const char *description, int opts)
206 {
207 int flags;
208
209 MPASS((opts & ~(SX_QUIET | SX_RECURSE | SX_NOWITNESS | SX_DUPOK |
210 SX_NOPROFILE | SX_NOADAPTIVE)) == 0);
211 ASSERT_ATOMIC_LOAD_PTR(sx->sx_lock,
212 ("%s: sx_lock not aligned for %s: %p", __func__, description,
213 &sx->sx_lock));
214
215 flags = LO_SLEEPABLE | LO_UPGRADABLE;
216 if (opts & SX_DUPOK)
217 flags |= LO_DUPOK;
218 if (opts & SX_NOPROFILE)
219 flags |= LO_NOPROFILE;
220 if (!(opts & SX_NOWITNESS))
221 flags |= LO_WITNESS;
222 if (opts & SX_RECURSE)
223 flags |= LO_RECURSABLE;
224 if (opts & SX_QUIET)
225 flags |= LO_QUIET;
226
227 flags |= opts & SX_NOADAPTIVE;
228 sx->sx_lock = SX_LOCK_UNLOCKED;
229 sx->sx_recurse = 0;
230 lock_init(&sx->lock_object, &lock_class_sx, description, NULL, flags);
231 }
232
233 void
234 sx_destroy(struct sx *sx)
235 {
236
237 KASSERT(sx->sx_lock == SX_LOCK_UNLOCKED, ("sx lock still held"));
238 KASSERT(sx->sx_recurse == 0, ("sx lock still recursed"));
239 sx->sx_lock = SX_LOCK_DESTROYED;
240 lock_destroy(&sx->lock_object);
241 }
242
243 int
244 _sx_slock(struct sx *sx, int opts, const char *file, int line)
245 {
246 int error = 0;
247
248 if (SCHEDULER_STOPPED())
249 return (0);
250 KASSERT(kdb_active != 0 || !TD_IS_IDLETHREAD(curthread),
251 ("sx_slock() by idle thread %p on sx %s @ %s:%d",
252 curthread, sx->lock_object.lo_name, file, line));
253 KASSERT(sx->sx_lock != SX_LOCK_DESTROYED,
254 ("sx_slock() of destroyed sx @ %s:%d", file, line));
255 WITNESS_CHECKORDER(&sx->lock_object, LOP_NEWORDER, file, line, NULL);
256 error = __sx_slock(sx, opts, file, line);
257 if (!error) {
258 LOCK_LOG_LOCK("SLOCK", &sx->lock_object, 0, 0, file, line);
259 WITNESS_LOCK(&sx->lock_object, 0, file, line);
260 curthread->td_locks++;
261 }
262
263 return (error);
264 }
265
266 int
267 _sx_try_slock(struct sx *sx, const char *file, int line)
268 {
269 uintptr_t x;
270
271 if (SCHEDULER_STOPPED())
272 return (1);
273
274 KASSERT(kdb_active != 0 || !TD_IS_IDLETHREAD(curthread),
275 ("sx_try_slock() by idle thread %p on sx %s @ %s:%d",
276 curthread, sx->lock_object.lo_name, file, line));
277
278 for (;;) {
279 x = sx->sx_lock;
280 KASSERT(x != SX_LOCK_DESTROYED,
281 ("sx_try_slock() of destroyed sx @ %s:%d", file, line));
282 if (!(x & SX_LOCK_SHARED))
283 break;
284 if (atomic_cmpset_acq_ptr(&sx->sx_lock, x, x + SX_ONE_SHARER)) {
285 LOCK_LOG_TRY("SLOCK", &sx->lock_object, 0, 1, file, line);
286 WITNESS_LOCK(&sx->lock_object, LOP_TRYLOCK, file, line);
287 LOCKSTAT_PROFILE_OBTAIN_LOCK_SUCCESS(LS_SX_SLOCK_ACQUIRE,
288 sx, 0, 0, file, line);
289 curthread->td_locks++;
290 return (1);
291 }
292 }
293
294 LOCK_LOG_TRY("SLOCK", &sx->lock_object, 0, 0, file, line);
295 return (0);
296 }
297
298 int
299 _sx_xlock(struct sx *sx, int opts, const char *file, int line)
300 {
301 int error = 0;
302
303 if (SCHEDULER_STOPPED())
304 return (0);
305 KASSERT(kdb_active != 0 || !TD_IS_IDLETHREAD(curthread),
306 ("sx_xlock() by idle thread %p on sx %s @ %s:%d",
307 curthread, sx->lock_object.lo_name, file, line));
308 KASSERT(sx->sx_lock != SX_LOCK_DESTROYED,
309 ("sx_xlock() of destroyed sx @ %s:%d", file, line));
310 WITNESS_CHECKORDER(&sx->lock_object, LOP_NEWORDER | LOP_EXCLUSIVE, file,
311 line, NULL);
312 error = __sx_xlock(sx, curthread, opts, file, line);
313 if (!error) {
314 LOCK_LOG_LOCK("XLOCK", &sx->lock_object, 0, sx->sx_recurse,
315 file, line);
316 WITNESS_LOCK(&sx->lock_object, LOP_EXCLUSIVE, file, line);
317 curthread->td_locks++;
318 }
319
320 return (error);
321 }
322
323 int
324 _sx_try_xlock(struct sx *sx, const char *file, int line)
325 {
326 int rval;
327
328 if (SCHEDULER_STOPPED())
329 return (1);
330
331 KASSERT(kdb_active != 0 || !TD_IS_IDLETHREAD(curthread),
332 ("sx_try_xlock() by idle thread %p on sx %s @ %s:%d",
333 curthread, sx->lock_object.lo_name, file, line));
334 KASSERT(sx->sx_lock != SX_LOCK_DESTROYED,
335 ("sx_try_xlock() of destroyed sx @ %s:%d", file, line));
336
337 if (sx_xlocked(sx) &&
338 (sx->lock_object.lo_flags & LO_RECURSABLE) != 0) {
339 sx->sx_recurse++;
340 atomic_set_ptr(&sx->sx_lock, SX_LOCK_RECURSED);
341 rval = 1;
342 } else
343 rval = atomic_cmpset_acq_ptr(&sx->sx_lock, SX_LOCK_UNLOCKED,
344 (uintptr_t)curthread);
345 LOCK_LOG_TRY("XLOCK", &sx->lock_object, 0, rval, file, line);
346 if (rval) {
347 WITNESS_LOCK(&sx->lock_object, LOP_EXCLUSIVE | LOP_TRYLOCK,
348 file, line);
349 if (!sx_recursed(sx))
350 LOCKSTAT_PROFILE_OBTAIN_LOCK_SUCCESS(LS_SX_XLOCK_ACQUIRE,
351 sx, 0, 0, file, line);
352 curthread->td_locks++;
353 }
354
355 return (rval);
356 }
357
358 void
359 _sx_sunlock(struct sx *sx, const char *file, int line)
360 {
361
362 if (SCHEDULER_STOPPED())
363 return;
364 KASSERT(sx->sx_lock != SX_LOCK_DESTROYED,
365 ("sx_sunlock() of destroyed sx @ %s:%d", file, line));
366 _sx_assert(sx, SA_SLOCKED, file, line);
367 curthread->td_locks--;
368 WITNESS_UNLOCK(&sx->lock_object, 0, file, line);
369 LOCK_LOG_LOCK("SUNLOCK", &sx->lock_object, 0, 0, file, line);
370 __sx_sunlock(sx, file, line);
371 LOCKSTAT_PROFILE_RELEASE_LOCK(LS_SX_SUNLOCK_RELEASE, sx);
372 }
373
374 void
375 _sx_xunlock(struct sx *sx, const char *file, int line)
376 {
377
378 if (SCHEDULER_STOPPED())
379 return;
380 KASSERT(sx->sx_lock != SX_LOCK_DESTROYED,
381 ("sx_xunlock() of destroyed sx @ %s:%d", file, line));
382 _sx_assert(sx, SA_XLOCKED, file, line);
383 curthread->td_locks--;
384 WITNESS_UNLOCK(&sx->lock_object, LOP_EXCLUSIVE, file, line);
385 LOCK_LOG_LOCK("XUNLOCK", &sx->lock_object, 0, sx->sx_recurse, file,
386 line);
387 if (!sx_recursed(sx))
388 LOCKSTAT_PROFILE_RELEASE_LOCK(LS_SX_XUNLOCK_RELEASE, sx);
389 __sx_xunlock(sx, curthread, file, line);
390 }
391
392 /*
393 * Try to do a non-blocking upgrade from a shared lock to an exclusive lock.
394 * This will only succeed if this thread holds a single shared lock.
395 * Return 1 if if the upgrade succeed, 0 otherwise.
396 */
397 int
398 _sx_try_upgrade(struct sx *sx, const char *file, int line)
399 {
400 uintptr_t x;
401 int success;
402
403 if (SCHEDULER_STOPPED())
404 return (1);
405
406 KASSERT(sx->sx_lock != SX_LOCK_DESTROYED,
407 ("sx_try_upgrade() of destroyed sx @ %s:%d", file, line));
408 _sx_assert(sx, SA_SLOCKED, file, line);
409
410 /*
411 * Try to switch from one shared lock to an exclusive lock. We need
412 * to maintain the SX_LOCK_EXCLUSIVE_WAITERS flag if set so that
413 * we will wake up the exclusive waiters when we drop the lock.
414 */
415 x = sx->sx_lock & SX_LOCK_EXCLUSIVE_WAITERS;
416 success = atomic_cmpset_ptr(&sx->sx_lock, SX_SHARERS_LOCK(1) | x,
417 (uintptr_t)curthread | x);
418 LOCK_LOG_TRY("XUPGRADE", &sx->lock_object, 0, success, file, line);
419 if (success) {
420 WITNESS_UPGRADE(&sx->lock_object, LOP_EXCLUSIVE | LOP_TRYLOCK,
421 file, line);
422 LOCKSTAT_RECORD0(LS_SX_TRYUPGRADE_UPGRADE, sx);
423 }
424 return (success);
425 }
426
427 /*
428 * Downgrade an unrecursed exclusive lock into a single shared lock.
429 */
430 void
431 _sx_downgrade(struct sx *sx, const char *file, int line)
432 {
433 uintptr_t x;
434 int wakeup_swapper;
435
436 if (SCHEDULER_STOPPED())
437 return;
438
439 KASSERT(sx->sx_lock != SX_LOCK_DESTROYED,
440 ("sx_downgrade() of destroyed sx @ %s:%d", file, line));
441 _sx_assert(sx, SA_XLOCKED | SA_NOTRECURSED, file, line);
442 #ifndef INVARIANTS
443 if (sx_recursed(sx))
444 panic("downgrade of a recursed lock");
445 #endif
446
447 WITNESS_DOWNGRADE(&sx->lock_object, 0, file, line);
448
449 /*
450 * Try to switch from an exclusive lock with no shared waiters
451 * to one sharer with no shared waiters. If there are
452 * exclusive waiters, we don't need to lock the sleep queue so
453 * long as we preserve the flag. We do one quick try and if
454 * that fails we grab the sleepq lock to keep the flags from
455 * changing and do it the slow way.
456 *
457 * We have to lock the sleep queue if there are shared waiters
458 * so we can wake them up.
459 */
460 x = sx->sx_lock;
461 if (!(x & SX_LOCK_SHARED_WAITERS) &&
462 atomic_cmpset_rel_ptr(&sx->sx_lock, x, SX_SHARERS_LOCK(1) |
463 (x & SX_LOCK_EXCLUSIVE_WAITERS))) {
464 LOCK_LOG_LOCK("XDOWNGRADE", &sx->lock_object, 0, 0, file, line);
465 return;
466 }
467
468 /*
469 * Lock the sleep queue so we can read the waiters bits
470 * without any races and wakeup any shared waiters.
471 */
472 sleepq_lock(&sx->lock_object);
473
474 /*
475 * Preserve SX_LOCK_EXCLUSIVE_WAITERS while downgraded to a single
476 * shared lock. If there are any shared waiters, wake them up.
477 */
478 wakeup_swapper = 0;
479 x = sx->sx_lock;
480 atomic_store_rel_ptr(&sx->sx_lock, SX_SHARERS_LOCK(1) |
481 (x & SX_LOCK_EXCLUSIVE_WAITERS));
482 if (x & SX_LOCK_SHARED_WAITERS)
483 wakeup_swapper = sleepq_broadcast(&sx->lock_object, SLEEPQ_SX,
484 0, SQ_SHARED_QUEUE);
485 sleepq_release(&sx->lock_object);
486
487 LOCK_LOG_LOCK("XDOWNGRADE", &sx->lock_object, 0, 0, file, line);
488 LOCKSTAT_RECORD0(LS_SX_DOWNGRADE_DOWNGRADE, sx);
489
490 if (wakeup_swapper)
491 kick_proc0();
492 }
493
494 /*
495 * This function represents the so-called 'hard case' for sx_xlock
496 * operation. All 'easy case' failures are redirected to this. Note
497 * that ideally this would be a static function, but it needs to be
498 * accessible from at least sx.h.
499 */
500 int
501 _sx_xlock_hard(struct sx *sx, uintptr_t tid, int opts, const char *file,
502 int line)
503 {
504 GIANT_DECLARE;
505 #ifdef ADAPTIVE_SX
506 volatile struct thread *owner;
507 u_int i, spintries = 0;
508 #endif
509 uintptr_t x;
510 #ifdef LOCK_PROFILING
511 uint64_t waittime = 0;
512 int contested = 0;
513 #endif
514 int error = 0;
515 #ifdef KDTRACE_HOOKS
516 uintptr_t state;
517 uint64_t spin_cnt = 0;
518 uint64_t sleep_cnt = 0;
519 int64_t sleep_time = 0;
520 int64_t all_time = 0;
521 #endif
522
523 if (SCHEDULER_STOPPED())
524 return (0);
525
526 /* If we already hold an exclusive lock, then recurse. */
527 if (sx_xlocked(sx)) {
528 KASSERT((sx->lock_object.lo_flags & LO_RECURSABLE) != 0,
529 ("_sx_xlock_hard: recursed on non-recursive sx %s @ %s:%d\n",
530 sx->lock_object.lo_name, file, line));
531 sx->sx_recurse++;
532 atomic_set_ptr(&sx->sx_lock, SX_LOCK_RECURSED);
533 if (LOCK_LOG_TEST(&sx->lock_object, 0))
534 CTR2(KTR_LOCK, "%s: %p recursing", __func__, sx);
535 return (0);
536 }
537
538 if (LOCK_LOG_TEST(&sx->lock_object, 0))
539 CTR5(KTR_LOCK, "%s: %s contested (lock=%p) at %s:%d", __func__,
540 sx->lock_object.lo_name, (void *)sx->sx_lock, file, line);
541
542 #ifdef KDTRACE_HOOKS
543 all_time -= lockstat_nsecs(&sx->lock_object);
544 state = sx->sx_lock;
545 #endif
546 while (!atomic_cmpset_acq_ptr(&sx->sx_lock, SX_LOCK_UNLOCKED, tid)) {
547 #ifdef KDTRACE_HOOKS
548 spin_cnt++;
549 #endif
550 #ifdef HWPMC_HOOKS
551 PMC_SOFT_CALL( , , lock, failed);
552 #endif
553 lock_profile_obtain_lock_failed(&sx->lock_object, &contested,
554 &waittime);
555 #ifdef ADAPTIVE_SX
556 /*
557 * If the lock is write locked and the owner is
558 * running on another CPU, spin until the owner stops
559 * running or the state of the lock changes.
560 */
561 x = sx->sx_lock;
562 if ((sx->lock_object.lo_flags & SX_NOADAPTIVE) == 0) {
563 if ((x & SX_LOCK_SHARED) == 0) {
564 x = SX_OWNER(x);
565 owner = (struct thread *)x;
566 if (TD_IS_RUNNING(owner)) {
567 if (LOCK_LOG_TEST(&sx->lock_object, 0))
568 CTR3(KTR_LOCK,
569 "%s: spinning on %p held by %p",
570 __func__, sx, owner);
571 GIANT_SAVE();
572 while (SX_OWNER(sx->sx_lock) == x &&
573 TD_IS_RUNNING(owner)) {
574 cpu_spinwait();
575 #ifdef KDTRACE_HOOKS
576 spin_cnt++;
577 #endif
578 }
579 continue;
580 }
581 } else if (SX_SHARERS(x) && spintries < ASX_RETRIES) {
582 GIANT_SAVE();
583 spintries++;
584 for (i = 0; i < ASX_LOOPS; i++) {
585 if (LOCK_LOG_TEST(&sx->lock_object, 0))
586 CTR4(KTR_LOCK,
587 "%s: shared spinning on %p with %u and %u",
588 __func__, sx, spintries, i);
589 x = sx->sx_lock;
590 if ((x & SX_LOCK_SHARED) == 0 ||
591 SX_SHARERS(x) == 0)
592 break;
593 cpu_spinwait();
594 #ifdef KDTRACE_HOOKS
595 spin_cnt++;
596 #endif
597 }
598 if (i != ASX_LOOPS)
599 continue;
600 }
601 }
602 #endif
603
604 sleepq_lock(&sx->lock_object);
605 x = sx->sx_lock;
606
607 /*
608 * If the lock was released while spinning on the
609 * sleep queue chain lock, try again.
610 */
611 if (x == SX_LOCK_UNLOCKED) {
612 sleepq_release(&sx->lock_object);
613 continue;
614 }
615
616 #ifdef ADAPTIVE_SX
617 /*
618 * The current lock owner might have started executing
619 * on another CPU (or the lock could have changed
620 * owners) while we were waiting on the sleep queue
621 * chain lock. If so, drop the sleep queue lock and try
622 * again.
623 */
624 if (!(x & SX_LOCK_SHARED) &&
625 (sx->lock_object.lo_flags & SX_NOADAPTIVE) == 0) {
626 owner = (struct thread *)SX_OWNER(x);
627 if (TD_IS_RUNNING(owner)) {
628 sleepq_release(&sx->lock_object);
629 continue;
630 }
631 }
632 #endif
633
634 /*
635 * If an exclusive lock was released with both shared
636 * and exclusive waiters and a shared waiter hasn't
637 * woken up and acquired the lock yet, sx_lock will be
638 * set to SX_LOCK_UNLOCKED | SX_LOCK_EXCLUSIVE_WAITERS.
639 * If we see that value, try to acquire it once. Note
640 * that we have to preserve SX_LOCK_EXCLUSIVE_WAITERS
641 * as there are other exclusive waiters still. If we
642 * fail, restart the loop.
643 */
644 if (x == (SX_LOCK_UNLOCKED | SX_LOCK_EXCLUSIVE_WAITERS)) {
645 if (atomic_cmpset_acq_ptr(&sx->sx_lock,
646 SX_LOCK_UNLOCKED | SX_LOCK_EXCLUSIVE_WAITERS,
647 tid | SX_LOCK_EXCLUSIVE_WAITERS)) {
648 sleepq_release(&sx->lock_object);
649 CTR2(KTR_LOCK, "%s: %p claimed by new writer",
650 __func__, sx);
651 break;
652 }
653 sleepq_release(&sx->lock_object);
654 continue;
655 }
656
657 /*
658 * Try to set the SX_LOCK_EXCLUSIVE_WAITERS. If we fail,
659 * than loop back and retry.
660 */
661 if (!(x & SX_LOCK_EXCLUSIVE_WAITERS)) {
662 if (!atomic_cmpset_ptr(&sx->sx_lock, x,
663 x | SX_LOCK_EXCLUSIVE_WAITERS)) {
664 sleepq_release(&sx->lock_object);
665 continue;
666 }
667 if (LOCK_LOG_TEST(&sx->lock_object, 0))
668 CTR2(KTR_LOCK, "%s: %p set excl waiters flag",
669 __func__, sx);
670 }
671
672 /*
673 * Since we have been unable to acquire the exclusive
674 * lock and the exclusive waiters flag is set, we have
675 * to sleep.
676 */
677 if (LOCK_LOG_TEST(&sx->lock_object, 0))
678 CTR2(KTR_LOCK, "%s: %p blocking on sleep queue",
679 __func__, sx);
680
681 #ifdef KDTRACE_HOOKS
682 sleep_time -= lockstat_nsecs(&sx->lock_object);
683 #endif
684 GIANT_SAVE();
685 sleepq_add(&sx->lock_object, NULL, sx->lock_object.lo_name,
686 SLEEPQ_SX | ((opts & SX_INTERRUPTIBLE) ?
687 SLEEPQ_INTERRUPTIBLE : 0), SQ_EXCLUSIVE_QUEUE);
688 if (!(opts & SX_INTERRUPTIBLE))
689 sleepq_wait(&sx->lock_object, 0);
690 else
691 error = sleepq_wait_sig(&sx->lock_object, 0);
692 #ifdef KDTRACE_HOOKS
693 sleep_time += lockstat_nsecs(&sx->lock_object);
694 sleep_cnt++;
695 #endif
696 if (error) {
697 if (LOCK_LOG_TEST(&sx->lock_object, 0))
698 CTR2(KTR_LOCK,
699 "%s: interruptible sleep by %p suspended by signal",
700 __func__, sx);
701 break;
702 }
703 if (LOCK_LOG_TEST(&sx->lock_object, 0))
704 CTR2(KTR_LOCK, "%s: %p resuming from sleep queue",
705 __func__, sx);
706 }
707 #ifdef KDTRACE_HOOKS
708 all_time += lockstat_nsecs(&sx->lock_object);
709 if (sleep_time)
710 LOCKSTAT_RECORD4(LS_SX_XLOCK_BLOCK, sx, sleep_time,
711 LOCKSTAT_WRITER, (state & SX_LOCK_SHARED) == 0,
712 (state & SX_LOCK_SHARED) == 0 ? 0 : SX_SHARERS(state));
713 if (spin_cnt > sleep_cnt)
714 LOCKSTAT_RECORD4(LS_SX_XLOCK_SPIN, sx, all_time - sleep_time,
715 LOCKSTAT_WRITER, (state & SX_LOCK_SHARED) == 0,
716 (state & SX_LOCK_SHARED) == 0 ? 0 : SX_SHARERS(state));
717 #endif
718 if (!error)
719 LOCKSTAT_PROFILE_OBTAIN_LOCK_SUCCESS(LS_SX_XLOCK_ACQUIRE, sx,
720 contested, waittime, file, line);
721 GIANT_RESTORE();
722 return (error);
723 }
724
725 /*
726 * This function represents the so-called 'hard case' for sx_xunlock
727 * operation. All 'easy case' failures are redirected to this. Note
728 * that ideally this would be a static function, but it needs to be
729 * accessible from at least sx.h.
730 */
731 void
732 _sx_xunlock_hard(struct sx *sx, uintptr_t tid, const char *file, int line)
733 {
734 uintptr_t x;
735 int queue, wakeup_swapper;
736
737 if (SCHEDULER_STOPPED())
738 return;
739
740 MPASS(!(sx->sx_lock & SX_LOCK_SHARED));
741
742 /* If the lock is recursed, then unrecurse one level. */
743 if (sx_xlocked(sx) && sx_recursed(sx)) {
744 if ((--sx->sx_recurse) == 0)
745 atomic_clear_ptr(&sx->sx_lock, SX_LOCK_RECURSED);
746 if (LOCK_LOG_TEST(&sx->lock_object, 0))
747 CTR2(KTR_LOCK, "%s: %p unrecursing", __func__, sx);
748 return;
749 }
750 MPASS(sx->sx_lock & (SX_LOCK_SHARED_WAITERS |
751 SX_LOCK_EXCLUSIVE_WAITERS));
752 if (LOCK_LOG_TEST(&sx->lock_object, 0))
753 CTR2(KTR_LOCK, "%s: %p contested", __func__, sx);
754
755 sleepq_lock(&sx->lock_object);
756 x = SX_LOCK_UNLOCKED;
757
758 /*
759 * The wake up algorithm here is quite simple and probably not
760 * ideal. It gives precedence to shared waiters if they are
761 * present. For this condition, we have to preserve the
762 * state of the exclusive waiters flag.
763 * If interruptible sleeps left the shared queue empty avoid a
764 * starvation for the threads sleeping on the exclusive queue by giving
765 * them precedence and cleaning up the shared waiters bit anyway.
766 */
767 if ((sx->sx_lock & SX_LOCK_SHARED_WAITERS) != 0 &&
768 sleepq_sleepcnt(&sx->lock_object, SQ_SHARED_QUEUE) != 0) {
769 queue = SQ_SHARED_QUEUE;
770 x |= (sx->sx_lock & SX_LOCK_EXCLUSIVE_WAITERS);
771 } else
772 queue = SQ_EXCLUSIVE_QUEUE;
773
774 /* Wake up all the waiters for the specific queue. */
775 if (LOCK_LOG_TEST(&sx->lock_object, 0))
776 CTR3(KTR_LOCK, "%s: %p waking up all threads on %s queue",
777 __func__, sx, queue == SQ_SHARED_QUEUE ? "shared" :
778 "exclusive");
779 atomic_store_rel_ptr(&sx->sx_lock, x);
780 wakeup_swapper = sleepq_broadcast(&sx->lock_object, SLEEPQ_SX, 0,
781 queue);
782 sleepq_release(&sx->lock_object);
783 if (wakeup_swapper)
784 kick_proc0();
785 }
786
787 /*
788 * This function represents the so-called 'hard case' for sx_slock
789 * operation. All 'easy case' failures are redirected to this. Note
790 * that ideally this would be a static function, but it needs to be
791 * accessible from at least sx.h.
792 */
793 int
794 _sx_slock_hard(struct sx *sx, int opts, const char *file, int line)
795 {
796 GIANT_DECLARE;
797 #ifdef ADAPTIVE_SX
798 volatile struct thread *owner;
799 #endif
800 #ifdef LOCK_PROFILING
801 uint64_t waittime = 0;
802 int contested = 0;
803 #endif
804 uintptr_t x;
805 int error = 0;
806 #ifdef KDTRACE_HOOKS
807 uintptr_t state;
808 uint64_t spin_cnt = 0;
809 uint64_t sleep_cnt = 0;
810 int64_t sleep_time = 0;
811 int64_t all_time = 0;
812 #endif
813
814 if (SCHEDULER_STOPPED())
815 return (0);
816
817 #ifdef KDTRACE_HOOKS
818 state = sx->sx_lock;
819 all_time -= lockstat_nsecs(&sx->lock_object);
820 #endif
821
822 /*
823 * As with rwlocks, we don't make any attempt to try to block
824 * shared locks once there is an exclusive waiter.
825 */
826 for (;;) {
827 #ifdef KDTRACE_HOOKS
828 spin_cnt++;
829 #endif
830 x = sx->sx_lock;
831
832 /*
833 * If no other thread has an exclusive lock then try to bump up
834 * the count of sharers. Since we have to preserve the state
835 * of SX_LOCK_EXCLUSIVE_WAITERS, if we fail to acquire the
836 * shared lock loop back and retry.
837 */
838 if (x & SX_LOCK_SHARED) {
839 MPASS(!(x & SX_LOCK_SHARED_WAITERS));
840 if (atomic_cmpset_acq_ptr(&sx->sx_lock, x,
841 x + SX_ONE_SHARER)) {
842 if (LOCK_LOG_TEST(&sx->lock_object, 0))
843 CTR4(KTR_LOCK,
844 "%s: %p succeed %p -> %p", __func__,
845 sx, (void *)x,
846 (void *)(x + SX_ONE_SHARER));
847 break;
848 }
849 continue;
850 }
851 #ifdef HWPMC_HOOKS
852 PMC_SOFT_CALL( , , lock, failed);
853 #endif
854 lock_profile_obtain_lock_failed(&sx->lock_object, &contested,
855 &waittime);
856
857 #ifdef ADAPTIVE_SX
858 /*
859 * If the owner is running on another CPU, spin until
860 * the owner stops running or the state of the lock
861 * changes.
862 */
863 if ((sx->lock_object.lo_flags & SX_NOADAPTIVE) == 0) {
864 x = SX_OWNER(x);
865 owner = (struct thread *)x;
866 if (TD_IS_RUNNING(owner)) {
867 if (LOCK_LOG_TEST(&sx->lock_object, 0))
868 CTR3(KTR_LOCK,
869 "%s: spinning on %p held by %p",
870 __func__, sx, owner);
871 GIANT_SAVE();
872 while (SX_OWNER(sx->sx_lock) == x &&
873 TD_IS_RUNNING(owner)) {
874 #ifdef KDTRACE_HOOKS
875 spin_cnt++;
876 #endif
877 cpu_spinwait();
878 }
879 continue;
880 }
881 }
882 #endif
883
884 /*
885 * Some other thread already has an exclusive lock, so
886 * start the process of blocking.
887 */
888 sleepq_lock(&sx->lock_object);
889 x = sx->sx_lock;
890
891 /*
892 * The lock could have been released while we spun.
893 * In this case loop back and retry.
894 */
895 if (x & SX_LOCK_SHARED) {
896 sleepq_release(&sx->lock_object);
897 continue;
898 }
899
900 #ifdef ADAPTIVE_SX
901 /*
902 * If the owner is running on another CPU, spin until
903 * the owner stops running or the state of the lock
904 * changes.
905 */
906 if (!(x & SX_LOCK_SHARED) &&
907 (sx->lock_object.lo_flags & SX_NOADAPTIVE) == 0) {
908 owner = (struct thread *)SX_OWNER(x);
909 if (TD_IS_RUNNING(owner)) {
910 sleepq_release(&sx->lock_object);
911 continue;
912 }
913 }
914 #endif
915
916 /*
917 * Try to set the SX_LOCK_SHARED_WAITERS flag. If we
918 * fail to set it drop the sleep queue lock and loop
919 * back.
920 */
921 if (!(x & SX_LOCK_SHARED_WAITERS)) {
922 if (!atomic_cmpset_ptr(&sx->sx_lock, x,
923 x | SX_LOCK_SHARED_WAITERS)) {
924 sleepq_release(&sx->lock_object);
925 continue;
926 }
927 if (LOCK_LOG_TEST(&sx->lock_object, 0))
928 CTR2(KTR_LOCK, "%s: %p set shared waiters flag",
929 __func__, sx);
930 }
931
932 /*
933 * Since we have been unable to acquire the shared lock,
934 * we have to sleep.
935 */
936 if (LOCK_LOG_TEST(&sx->lock_object, 0))
937 CTR2(KTR_LOCK, "%s: %p blocking on sleep queue",
938 __func__, sx);
939
940 #ifdef KDTRACE_HOOKS
941 sleep_time -= lockstat_nsecs(&sx->lock_object);
942 #endif
943 GIANT_SAVE();
944 sleepq_add(&sx->lock_object, NULL, sx->lock_object.lo_name,
945 SLEEPQ_SX | ((opts & SX_INTERRUPTIBLE) ?
946 SLEEPQ_INTERRUPTIBLE : 0), SQ_SHARED_QUEUE);
947 if (!(opts & SX_INTERRUPTIBLE))
948 sleepq_wait(&sx->lock_object, 0);
949 else
950 error = sleepq_wait_sig(&sx->lock_object, 0);
951 #ifdef KDTRACE_HOOKS
952 sleep_time += lockstat_nsecs(&sx->lock_object);
953 sleep_cnt++;
954 #endif
955 if (error) {
956 if (LOCK_LOG_TEST(&sx->lock_object, 0))
957 CTR2(KTR_LOCK,
958 "%s: interruptible sleep by %p suspended by signal",
959 __func__, sx);
960 break;
961 }
962 if (LOCK_LOG_TEST(&sx->lock_object, 0))
963 CTR2(KTR_LOCK, "%s: %p resuming from sleep queue",
964 __func__, sx);
965 }
966 #ifdef KDTRACE_HOOKS
967 all_time += lockstat_nsecs(&sx->lock_object);
968 if (sleep_time)
969 LOCKSTAT_RECORD4(LS_SX_SLOCK_BLOCK, sx, sleep_time,
970 LOCKSTAT_READER, (state & SX_LOCK_SHARED) == 0,
971 (state & SX_LOCK_SHARED) == 0 ? 0 : SX_SHARERS(state));
972 if (spin_cnt > sleep_cnt)
973 LOCKSTAT_RECORD4(LS_SX_SLOCK_SPIN, sx, all_time - sleep_time,
974 LOCKSTAT_READER, (state & SX_LOCK_SHARED) == 0,
975 (state & SX_LOCK_SHARED) == 0 ? 0 : SX_SHARERS(state));
976 #endif
977 if (error == 0)
978 LOCKSTAT_PROFILE_OBTAIN_LOCK_SUCCESS(LS_SX_SLOCK_ACQUIRE, sx,
979 contested, waittime, file, line);
980 GIANT_RESTORE();
981 return (error);
982 }
983
984 /*
985 * This function represents the so-called 'hard case' for sx_sunlock
986 * operation. All 'easy case' failures are redirected to this. Note
987 * that ideally this would be a static function, but it needs to be
988 * accessible from at least sx.h.
989 */
990 void
991 _sx_sunlock_hard(struct sx *sx, const char *file, int line)
992 {
993 uintptr_t x;
994 int wakeup_swapper;
995
996 if (SCHEDULER_STOPPED())
997 return;
998
999 for (;;) {
1000 x = sx->sx_lock;
1001
1002 /*
1003 * We should never have sharers while at least one thread
1004 * holds a shared lock.
1005 */
1006 KASSERT(!(x & SX_LOCK_SHARED_WAITERS),
1007 ("%s: waiting sharers", __func__));
1008
1009 /*
1010 * See if there is more than one shared lock held. If
1011 * so, just drop one and return.
1012 */
1013 if (SX_SHARERS(x) > 1) {
1014 if (atomic_cmpset_rel_ptr(&sx->sx_lock, x,
1015 x - SX_ONE_SHARER)) {
1016 if (LOCK_LOG_TEST(&sx->lock_object, 0))
1017 CTR4(KTR_LOCK,
1018 "%s: %p succeeded %p -> %p",
1019 __func__, sx, (void *)x,
1020 (void *)(x - SX_ONE_SHARER));
1021 break;
1022 }
1023 continue;
1024 }
1025
1026 /*
1027 * If there aren't any waiters for an exclusive lock,
1028 * then try to drop it quickly.
1029 */
1030 if (!(x & SX_LOCK_EXCLUSIVE_WAITERS)) {
1031 MPASS(x == SX_SHARERS_LOCK(1));
1032 if (atomic_cmpset_rel_ptr(&sx->sx_lock,
1033 SX_SHARERS_LOCK(1), SX_LOCK_UNLOCKED)) {
1034 if (LOCK_LOG_TEST(&sx->lock_object, 0))
1035 CTR2(KTR_LOCK, "%s: %p last succeeded",
1036 __func__, sx);
1037 break;
1038 }
1039 continue;
1040 }
1041
1042 /*
1043 * At this point, there should just be one sharer with
1044 * exclusive waiters.
1045 */
1046 MPASS(x == (SX_SHARERS_LOCK(1) | SX_LOCK_EXCLUSIVE_WAITERS));
1047
1048 sleepq_lock(&sx->lock_object);
1049
1050 /*
1051 * Wake up semantic here is quite simple:
1052 * Just wake up all the exclusive waiters.
1053 * Note that the state of the lock could have changed,
1054 * so if it fails loop back and retry.
1055 */
1056 if (!atomic_cmpset_rel_ptr(&sx->sx_lock,
1057 SX_SHARERS_LOCK(1) | SX_LOCK_EXCLUSIVE_WAITERS,
1058 SX_LOCK_UNLOCKED)) {
1059 sleepq_release(&sx->lock_object);
1060 continue;
1061 }
1062 if (LOCK_LOG_TEST(&sx->lock_object, 0))
1063 CTR2(KTR_LOCK, "%s: %p waking up all thread on"
1064 "exclusive queue", __func__, sx);
1065 wakeup_swapper = sleepq_broadcast(&sx->lock_object, SLEEPQ_SX,
1066 0, SQ_EXCLUSIVE_QUEUE);
1067 sleepq_release(&sx->lock_object);
1068 if (wakeup_swapper)
1069 kick_proc0();
1070 break;
1071 }
1072 }
1073
1074 #ifdef INVARIANT_SUPPORT
1075 #ifndef INVARIANTS
1076 #undef _sx_assert
1077 #endif
1078
1079 /*
1080 * In the non-WITNESS case, sx_assert() can only detect that at least
1081 * *some* thread owns an slock, but it cannot guarantee that *this*
1082 * thread owns an slock.
1083 */
1084 void
1085 _sx_assert(struct sx *sx, int what, const char *file, int line)
1086 {
1087 #ifndef WITNESS
1088 int slocked = 0;
1089 #endif
1090
1091 if (panicstr != NULL)
1092 return;
1093 switch (what) {
1094 case SA_SLOCKED:
1095 case SA_SLOCKED | SA_NOTRECURSED:
1096 case SA_SLOCKED | SA_RECURSED:
1097 #ifndef WITNESS
1098 slocked = 1;
1099 /* FALLTHROUGH */
1100 #endif
1101 case SA_LOCKED:
1102 case SA_LOCKED | SA_NOTRECURSED:
1103 case SA_LOCKED | SA_RECURSED:
1104 #ifdef WITNESS
1105 witness_assert(&sx->lock_object, what, file, line);
1106 #else
1107 /*
1108 * If some other thread has an exclusive lock or we
1109 * have one and are asserting a shared lock, fail.
1110 * Also, if no one has a lock at all, fail.
1111 */
1112 if (sx->sx_lock == SX_LOCK_UNLOCKED ||
1113 (!(sx->sx_lock & SX_LOCK_SHARED) && (slocked ||
1114 sx_xholder(sx) != curthread)))
1115 panic("Lock %s not %slocked @ %s:%d\n",
1116 sx->lock_object.lo_name, slocked ? "share " : "",
1117 file, line);
1118
1119 if (!(sx->sx_lock & SX_LOCK_SHARED)) {
1120 if (sx_recursed(sx)) {
1121 if (what & SA_NOTRECURSED)
1122 panic("Lock %s recursed @ %s:%d\n",
1123 sx->lock_object.lo_name, file,
1124 line);
1125 } else if (what & SA_RECURSED)
1126 panic("Lock %s not recursed @ %s:%d\n",
1127 sx->lock_object.lo_name, file, line);
1128 }
1129 #endif
1130 break;
1131 case SA_XLOCKED:
1132 case SA_XLOCKED | SA_NOTRECURSED:
1133 case SA_XLOCKED | SA_RECURSED:
1134 if (sx_xholder(sx) != curthread)
1135 panic("Lock %s not exclusively locked @ %s:%d\n",
1136 sx->lock_object.lo_name, file, line);
1137 if (sx_recursed(sx)) {
1138 if (what & SA_NOTRECURSED)
1139 panic("Lock %s recursed @ %s:%d\n",
1140 sx->lock_object.lo_name, file, line);
1141 } else if (what & SA_RECURSED)
1142 panic("Lock %s not recursed @ %s:%d\n",
1143 sx->lock_object.lo_name, file, line);
1144 break;
1145 case SA_UNLOCKED:
1146 #ifdef WITNESS
1147 witness_assert(&sx->lock_object, what, file, line);
1148 #else
1149 /*
1150 * If we hold an exclusve lock fail. We can't
1151 * reliably check to see if we hold a shared lock or
1152 * not.
1153 */
1154 if (sx_xholder(sx) == curthread)
1155 panic("Lock %s exclusively locked @ %s:%d\n",
1156 sx->lock_object.lo_name, file, line);
1157 #endif
1158 break;
1159 default:
1160 panic("Unknown sx lock assertion: %d @ %s:%d", what, file,
1161 line);
1162 }
1163 }
1164 #endif /* INVARIANT_SUPPORT */
1165
1166 #ifdef DDB
1167 static void
1168 db_show_sx(struct lock_object *lock)
1169 {
1170 struct thread *td;
1171 struct sx *sx;
1172
1173 sx = (struct sx *)lock;
1174
1175 db_printf(" state: ");
1176 if (sx->sx_lock == SX_LOCK_UNLOCKED)
1177 db_printf("UNLOCKED\n");
1178 else if (sx->sx_lock == SX_LOCK_DESTROYED) {
1179 db_printf("DESTROYED\n");
1180 return;
1181 } else if (sx->sx_lock & SX_LOCK_SHARED)
1182 db_printf("SLOCK: %ju\n", (uintmax_t)SX_SHARERS(sx->sx_lock));
1183 else {
1184 td = sx_xholder(sx);
1185 db_printf("XLOCK: %p (tid %d, pid %d, \"%s\")\n", td,
1186 td->td_tid, td->td_proc->p_pid, td->td_name);
1187 if (sx_recursed(sx))
1188 db_printf(" recursed: %d\n", sx->sx_recurse);
1189 }
1190
1191 db_printf(" waiters: ");
1192 switch(sx->sx_lock &
1193 (SX_LOCK_SHARED_WAITERS | SX_LOCK_EXCLUSIVE_WAITERS)) {
1194 case SX_LOCK_SHARED_WAITERS:
1195 db_printf("shared\n");
1196 break;
1197 case SX_LOCK_EXCLUSIVE_WAITERS:
1198 db_printf("exclusive\n");
1199 break;
1200 case SX_LOCK_SHARED_WAITERS | SX_LOCK_EXCLUSIVE_WAITERS:
1201 db_printf("exclusive and shared\n");
1202 break;
1203 default:
1204 db_printf("none\n");
1205 }
1206 }
1207
1208 /*
1209 * Check to see if a thread that is blocked on a sleep queue is actually
1210 * blocked on an sx lock. If so, output some details and return true.
1211 * If the lock has an exclusive owner, return that in *ownerp.
1212 */
1213 int
1214 sx_chain(struct thread *td, struct thread **ownerp)
1215 {
1216 struct sx *sx;
1217
1218 /*
1219 * Check to see if this thread is blocked on an sx lock.
1220 * First, we check the lock class. If that is ok, then we
1221 * compare the lock name against the wait message.
1222 */
1223 sx = td->td_wchan;
1224 if (LOCK_CLASS(&sx->lock_object) != &lock_class_sx ||
1225 sx->lock_object.lo_name != td->td_wmesg)
1226 return (0);
1227
1228 /* We think we have an sx lock, so output some details. */
1229 db_printf("blocked on sx \"%s\" ", td->td_wmesg);
1230 *ownerp = sx_xholder(sx);
1231 if (sx->sx_lock & SX_LOCK_SHARED)
1232 db_printf("SLOCK (count %ju)\n",
1233 (uintmax_t)SX_SHARERS(sx->sx_lock));
1234 else
1235 db_printf("XLOCK\n");
1236 return (1);
1237 }
1238 #endif
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