2 * SPDX-License-Identifier: BSD-2-Clause
4 * Copyright (c) 2007 Attilio Rao <attilio@freebsd.org>
5 * Copyright (c) 2001 Jason Evans <jasone@freebsd.org>
8 * Redistribution and use in source and binary forms, with or without
9 * modification, are permitted provided that the following conditions
11 * 1. Redistributions of source code must retain the above copyright
12 * notice(s), this list of conditions and the following disclaimer as
13 * the first lines of this file unmodified other than the possible
14 * addition of one or more copyright notices.
15 * 2. Redistributions in binary form must reproduce the above copyright
16 * notice(s), this list of conditions and the following disclaimer in the
17 * documentation and/or other materials provided with the distribution.
19 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDER(S) ``AS IS'' AND ANY
20 * EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED
21 * WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
22 * DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER(S) BE LIABLE FOR ANY
23 * DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES
24 * (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
25 * SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
26 * CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
27 * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
28 * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH
33 * Shared/exclusive locks. This implementation attempts to ensure
34 * deterministic lock granting behavior, so that slocks and xlocks are
37 * Priority propagation will not generally raise the priority of lock holders,
38 * so should not be relied upon in combination with sx locks.
42 #include "opt_hwpmc_hooks.h"
43 #include "opt_no_adaptive_sx.h"
45 #include <sys/param.h>
46 #include <sys/systm.h>
48 #include <sys/kernel.h>
51 #include <sys/mutex.h>
53 #include <sys/sched.h>
54 #include <sys/sleepqueue.h>
57 #include <sys/sysctl.h>
59 #if defined(SMP) && !defined(NO_ADAPTIVE_SX)
60 #include <machine/cpu.h>
67 #if defined(SMP) && !defined(NO_ADAPTIVE_SX)
72 #include <sys/pmckern.h>
73 PMC_SOFT_DECLARE( , , lock, failed);
76 /* Handy macros for sleep queues. */
77 #define SQ_EXCLUSIVE_QUEUE 0
78 #define SQ_SHARED_QUEUE 1
81 * Variations on DROP_GIANT()/PICKUP_GIANT() for use in this file. We
82 * drop Giant anytime we have to sleep or if we adaptively spin.
84 #define GIANT_DECLARE \
86 WITNESS_SAVE_DECL(Giant) \
88 #define GIANT_SAVE(work) do { \
89 if (__predict_false(mtx_owned(&Giant))) { \
91 WITNESS_SAVE(&Giant.lock_object, Giant); \
92 while (mtx_owned(&Giant)) { \
99 #define GIANT_RESTORE() do { \
100 if (_giantcnt > 0) { \
101 mtx_assert(&Giant, MA_NOTOWNED); \
102 while (_giantcnt--) \
104 WITNESS_RESTORE(&Giant.lock_object, Giant); \
109 * Returns true if an exclusive lock is recursed. It assumes
110 * curthread currently has an exclusive lock.
112 #define sx_recursed(sx) ((sx)->sx_recurse != 0)
114 static void assert_sx(const struct lock_object *lock, int what);
116 static void db_show_sx(const struct lock_object *lock);
118 static void lock_sx(struct lock_object *lock, uintptr_t how);
120 static int owner_sx(const struct lock_object *lock, struct thread **owner);
122 static uintptr_t unlock_sx(struct lock_object *lock);
124 struct lock_class lock_class_sx = {
126 .lc_flags = LC_SLEEPLOCK | LC_SLEEPABLE | LC_RECURSABLE | LC_UPGRADABLE,
127 .lc_assert = assert_sx,
129 .lc_ddb_show = db_show_sx,
132 .lc_unlock = unlock_sx,
134 .lc_owner = owner_sx,
139 #define _sx_assert(sx, what, file, line)
143 #ifdef SX_CUSTOM_BACKOFF
144 static u_short __read_frequently asx_retries;
145 static u_short __read_frequently asx_loops;
146 static SYSCTL_NODE(_debug, OID_AUTO, sx, CTLFLAG_RD | CTLFLAG_MPSAFE, NULL,
148 SYSCTL_U16(_debug_sx, OID_AUTO, retries, CTLFLAG_RW, &asx_retries, 0, "");
149 SYSCTL_U16(_debug_sx, OID_AUTO, loops, CTLFLAG_RW, &asx_loops, 0, "");
151 static struct lock_delay_config __read_frequently sx_delay;
153 SYSCTL_U16(_debug_sx, OID_AUTO, delay_base, CTLFLAG_RW, &sx_delay.base,
155 SYSCTL_U16(_debug_sx, OID_AUTO, delay_max, CTLFLAG_RW, &sx_delay.max,
159 sx_lock_delay_init(void *arg __unused)
162 lock_delay_default_init(&sx_delay);
164 asx_loops = max(10000, sx_delay.max);
166 LOCK_DELAY_SYSINIT(sx_lock_delay_init);
168 #define sx_delay locks_delay
169 #define asx_retries locks_delay_retries
170 #define asx_loops locks_delay_loops
175 assert_sx(const struct lock_object *lock, int what)
178 sx_assert((const struct sx *)lock, what);
182 lock_sx(struct lock_object *lock, uintptr_t how)
186 sx = (struct sx *)lock;
194 unlock_sx(struct lock_object *lock)
198 sx = (struct sx *)lock;
199 sx_assert(sx, SA_LOCKED | SA_NOTRECURSED);
200 if (sx_xlocked(sx)) {
211 owner_sx(const struct lock_object *lock, struct thread **owner)
216 sx = (const struct sx *)lock;
219 return ((x & SX_LOCK_SHARED) != 0 ? (SX_SHARERS(x) != 0) :
220 ((*owner = (struct thread *)SX_OWNER(x)) != NULL));
225 sx_sysinit(void *arg)
227 struct sx_args *sargs = arg;
229 sx_init_flags(sargs->sa_sx, sargs->sa_desc, sargs->sa_flags);
233 sx_init_flags(struct sx *sx, const char *description, int opts)
237 MPASS((opts & ~(SX_QUIET | SX_RECURSE | SX_NOWITNESS | SX_DUPOK |
238 SX_NOPROFILE | SX_NEW)) == 0);
239 ASSERT_ATOMIC_LOAD_PTR(sx->sx_lock,
240 ("%s: sx_lock not aligned for %s: %p", __func__, description,
243 flags = LO_SLEEPABLE | LO_UPGRADABLE;
246 if (opts & SX_NOPROFILE)
247 flags |= LO_NOPROFILE;
248 if (!(opts & SX_NOWITNESS))
250 if (opts & SX_RECURSE)
251 flags |= LO_RECURSABLE;
257 lock_init(&sx->lock_object, &lock_class_sx, description, NULL, flags);
258 sx->sx_lock = SX_LOCK_UNLOCKED;
263 sx_destroy(struct sx *sx)
266 KASSERT(sx->sx_lock == SX_LOCK_UNLOCKED, ("sx lock still held"));
267 KASSERT(sx->sx_recurse == 0, ("sx lock still recursed"));
268 sx->sx_lock = SX_LOCK_DESTROYED;
269 lock_destroy(&sx->lock_object);
273 sx_try_slock_int(struct sx *sx LOCK_FILE_LINE_ARG_DEF)
277 if (SCHEDULER_STOPPED())
280 KASSERT(kdb_active != 0 || !TD_IS_IDLETHREAD(curthread),
281 ("sx_try_slock() by idle thread %p on sx %s @ %s:%d",
282 curthread, sx->lock_object.lo_name, file, line));
286 KASSERT(x != SX_LOCK_DESTROYED,
287 ("sx_try_slock() of destroyed sx @ %s:%d", file, line));
288 if (!(x & SX_LOCK_SHARED))
290 if (atomic_fcmpset_acq_ptr(&sx->sx_lock, &x, x + SX_ONE_SHARER)) {
291 LOCK_LOG_TRY("SLOCK", &sx->lock_object, 0, 1, file, line);
292 WITNESS_LOCK(&sx->lock_object, LOP_TRYLOCK, file, line);
293 LOCKSTAT_PROFILE_OBTAIN_RWLOCK_SUCCESS(sx__acquire,
294 sx, 0, 0, file, line, LOCKSTAT_READER);
295 TD_LOCKS_INC(curthread);
296 curthread->td_sx_slocks++;
301 LOCK_LOG_TRY("SLOCK", &sx->lock_object, 0, 0, file, line);
306 sx_try_slock_(struct sx *sx, const char *file, int line)
309 return (sx_try_slock_int(sx LOCK_FILE_LINE_ARG));
313 _sx_xlock(struct sx *sx, int opts, const char *file, int line)
318 KASSERT(kdb_active != 0 || SCHEDULER_STOPPED() ||
319 !TD_IS_IDLETHREAD(curthread),
320 ("sx_xlock() by idle thread %p on sx %s @ %s:%d",
321 curthread, sx->lock_object.lo_name, file, line));
322 KASSERT(sx->sx_lock != SX_LOCK_DESTROYED,
323 ("sx_xlock() of destroyed sx @ %s:%d", file, line));
324 WITNESS_CHECKORDER(&sx->lock_object, LOP_NEWORDER | LOP_EXCLUSIVE, file,
326 tid = (uintptr_t)curthread;
327 x = SX_LOCK_UNLOCKED;
328 if (!atomic_fcmpset_acq_ptr(&sx->sx_lock, &x, tid))
329 error = _sx_xlock_hard(sx, x, opts LOCK_FILE_LINE_ARG);
331 LOCKSTAT_PROFILE_OBTAIN_RWLOCK_SUCCESS(sx__acquire, sx,
332 0, 0, file, line, LOCKSTAT_WRITER);
334 LOCK_LOG_LOCK("XLOCK", &sx->lock_object, 0, sx->sx_recurse,
336 WITNESS_LOCK(&sx->lock_object, LOP_EXCLUSIVE, file, line);
337 TD_LOCKS_INC(curthread);
344 sx_try_xlock_int(struct sx *sx LOCK_FILE_LINE_ARG_DEF)
353 if (SCHEDULER_STOPPED_TD(td))
356 KASSERT(kdb_active != 0 || !TD_IS_IDLETHREAD(td),
357 ("sx_try_xlock() by idle thread %p on sx %s @ %s:%d",
358 curthread, sx->lock_object.lo_name, file, line));
359 KASSERT(sx->sx_lock != SX_LOCK_DESTROYED,
360 ("sx_try_xlock() of destroyed sx @ %s:%d", file, line));
364 x = SX_LOCK_UNLOCKED;
366 if (atomic_fcmpset_acq_ptr(&sx->sx_lock, &x, tid))
368 if (x == SX_LOCK_UNLOCKED)
370 if (x == tid && (sx->lock_object.lo_flags & LO_RECURSABLE)) {
372 atomic_set_ptr(&sx->sx_lock, SX_LOCK_RECURSED);
379 LOCK_LOG_TRY("XLOCK", &sx->lock_object, 0, rval, file, line);
381 WITNESS_LOCK(&sx->lock_object, LOP_EXCLUSIVE | LOP_TRYLOCK,
384 LOCKSTAT_PROFILE_OBTAIN_RWLOCK_SUCCESS(sx__acquire,
385 sx, 0, 0, file, line, LOCKSTAT_WRITER);
386 TD_LOCKS_INC(curthread);
393 sx_try_xlock_(struct sx *sx, const char *file, int line)
396 return (sx_try_xlock_int(sx LOCK_FILE_LINE_ARG));
400 _sx_xunlock(struct sx *sx, const char *file, int line)
403 KASSERT(sx->sx_lock != SX_LOCK_DESTROYED,
404 ("sx_xunlock() of destroyed sx @ %s:%d", file, line));
405 _sx_assert(sx, SA_XLOCKED, file, line);
406 WITNESS_UNLOCK(&sx->lock_object, LOP_EXCLUSIVE, file, line);
407 LOCK_LOG_LOCK("XUNLOCK", &sx->lock_object, 0, sx->sx_recurse, file,
410 _sx_xunlock_hard(sx, (uintptr_t)curthread, file, line);
412 __sx_xunlock(sx, curthread, file, line);
414 TD_LOCKS_DEC(curthread);
418 * Try to do a non-blocking upgrade from a shared lock to an exclusive lock.
419 * This will only succeed if this thread holds a single shared lock.
420 * Return 1 if if the upgrade succeed, 0 otherwise.
423 sx_try_upgrade_int(struct sx *sx LOCK_FILE_LINE_ARG_DEF)
429 if (SCHEDULER_STOPPED())
432 KASSERT(sx->sx_lock != SX_LOCK_DESTROYED,
433 ("sx_try_upgrade() of destroyed sx @ %s:%d", file, line));
434 _sx_assert(sx, SA_SLOCKED, file, line);
437 * Try to switch from one shared lock to an exclusive lock. We need
438 * to maintain the SX_LOCK_EXCLUSIVE_WAITERS flag if set so that
439 * we will wake up the exclusive waiters when we drop the lock.
442 x = SX_READ_VALUE(sx);
444 if (SX_SHARERS(x) > 1)
446 waiters = (x & SX_LOCK_WAITERS);
447 if (atomic_fcmpset_acq_ptr(&sx->sx_lock, &x,
448 (uintptr_t)curthread | waiters)) {
453 LOCK_LOG_TRY("XUPGRADE", &sx->lock_object, 0, success, file, line);
455 curthread->td_sx_slocks--;
456 WITNESS_UPGRADE(&sx->lock_object, LOP_EXCLUSIVE | LOP_TRYLOCK,
458 LOCKSTAT_RECORD0(sx__upgrade, sx);
464 sx_try_upgrade_(struct sx *sx, const char *file, int line)
467 return (sx_try_upgrade_int(sx LOCK_FILE_LINE_ARG));
471 * Downgrade an unrecursed exclusive lock into a single shared lock.
474 sx_downgrade_int(struct sx *sx LOCK_FILE_LINE_ARG_DEF)
479 if (SCHEDULER_STOPPED())
482 KASSERT(sx->sx_lock != SX_LOCK_DESTROYED,
483 ("sx_downgrade() of destroyed sx @ %s:%d", file, line));
484 _sx_assert(sx, SA_XLOCKED | SA_NOTRECURSED, file, line);
487 panic("downgrade of a recursed lock");
490 WITNESS_DOWNGRADE(&sx->lock_object, 0, file, line);
493 * Try to switch from an exclusive lock with no shared waiters
494 * to one sharer with no shared waiters. If there are
495 * exclusive waiters, we don't need to lock the sleep queue so
496 * long as we preserve the flag. We do one quick try and if
497 * that fails we grab the sleepq lock to keep the flags from
498 * changing and do it the slow way.
500 * We have to lock the sleep queue if there are shared waiters
501 * so we can wake them up.
504 if (!(x & SX_LOCK_SHARED_WAITERS) &&
505 atomic_cmpset_rel_ptr(&sx->sx_lock, x, SX_SHARERS_LOCK(1) |
506 (x & SX_LOCK_EXCLUSIVE_WAITERS)))
510 * Lock the sleep queue so we can read the waiters bits
511 * without any races and wakeup any shared waiters.
513 sleepq_lock(&sx->lock_object);
516 * Preserve SX_LOCK_EXCLUSIVE_WAITERS while downgraded to a single
517 * shared lock. If there are any shared waiters, wake them up.
521 atomic_store_rel_ptr(&sx->sx_lock, SX_SHARERS_LOCK(1) |
522 (x & SX_LOCK_EXCLUSIVE_WAITERS));
523 if (x & SX_LOCK_SHARED_WAITERS)
524 wakeup_swapper = sleepq_broadcast(&sx->lock_object, SLEEPQ_SX,
526 sleepq_release(&sx->lock_object);
532 curthread->td_sx_slocks++;
533 LOCK_LOG_LOCK("XDOWNGRADE", &sx->lock_object, 0, 0, file, line);
534 LOCKSTAT_RECORD0(sx__downgrade, sx);
538 sx_downgrade_(struct sx *sx, const char *file, int line)
541 sx_downgrade_int(sx LOCK_FILE_LINE_ARG);
546 sx_drop_critical(uintptr_t x, bool *in_critical, int *extra_work)
549 if (x & SX_LOCK_WRITE_SPINNER)
553 *in_critical = false;
558 #define sx_drop_critical(x, in_critical, extra_work) do { } while (0)
562 * This function represents the so-called 'hard case' for sx_xlock
563 * operation. All 'easy case' failures are redirected to this. Note
564 * that ideally this would be a static function, but it needs to be
565 * accessible from at least sx.h.
568 _sx_xlock_hard(struct sx *sx, uintptr_t x, int opts LOCK_FILE_LINE_ARG_DEF)
573 struct thread *owner;
574 u_int i, n, spintries = 0;
575 enum { READERS, WRITER } sleep_reason = READERS;
576 bool in_critical = false;
578 #ifdef LOCK_PROFILING
579 uint64_t waittime = 0;
583 #if defined(ADAPTIVE_SX) || defined(KDTRACE_HOOKS)
584 struct lock_delay_arg lda;
588 int64_t sleep_time = 0;
589 int64_t all_time = 0;
591 #if defined(KDTRACE_HOOKS) || defined(LOCK_PROFILING)
593 int doing_lockprof = 0;
597 tid = (uintptr_t)curthread;
600 if (LOCKSTAT_PROFILE_ENABLED(sx__acquire)) {
601 while (x == SX_LOCK_UNLOCKED) {
602 if (atomic_fcmpset_acq_ptr(&sx->sx_lock, &x, tid))
607 all_time -= lockstat_nsecs(&sx->lock_object);
611 #ifdef LOCK_PROFILING
617 if (SCHEDULER_STOPPED())
620 if (__predict_false(x == SX_LOCK_UNLOCKED))
621 x = SX_READ_VALUE(sx);
623 /* If we already hold an exclusive lock, then recurse. */
624 if (__predict_false(lv_sx_owner(x) == (struct thread *)tid)) {
625 KASSERT((sx->lock_object.lo_flags & LO_RECURSABLE) != 0,
626 ("_sx_xlock_hard: recursed on non-recursive sx %s @ %s:%d\n",
627 sx->lock_object.lo_name, file, line));
629 atomic_set_ptr(&sx->sx_lock, SX_LOCK_RECURSED);
630 if (LOCK_LOG_TEST(&sx->lock_object, 0))
631 CTR2(KTR_LOCK, "%s: %p recursing", __func__, sx);
635 if (LOCK_LOG_TEST(&sx->lock_object, 0))
636 CTR5(KTR_LOCK, "%s: %s contested (lock=%p) at %s:%d", __func__,
637 sx->lock_object.lo_name, (void *)sx->sx_lock, file, line);
639 #if defined(ADAPTIVE_SX)
640 lock_delay_arg_init(&lda, &sx_delay);
641 #elif defined(KDTRACE_HOOKS)
642 lock_delay_arg_init_noadapt(&lda);
646 PMC_SOFT_CALL( , , lock, failed);
648 lock_profile_obtain_lock_failed(&sx->lock_object, false, &contested,
652 GIANT_SAVE(extra_work);
655 THREAD_CONTENDS_ON_LOCK(&sx->lock_object);
658 if (x == SX_LOCK_UNLOCKED) {
659 if (atomic_fcmpset_acq_ptr(&sx->sx_lock, &x, tid))
664 GIANT_SAVE(extra_work);
670 if (x == (SX_LOCK_SHARED | SX_LOCK_WRITE_SPINNER)) {
671 if (atomic_fcmpset_acq_ptr(&sx->sx_lock, &x, tid))
677 * If the lock is write locked and the owner is
678 * running on another CPU, spin until the owner stops
679 * running or the state of the lock changes.
681 if ((x & SX_LOCK_SHARED) == 0) {
682 sx_drop_critical(x, &in_critical, &extra_work);
683 sleep_reason = WRITER;
684 owner = lv_sx_owner(x);
685 if (!TD_IS_RUNNING(owner))
687 if (LOCK_LOG_TEST(&sx->lock_object, 0))
688 CTR3(KTR_LOCK, "%s: spinning on %p held by %p",
689 __func__, sx, owner);
690 KTR_STATE1(KTR_SCHED, "thread", sched_tdname(curthread),
691 "spinning", "lockname:\"%s\"",
692 sx->lock_object.lo_name);
695 x = SX_READ_VALUE(sx);
696 owner = lv_sx_owner(x);
697 } while (owner != NULL && TD_IS_RUNNING(owner));
698 KTR_STATE0(KTR_SCHED, "thread", sched_tdname(curthread),
701 } else if (SX_SHARERS(x) > 0) {
702 sleep_reason = READERS;
703 if (spintries == asx_retries)
705 if (!(x & SX_LOCK_WRITE_SPINNER)) {
711 if (!atomic_fcmpset_ptr(&sx->sx_lock, &x,
712 x | SX_LOCK_WRITE_SPINNER)) {
720 KTR_STATE1(KTR_SCHED, "thread", sched_tdname(curthread),
721 "spinning", "lockname:\"%s\"",
722 sx->lock_object.lo_name);
724 for (i = 0; i < asx_loops; i += n) {
726 x = SX_READ_VALUE(sx);
727 if (!(x & SX_LOCK_WRITE_SPINNER))
729 if (!(x & SX_LOCK_SHARED))
738 KTR_STATE0(KTR_SCHED, "thread", sched_tdname(curthread),
745 sleepq_lock(&sx->lock_object);
746 x = SX_READ_VALUE(sx);
750 * If the lock was released while spinning on the
751 * sleep queue chain lock, try again.
753 if (x == SX_LOCK_UNLOCKED) {
754 sleepq_release(&sx->lock_object);
755 sx_drop_critical(x, &in_critical, &extra_work);
761 * The current lock owner might have started executing
762 * on another CPU (or the lock could have changed
763 * owners) while we were waiting on the sleep queue
764 * chain lock. If so, drop the sleep queue lock and try
767 if (!(x & SX_LOCK_SHARED)) {
768 owner = (struct thread *)SX_OWNER(x);
769 if (TD_IS_RUNNING(owner)) {
770 sleepq_release(&sx->lock_object);
771 sx_drop_critical(x, &in_critical,
775 } else if (SX_SHARERS(x) > 0 && sleep_reason == WRITER) {
776 sleepq_release(&sx->lock_object);
777 sx_drop_critical(x, &in_critical, &extra_work);
783 * If an exclusive lock was released with both shared
784 * and exclusive waiters and a shared waiter hasn't
785 * woken up and acquired the lock yet, sx_lock will be
786 * set to SX_LOCK_UNLOCKED | SX_LOCK_EXCLUSIVE_WAITERS.
787 * If we see that value, try to acquire it once. Note
788 * that we have to preserve SX_LOCK_EXCLUSIVE_WAITERS
789 * as there are other exclusive waiters still. If we
790 * fail, restart the loop.
792 setx = x & (SX_LOCK_WAITERS | SX_LOCK_WRITE_SPINNER);
793 if ((x & ~setx) == SX_LOCK_SHARED) {
794 setx &= ~SX_LOCK_WRITE_SPINNER;
795 if (!atomic_fcmpset_acq_ptr(&sx->sx_lock, &x, tid | setx))
797 sleepq_release(&sx->lock_object);
798 CTR2(KTR_LOCK, "%s: %p claimed by new writer",
805 * It is possible we set the SX_LOCK_WRITE_SPINNER bit.
806 * It is an invariant that when the bit is set, there is
807 * a writer ready to grab the lock. Thus clear the bit since
808 * we are going to sleep.
811 if ((x & SX_LOCK_WRITE_SPINNER) ||
812 !((x & SX_LOCK_EXCLUSIVE_WAITERS))) {
813 setx = x & ~SX_LOCK_WRITE_SPINNER;
814 setx |= SX_LOCK_EXCLUSIVE_WAITERS;
815 if (!atomic_fcmpset_ptr(&sx->sx_lock, &x,
825 * Try to set the SX_LOCK_EXCLUSIVE_WAITERS. If we fail,
826 * than loop back and retry.
828 if (!(x & SX_LOCK_EXCLUSIVE_WAITERS)) {
829 if (!atomic_fcmpset_ptr(&sx->sx_lock, &x,
830 x | SX_LOCK_EXCLUSIVE_WAITERS)) {
833 if (LOCK_LOG_TEST(&sx->lock_object, 0))
834 CTR2(KTR_LOCK, "%s: %p set excl waiters flag",
842 * Since we have been unable to acquire the exclusive
843 * lock and the exclusive waiters flag is set, we have
846 if (LOCK_LOG_TEST(&sx->lock_object, 0))
847 CTR2(KTR_LOCK, "%s: %p blocking on sleep queue",
851 sleep_time -= lockstat_nsecs(&sx->lock_object);
853 sleepq_add(&sx->lock_object, NULL, sx->lock_object.lo_name,
854 SLEEPQ_SX | ((opts & SX_INTERRUPTIBLE) ?
855 SLEEPQ_INTERRUPTIBLE : 0), SQ_EXCLUSIVE_QUEUE);
857 * Hack: this can land in thread_suspend_check which will
858 * conditionally take a mutex, tripping over an assert if a
859 * lock we are waiting for is set.
861 THREAD_CONTENTION_DONE(&sx->lock_object);
862 if (!(opts & SX_INTERRUPTIBLE))
863 sleepq_wait(&sx->lock_object, 0);
865 error = sleepq_wait_sig(&sx->lock_object, 0);
866 THREAD_CONTENDS_ON_LOCK(&sx->lock_object);
868 sleep_time += lockstat_nsecs(&sx->lock_object);
872 if (LOCK_LOG_TEST(&sx->lock_object, 0))
874 "%s: interruptible sleep by %p suspended by signal",
878 if (LOCK_LOG_TEST(&sx->lock_object, 0))
879 CTR2(KTR_LOCK, "%s: %p resuming from sleep queue",
881 x = SX_READ_VALUE(sx);
883 THREAD_CONTENTION_DONE(&sx->lock_object);
884 if (__predict_true(!extra_work))
891 #if defined(KDTRACE_HOOKS) || defined(LOCK_PROFILING)
892 if (__predict_true(!doing_lockprof))
896 all_time += lockstat_nsecs(&sx->lock_object);
898 LOCKSTAT_RECORD4(sx__block, sx, sleep_time,
899 LOCKSTAT_WRITER, (state & SX_LOCK_SHARED) == 0,
900 (state & SX_LOCK_SHARED) == 0 ? 0 : SX_SHARERS(state));
901 if (lda.spin_cnt > sleep_cnt)
902 LOCKSTAT_RECORD4(sx__spin, sx, all_time - sleep_time,
903 LOCKSTAT_WRITER, (state & SX_LOCK_SHARED) == 0,
904 (state & SX_LOCK_SHARED) == 0 ? 0 : SX_SHARERS(state));
908 LOCKSTAT_PROFILE_OBTAIN_RWLOCK_SUCCESS(sx__acquire, sx,
909 contested, waittime, file, line, LOCKSTAT_WRITER);
914 * This function represents the so-called 'hard case' for sx_xunlock
915 * operation. All 'easy case' failures are redirected to this. Note
916 * that ideally this would be a static function, but it needs to be
917 * accessible from at least sx.h.
920 _sx_xunlock_hard(struct sx *sx, uintptr_t x LOCK_FILE_LINE_ARG_DEF)
923 int queue, wakeup_swapper;
925 if (SCHEDULER_STOPPED())
928 tid = (uintptr_t)curthread;
930 if (__predict_false(x == tid))
931 x = SX_READ_VALUE(sx);
933 MPASS(!(x & SX_LOCK_SHARED));
935 if (__predict_false(x & SX_LOCK_RECURSED)) {
936 /* The lock is recursed, unrecurse one level. */
937 if ((--sx->sx_recurse) == 0)
938 atomic_clear_ptr(&sx->sx_lock, SX_LOCK_RECURSED);
939 if (LOCK_LOG_TEST(&sx->lock_object, 0))
940 CTR2(KTR_LOCK, "%s: %p unrecursing", __func__, sx);
944 LOCKSTAT_PROFILE_RELEASE_RWLOCK(sx__release, sx, LOCKSTAT_WRITER);
946 atomic_cmpset_rel_ptr(&sx->sx_lock, tid, SX_LOCK_UNLOCKED))
949 if (LOCK_LOG_TEST(&sx->lock_object, 0))
950 CTR2(KTR_LOCK, "%s: %p contested", __func__, sx);
952 sleepq_lock(&sx->lock_object);
953 x = SX_READ_VALUE(sx);
954 MPASS(x & (SX_LOCK_SHARED_WAITERS | SX_LOCK_EXCLUSIVE_WAITERS));
957 * The wake up algorithm here is quite simple and probably not
958 * ideal. It gives precedence to shared waiters if they are
959 * present. For this condition, we have to preserve the
960 * state of the exclusive waiters flag.
961 * If interruptible sleeps left the shared queue empty avoid a
962 * starvation for the threads sleeping on the exclusive queue by giving
963 * them precedence and cleaning up the shared waiters bit anyway.
965 setx = SX_LOCK_UNLOCKED;
966 queue = SQ_SHARED_QUEUE;
967 if ((x & SX_LOCK_EXCLUSIVE_WAITERS) != 0 &&
968 sleepq_sleepcnt(&sx->lock_object, SQ_EXCLUSIVE_QUEUE) != 0) {
969 queue = SQ_EXCLUSIVE_QUEUE;
970 setx |= (x & SX_LOCK_SHARED_WAITERS);
972 atomic_store_rel_ptr(&sx->sx_lock, setx);
974 /* Wake up all the waiters for the specific queue. */
975 if (LOCK_LOG_TEST(&sx->lock_object, 0))
976 CTR3(KTR_LOCK, "%s: %p waking up all threads on %s queue",
977 __func__, sx, queue == SQ_SHARED_QUEUE ? "shared" :
980 wakeup_swapper = sleepq_broadcast(&sx->lock_object, SLEEPQ_SX, 0,
982 sleepq_release(&sx->lock_object);
987 static bool __always_inline
988 __sx_can_read(struct thread *td, uintptr_t x, bool fp)
991 if ((x & (SX_LOCK_SHARED | SX_LOCK_EXCLUSIVE_WAITERS | SX_LOCK_WRITE_SPINNER))
994 if (!fp && td->td_sx_slocks && (x & SX_LOCK_SHARED))
999 static bool __always_inline
1000 __sx_slock_try(struct sx *sx, struct thread *td, uintptr_t *xp, bool fp
1001 LOCK_FILE_LINE_ARG_DEF)
1005 * If no other thread has an exclusive lock then try to bump up
1006 * the count of sharers. Since we have to preserve the state
1007 * of SX_LOCK_EXCLUSIVE_WAITERS, if we fail to acquire the
1008 * shared lock loop back and retry.
1010 while (__sx_can_read(td, *xp, fp)) {
1011 if (atomic_fcmpset_acq_ptr(&sx->sx_lock, xp,
1012 *xp + SX_ONE_SHARER)) {
1013 if (LOCK_LOG_TEST(&sx->lock_object, 0))
1014 CTR4(KTR_LOCK, "%s: %p succeed %p -> %p",
1015 __func__, sx, (void *)*xp,
1016 (void *)(*xp + SX_ONE_SHARER));
1024 static int __noinline
1025 _sx_slock_hard(struct sx *sx, int opts, uintptr_t x LOCK_FILE_LINE_ARG_DEF)
1030 struct thread *owner;
1031 u_int i, n, spintries = 0;
1033 #ifdef LOCK_PROFILING
1034 uint64_t waittime = 0;
1038 #if defined(ADAPTIVE_SX) || defined(KDTRACE_HOOKS)
1039 struct lock_delay_arg lda;
1041 #ifdef KDTRACE_HOOKS
1042 u_int sleep_cnt = 0;
1043 int64_t sleep_time = 0;
1044 int64_t all_time = 0;
1046 #if defined(KDTRACE_HOOKS) || defined(LOCK_PROFILING)
1047 uintptr_t state = 0;
1049 int extra_work __sdt_used = 0;
1053 #ifdef KDTRACE_HOOKS
1054 if (LOCKSTAT_PROFILE_ENABLED(sx__acquire)) {
1055 if (__sx_slock_try(sx, td, &x, false LOCK_FILE_LINE_ARG))
1058 all_time -= lockstat_nsecs(&sx->lock_object);
1062 #ifdef LOCK_PROFILING
1067 if (SCHEDULER_STOPPED())
1070 #if defined(ADAPTIVE_SX)
1071 lock_delay_arg_init(&lda, &sx_delay);
1072 #elif defined(KDTRACE_HOOKS)
1073 lock_delay_arg_init_noadapt(&lda);
1077 PMC_SOFT_CALL( , , lock, failed);
1079 lock_profile_obtain_lock_failed(&sx->lock_object, false, &contested,
1083 GIANT_SAVE(extra_work);
1086 THREAD_CONTENDS_ON_LOCK(&sx->lock_object);
1089 * As with rwlocks, we don't make any attempt to try to block
1090 * shared locks once there is an exclusive waiter.
1093 if (__sx_slock_try(sx, td, &x, false LOCK_FILE_LINE_ARG))
1096 GIANT_SAVE(extra_work);
1098 #ifdef KDTRACE_HOOKS
1104 * If the owner is running on another CPU, spin until
1105 * the owner stops running or the state of the lock
1108 if ((x & SX_LOCK_SHARED) == 0) {
1109 owner = lv_sx_owner(x);
1110 if (TD_IS_RUNNING(owner)) {
1111 if (LOCK_LOG_TEST(&sx->lock_object, 0))
1113 "%s: spinning on %p held by %p",
1114 __func__, sx, owner);
1115 KTR_STATE1(KTR_SCHED, "thread",
1116 sched_tdname(curthread), "spinning",
1117 "lockname:\"%s\"", sx->lock_object.lo_name);
1120 x = SX_READ_VALUE(sx);
1121 owner = lv_sx_owner(x);
1122 } while (owner != NULL && TD_IS_RUNNING(owner));
1123 KTR_STATE0(KTR_SCHED, "thread",
1124 sched_tdname(curthread), "running");
1128 if ((x & SX_LOCK_WRITE_SPINNER) && SX_SHARERS(x) == 0) {
1129 MPASS(!__sx_can_read(td, x, false));
1131 x = SX_READ_VALUE(sx);
1134 if (spintries < asx_retries) {
1135 KTR_STATE1(KTR_SCHED, "thread", sched_tdname(curthread),
1136 "spinning", "lockname:\"%s\"",
1137 sx->lock_object.lo_name);
1139 for (i = 0; i < asx_loops; i += n) {
1141 x = SX_READ_VALUE(sx);
1142 if (!(x & SX_LOCK_SHARED))
1147 if (__sx_can_read(td, x, false))
1150 #ifdef KDTRACE_HOOKS
1153 KTR_STATE0(KTR_SCHED, "thread", sched_tdname(curthread),
1162 * Some other thread already has an exclusive lock, so
1163 * start the process of blocking.
1165 sleepq_lock(&sx->lock_object);
1166 x = SX_READ_VALUE(sx);
1168 if (((x & SX_LOCK_WRITE_SPINNER) && SX_SHARERS(x) == 0) ||
1169 __sx_can_read(td, x, false)) {
1170 sleepq_release(&sx->lock_object);
1176 * If the owner is running on another CPU, spin until
1177 * the owner stops running or the state of the lock
1180 if (!(x & SX_LOCK_SHARED)) {
1181 owner = (struct thread *)SX_OWNER(x);
1182 if (TD_IS_RUNNING(owner)) {
1183 sleepq_release(&sx->lock_object);
1184 x = SX_READ_VALUE(sx);
1191 * Try to set the SX_LOCK_SHARED_WAITERS flag. If we
1192 * fail to set it drop the sleep queue lock and loop
1195 if (!(x & SX_LOCK_SHARED_WAITERS)) {
1196 if (!atomic_fcmpset_ptr(&sx->sx_lock, &x,
1197 x | SX_LOCK_SHARED_WAITERS))
1199 if (LOCK_LOG_TEST(&sx->lock_object, 0))
1200 CTR2(KTR_LOCK, "%s: %p set shared waiters flag",
1205 * Since we have been unable to acquire the shared lock,
1208 if (LOCK_LOG_TEST(&sx->lock_object, 0))
1209 CTR2(KTR_LOCK, "%s: %p blocking on sleep queue",
1212 #ifdef KDTRACE_HOOKS
1213 sleep_time -= lockstat_nsecs(&sx->lock_object);
1215 sleepq_add(&sx->lock_object, NULL, sx->lock_object.lo_name,
1216 SLEEPQ_SX | ((opts & SX_INTERRUPTIBLE) ?
1217 SLEEPQ_INTERRUPTIBLE : 0), SQ_SHARED_QUEUE);
1219 * Hack: this can land in thread_suspend_check which will
1220 * conditionally take a mutex, tripping over an assert if a
1221 * lock we are waiting for is set.
1223 THREAD_CONTENTION_DONE(&sx->lock_object);
1224 if (!(opts & SX_INTERRUPTIBLE))
1225 sleepq_wait(&sx->lock_object, 0);
1227 error = sleepq_wait_sig(&sx->lock_object, 0);
1228 THREAD_CONTENDS_ON_LOCK(&sx->lock_object);
1229 #ifdef KDTRACE_HOOKS
1230 sleep_time += lockstat_nsecs(&sx->lock_object);
1234 if (LOCK_LOG_TEST(&sx->lock_object, 0))
1236 "%s: interruptible sleep by %p suspended by signal",
1240 if (LOCK_LOG_TEST(&sx->lock_object, 0))
1241 CTR2(KTR_LOCK, "%s: %p resuming from sleep queue",
1243 x = SX_READ_VALUE(sx);
1245 THREAD_CONTENTION_DONE(&sx->lock_object);
1246 #if defined(KDTRACE_HOOKS) || defined(LOCK_PROFILING)
1247 if (__predict_true(!extra_work))
1250 #ifdef KDTRACE_HOOKS
1251 all_time += lockstat_nsecs(&sx->lock_object);
1253 LOCKSTAT_RECORD4(sx__block, sx, sleep_time,
1254 LOCKSTAT_READER, (state & SX_LOCK_SHARED) == 0,
1255 (state & SX_LOCK_SHARED) == 0 ? 0 : SX_SHARERS(state));
1256 if (lda.spin_cnt > sleep_cnt)
1257 LOCKSTAT_RECORD4(sx__spin, sx, all_time - sleep_time,
1258 LOCKSTAT_READER, (state & SX_LOCK_SHARED) == 0,
1259 (state & SX_LOCK_SHARED) == 0 ? 0 : SX_SHARERS(state));
1263 LOCKSTAT_PROFILE_OBTAIN_RWLOCK_SUCCESS(sx__acquire, sx,
1264 contested, waittime, file, line, LOCKSTAT_READER);
1271 _sx_slock_int(struct sx *sx, int opts LOCK_FILE_LINE_ARG_DEF)
1277 KASSERT(kdb_active != 0 || SCHEDULER_STOPPED() ||
1278 !TD_IS_IDLETHREAD(curthread),
1279 ("sx_slock() by idle thread %p on sx %s @ %s:%d",
1280 curthread, sx->lock_object.lo_name, file, line));
1281 KASSERT(sx->sx_lock != SX_LOCK_DESTROYED,
1282 ("sx_slock() of destroyed sx @ %s:%d", file, line));
1283 WITNESS_CHECKORDER(&sx->lock_object, LOP_NEWORDER, file, line, NULL);
1287 x = SX_READ_VALUE(sx);
1288 if (__predict_false(LOCKSTAT_PROFILE_ENABLED(sx__acquire) ||
1289 !__sx_slock_try(sx, td, &x, true LOCK_FILE_LINE_ARG)))
1290 error = _sx_slock_hard(sx, opts, x LOCK_FILE_LINE_ARG);
1292 lock_profile_obtain_lock_success(&sx->lock_object, false, 0, 0,
1295 LOCK_LOG_LOCK("SLOCK", &sx->lock_object, 0, 0, file, line);
1296 WITNESS_LOCK(&sx->lock_object, 0, file, line);
1297 TD_LOCKS_INC(curthread);
1303 _sx_slock(struct sx *sx, int opts, const char *file, int line)
1306 return (_sx_slock_int(sx, opts LOCK_FILE_LINE_ARG));
1309 static bool __always_inline
1310 _sx_sunlock_try(struct sx *sx, struct thread *td, uintptr_t *xp)
1314 if (SX_SHARERS(*xp) > 1 || !(*xp & SX_LOCK_WAITERS)) {
1315 if (atomic_fcmpset_rel_ptr(&sx->sx_lock, xp,
1316 *xp - SX_ONE_SHARER)) {
1317 if (LOCK_LOG_TEST(&sx->lock_object, 0))
1319 "%s: %p succeeded %p -> %p",
1320 __func__, sx, (void *)*xp,
1321 (void *)(*xp - SX_ONE_SHARER));
1332 static void __noinline
1333 _sx_sunlock_hard(struct sx *sx, struct thread *td, uintptr_t x
1334 LOCK_FILE_LINE_ARG_DEF)
1336 int wakeup_swapper = 0;
1337 uintptr_t setx, queue;
1339 if (SCHEDULER_STOPPED())
1342 if (_sx_sunlock_try(sx, td, &x))
1345 sleepq_lock(&sx->lock_object);
1346 x = SX_READ_VALUE(sx);
1348 if (_sx_sunlock_try(sx, td, &x))
1352 * Wake up semantic here is quite simple:
1353 * Just wake up all the exclusive waiters.
1354 * Note that the state of the lock could have changed,
1355 * so if it fails loop back and retry.
1357 setx = SX_LOCK_UNLOCKED;
1358 queue = SQ_SHARED_QUEUE;
1359 if (x & SX_LOCK_EXCLUSIVE_WAITERS) {
1360 setx |= (x & SX_LOCK_SHARED_WAITERS);
1361 queue = SQ_EXCLUSIVE_QUEUE;
1363 setx |= (x & SX_LOCK_WRITE_SPINNER);
1364 if (!atomic_fcmpset_rel_ptr(&sx->sx_lock, &x, setx))
1366 if (LOCK_LOG_TEST(&sx->lock_object, 0))
1367 CTR2(KTR_LOCK, "%s: %p waking up all thread on"
1368 "exclusive queue", __func__, sx);
1369 wakeup_swapper = sleepq_broadcast(&sx->lock_object, SLEEPQ_SX,
1374 sleepq_release(&sx->lock_object);
1378 LOCKSTAT_PROFILE_RELEASE_RWLOCK(sx__release, sx, LOCKSTAT_READER);
1382 _sx_sunlock_int(struct sx *sx LOCK_FILE_LINE_ARG_DEF)
1387 KASSERT(sx->sx_lock != SX_LOCK_DESTROYED,
1388 ("sx_sunlock() of destroyed sx @ %s:%d", file, line));
1389 _sx_assert(sx, SA_SLOCKED, file, line);
1390 WITNESS_UNLOCK(&sx->lock_object, 0, file, line);
1391 LOCK_LOG_LOCK("SUNLOCK", &sx->lock_object, 0, 0, file, line);
1394 x = SX_READ_VALUE(sx);
1395 if (__predict_false(LOCKSTAT_PROFILE_ENABLED(sx__release) ||
1396 !_sx_sunlock_try(sx, td, &x)))
1397 _sx_sunlock_hard(sx, td, x LOCK_FILE_LINE_ARG);
1399 lock_profile_release_lock(&sx->lock_object, false);
1401 TD_LOCKS_DEC(curthread);
1405 _sx_sunlock(struct sx *sx, const char *file, int line)
1408 _sx_sunlock_int(sx LOCK_FILE_LINE_ARG);
1411 #ifdef INVARIANT_SUPPORT
1417 * In the non-WITNESS case, sx_assert() can only detect that at least
1418 * *some* thread owns an slock, but it cannot guarantee that *this*
1419 * thread owns an slock.
1422 _sx_assert(const struct sx *sx, int what, const char *file, int line)
1428 if (SCHEDULER_STOPPED())
1432 case SA_SLOCKED | SA_NOTRECURSED:
1433 case SA_SLOCKED | SA_RECURSED:
1439 case SA_LOCKED | SA_NOTRECURSED:
1440 case SA_LOCKED | SA_RECURSED:
1442 witness_assert(&sx->lock_object, what, file, line);
1445 * If some other thread has an exclusive lock or we
1446 * have one and are asserting a shared lock, fail.
1447 * Also, if no one has a lock at all, fail.
1449 if (sx->sx_lock == SX_LOCK_UNLOCKED ||
1450 (!(sx->sx_lock & SX_LOCK_SHARED) && (slocked ||
1451 sx_xholder(sx) != curthread)))
1452 panic("Lock %s not %slocked @ %s:%d\n",
1453 sx->lock_object.lo_name, slocked ? "share " : "",
1456 if (!(sx->sx_lock & SX_LOCK_SHARED)) {
1457 if (sx_recursed(sx)) {
1458 if (what & SA_NOTRECURSED)
1459 panic("Lock %s recursed @ %s:%d\n",
1460 sx->lock_object.lo_name, file,
1462 } else if (what & SA_RECURSED)
1463 panic("Lock %s not recursed @ %s:%d\n",
1464 sx->lock_object.lo_name, file, line);
1469 case SA_XLOCKED | SA_NOTRECURSED:
1470 case SA_XLOCKED | SA_RECURSED:
1471 if (sx_xholder(sx) != curthread)
1472 panic("Lock %s not exclusively locked @ %s:%d\n",
1473 sx->lock_object.lo_name, file, line);
1474 if (sx_recursed(sx)) {
1475 if (what & SA_NOTRECURSED)
1476 panic("Lock %s recursed @ %s:%d\n",
1477 sx->lock_object.lo_name, file, line);
1478 } else if (what & SA_RECURSED)
1479 panic("Lock %s not recursed @ %s:%d\n",
1480 sx->lock_object.lo_name, file, line);
1484 witness_assert(&sx->lock_object, what, file, line);
1487 * If we hold an exclusve lock fail. We can't
1488 * reliably check to see if we hold a shared lock or
1491 if (sx_xholder(sx) == curthread)
1492 panic("Lock %s exclusively locked @ %s:%d\n",
1493 sx->lock_object.lo_name, file, line);
1497 panic("Unknown sx lock assertion: %d @ %s:%d", what, file,
1501 #endif /* INVARIANT_SUPPORT */
1505 db_show_sx(const struct lock_object *lock)
1508 const struct sx *sx;
1510 sx = (const struct sx *)lock;
1512 db_printf(" state: ");
1513 if (sx->sx_lock == SX_LOCK_UNLOCKED)
1514 db_printf("UNLOCKED\n");
1515 else if (sx->sx_lock == SX_LOCK_DESTROYED) {
1516 db_printf("DESTROYED\n");
1518 } else if (sx->sx_lock & SX_LOCK_SHARED)
1519 db_printf("SLOCK: %ju\n", (uintmax_t)SX_SHARERS(sx->sx_lock));
1521 td = sx_xholder(sx);
1522 db_printf("XLOCK: %p (tid %d, pid %d, \"%s\")\n", td,
1523 td->td_tid, td->td_proc->p_pid, td->td_name);
1524 if (sx_recursed(sx))
1525 db_printf(" recursed: %d\n", sx->sx_recurse);
1528 db_printf(" waiters: ");
1529 switch(sx->sx_lock &
1530 (SX_LOCK_SHARED_WAITERS | SX_LOCK_EXCLUSIVE_WAITERS)) {
1531 case SX_LOCK_SHARED_WAITERS:
1532 db_printf("shared\n");
1534 case SX_LOCK_EXCLUSIVE_WAITERS:
1535 db_printf("exclusive\n");
1537 case SX_LOCK_SHARED_WAITERS | SX_LOCK_EXCLUSIVE_WAITERS:
1538 db_printf("exclusive and shared\n");
1541 db_printf("none\n");
1546 * Check to see if a thread that is blocked on a sleep queue is actually
1547 * blocked on an sx lock. If so, output some details and return true.
1548 * If the lock has an exclusive owner, return that in *ownerp.
1551 sx_chain(struct thread *td, struct thread **ownerp)
1553 const struct sx *sx;
1556 * Check to see if this thread is blocked on an sx lock.
1557 * First, we check the lock class. If that is ok, then we
1558 * compare the lock name against the wait message.
1561 if (LOCK_CLASS(&sx->lock_object) != &lock_class_sx ||
1562 sx->lock_object.lo_name != td->td_wmesg)
1565 /* We think we have an sx lock, so output some details. */
1566 db_printf("blocked on sx \"%s\" ", td->td_wmesg);
1567 *ownerp = sx_xholder(sx);
1568 if (sx->sx_lock & SX_LOCK_SHARED)
1569 db_printf("SLOCK (count %ju)\n",
1570 (uintmax_t)SX_SHARERS(sx->sx_lock));
1572 db_printf("XLOCK\n");