xref: /freebsd/sys/kern/kern_sx.c (revision 4ec234c813eed05c166859bba82c882e40826eb9)
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_no_adaptive_sx.h"
42 
43 #include <sys/cdefs.h>
44 __FBSDID("$FreeBSD$");
45 
46 #include <sys/param.h>
47 #include <sys/systm.h>
48 #include <sys/kdb.h>
49 #include <sys/ktr.h>
50 #include <sys/lock.h>
51 #include <sys/mutex.h>
52 #include <sys/proc.h>
53 #include <sys/sleepqueue.h>
54 #include <sys/sx.h>
55 #include <sys/sysctl.h>
56 
57 #if defined(SMP) && !defined(NO_ADAPTIVE_SX)
58 #include <machine/cpu.h>
59 #endif
60 
61 #ifdef DDB
62 #include <ddb/ddb.h>
63 #endif
64 
65 #if defined(SMP) && !defined(NO_ADAPTIVE_SX)
66 #define	ADAPTIVE_SX
67 #endif
68 
69 CTASSERT((SX_NOADAPTIVE & LO_CLASSFLAGS) == SX_NOADAPTIVE);
70 
71 #ifdef HWPMC_HOOKS
72 #include <sys/pmckern.h>
73 PMC_SOFT_DECLARE( , , lock, failed);
74 #endif
75 
76 /* Handy macros for sleep queues. */
77 #define	SQ_EXCLUSIVE_QUEUE	0
78 #define	SQ_SHARED_QUEUE		1
79 
80 /*
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.
83  */
84 #define	GIANT_DECLARE							\
85 	int _giantcnt = 0;						\
86 	WITNESS_SAVE_DECL(Giant)					\
87 
88 #define	GIANT_SAVE() do {						\
89 	if (mtx_owned(&Giant)) {					\
90 		WITNESS_SAVE(&Giant.lock_object, Giant);		\
91 		while (mtx_owned(&Giant)) {				\
92 			_giantcnt++;					\
93 			mtx_unlock(&Giant);				\
94 		}							\
95 	}								\
96 } while (0)
97 
98 #define GIANT_RESTORE() do {						\
99 	if (_giantcnt > 0) {						\
100 		mtx_assert(&Giant, MA_NOTOWNED);			\
101 		while (_giantcnt--)					\
102 			mtx_lock(&Giant);				\
103 		WITNESS_RESTORE(&Giant.lock_object, Giant);		\
104 	}								\
105 } while (0)
106 
107 /*
108  * Returns true if an exclusive lock is recursed.  It assumes
109  * curthread currently has an exclusive lock.
110  */
111 #define	sx_recursed(sx)		((sx)->sx_recurse != 0)
112 
113 static void	assert_sx(const struct lock_object *lock, int what);
114 #ifdef DDB
115 static void	db_show_sx(const struct lock_object *lock);
116 #endif
117 static void	lock_sx(struct lock_object *lock, uintptr_t how);
118 #ifdef KDTRACE_HOOKS
119 static int	owner_sx(const struct lock_object *lock, struct thread **owner);
120 #endif
121 static uintptr_t unlock_sx(struct lock_object *lock);
122 
123 struct lock_class lock_class_sx = {
124 	.lc_name = "sx",
125 	.lc_flags = LC_SLEEPLOCK | LC_SLEEPABLE | LC_RECURSABLE | LC_UPGRADABLE,
126 	.lc_assert = assert_sx,
127 #ifdef DDB
128 	.lc_ddb_show = db_show_sx,
129 #endif
130 	.lc_lock = lock_sx,
131 	.lc_unlock = unlock_sx,
132 #ifdef KDTRACE_HOOKS
133 	.lc_owner = owner_sx,
134 #endif
135 };
136 
137 #ifndef INVARIANTS
138 #define	_sx_assert(sx, what, file, line)
139 #endif
140 
141 #ifdef ADAPTIVE_SX
142 static u_int asx_retries = 10;
143 static u_int asx_loops = 10000;
144 static SYSCTL_NODE(_debug, OID_AUTO, sx, CTLFLAG_RD, NULL, "sxlock debugging");
145 SYSCTL_UINT(_debug_sx, OID_AUTO, retries, CTLFLAG_RW, &asx_retries, 0, "");
146 SYSCTL_UINT(_debug_sx, OID_AUTO, loops, CTLFLAG_RW, &asx_loops, 0, "");
147 #endif
148 
149 void
150 assert_sx(const struct lock_object *lock, int what)
151 {
152 
153 	sx_assert((const struct sx *)lock, what);
154 }
155 
156 void
157 lock_sx(struct lock_object *lock, uintptr_t how)
158 {
159 	struct sx *sx;
160 
161 	sx = (struct sx *)lock;
162 	if (how)
163 		sx_slock(sx);
164 	else
165 		sx_xlock(sx);
166 }
167 
168 uintptr_t
169 unlock_sx(struct lock_object *lock)
170 {
171 	struct sx *sx;
172 
173 	sx = (struct sx *)lock;
174 	sx_assert(sx, SA_LOCKED | SA_NOTRECURSED);
175 	if (sx_xlocked(sx)) {
176 		sx_xunlock(sx);
177 		return (0);
178 	} else {
179 		sx_sunlock(sx);
180 		return (1);
181 	}
182 }
183 
184 #ifdef KDTRACE_HOOKS
185 int
186 owner_sx(const struct lock_object *lock, struct thread **owner)
187 {
188         const struct sx *sx = (const struct sx *)lock;
189 	uintptr_t x = sx->sx_lock;
190 
191         *owner = (struct thread *)SX_OWNER(x);
192         return ((x & SX_LOCK_SHARED) != 0 ? (SX_SHARERS(x) != 0) :
193 	    (*owner != NULL));
194 }
195 #endif
196 
197 void
198 sx_sysinit(void *arg)
199 {
200 	struct sx_args *sargs = arg;
201 
202 	sx_init_flags(sargs->sa_sx, sargs->sa_desc, sargs->sa_flags);
203 }
204 
205 void
206 sx_init_flags(struct sx *sx, const char *description, int opts)
207 {
208 	int flags;
209 
210 	MPASS((opts & ~(SX_QUIET | SX_RECURSE | SX_NOWITNESS | SX_DUPOK |
211 	    SX_NOPROFILE | SX_NOADAPTIVE)) == 0);
212 	ASSERT_ATOMIC_LOAD_PTR(sx->sx_lock,
213 	    ("%s: sx_lock not aligned for %s: %p", __func__, description,
214 	    &sx->sx_lock));
215 
216 	flags = LO_SLEEPABLE | LO_UPGRADABLE;
217 	if (opts & SX_DUPOK)
218 		flags |= LO_DUPOK;
219 	if (opts & SX_NOPROFILE)
220 		flags |= LO_NOPROFILE;
221 	if (!(opts & SX_NOWITNESS))
222 		flags |= LO_WITNESS;
223 	if (opts & SX_RECURSE)
224 		flags |= LO_RECURSABLE;
225 	if (opts & SX_QUIET)
226 		flags |= LO_QUIET;
227 
228 	flags |= opts & SX_NOADAPTIVE;
229 	lock_init(&sx->lock_object, &lock_class_sx, description, NULL, flags);
230 	sx->sx_lock = SX_LOCK_UNLOCKED;
231 	sx->sx_recurse = 0;
232 }
233 
234 void
235 sx_destroy(struct sx *sx)
236 {
237 
238 	KASSERT(sx->sx_lock == SX_LOCK_UNLOCKED, ("sx lock still held"));
239 	KASSERT(sx->sx_recurse == 0, ("sx lock still recursed"));
240 	sx->sx_lock = SX_LOCK_DESTROYED;
241 	lock_destroy(&sx->lock_object);
242 }
243 
244 int
245 _sx_slock(struct sx *sx, int opts, const char *file, int line)
246 {
247 	int error = 0;
248 
249 	if (SCHEDULER_STOPPED())
250 		return (0);
251 	KASSERT(kdb_active != 0 || !TD_IS_IDLETHREAD(curthread),
252 	    ("sx_slock() by idle thread %p on sx %s @ %s:%d",
253 	    curthread, sx->lock_object.lo_name, file, line));
254 	KASSERT(sx->sx_lock != SX_LOCK_DESTROYED,
255 	    ("sx_slock() of destroyed sx @ %s:%d", file, line));
256 	WITNESS_CHECKORDER(&sx->lock_object, LOP_NEWORDER, file, line, NULL);
257 	error = __sx_slock(sx, opts, file, line);
258 	if (!error) {
259 		LOCK_LOG_LOCK("SLOCK", &sx->lock_object, 0, 0, file, line);
260 		WITNESS_LOCK(&sx->lock_object, 0, file, line);
261 		curthread->td_locks++;
262 	}
263 
264 	return (error);
265 }
266 
267 int
268 sx_try_slock_(struct sx *sx, const char *file, int line)
269 {
270 	uintptr_t x;
271 
272 	if (SCHEDULER_STOPPED())
273 		return (1);
274 
275 	KASSERT(kdb_active != 0 || !TD_IS_IDLETHREAD(curthread),
276 	    ("sx_try_slock() by idle thread %p on sx %s @ %s:%d",
277 	    curthread, sx->lock_object.lo_name, file, line));
278 
279 	for (;;) {
280 		x = sx->sx_lock;
281 		KASSERT(x != SX_LOCK_DESTROYED,
282 		    ("sx_try_slock() of destroyed sx @ %s:%d", file, line));
283 		if (!(x & SX_LOCK_SHARED))
284 			break;
285 		if (atomic_cmpset_acq_ptr(&sx->sx_lock, x, x + SX_ONE_SHARER)) {
286 			LOCK_LOG_TRY("SLOCK", &sx->lock_object, 0, 1, file, line);
287 			WITNESS_LOCK(&sx->lock_object, LOP_TRYLOCK, file, line);
288 			curthread->td_locks++;
289 			return (1);
290 		}
291 	}
292 
293 	LOCK_LOG_TRY("SLOCK", &sx->lock_object, 0, 0, file, line);
294 	return (0);
295 }
296 
297 int
298 _sx_xlock(struct sx *sx, int opts, const char *file, int line)
299 {
300 	int error = 0;
301 
302 	if (SCHEDULER_STOPPED())
303 		return (0);
304 	KASSERT(kdb_active != 0 || !TD_IS_IDLETHREAD(curthread),
305 	    ("sx_xlock() by idle thread %p on sx %s @ %s:%d",
306 	    curthread, sx->lock_object.lo_name, file, line));
307 	KASSERT(sx->sx_lock != SX_LOCK_DESTROYED,
308 	    ("sx_xlock() of destroyed sx @ %s:%d", file, line));
309 	WITNESS_CHECKORDER(&sx->lock_object, LOP_NEWORDER | LOP_EXCLUSIVE, file,
310 	    line, NULL);
311 	error = __sx_xlock(sx, curthread, opts, file, line);
312 	if (!error) {
313 		LOCK_LOG_LOCK("XLOCK", &sx->lock_object, 0, sx->sx_recurse,
314 		    file, line);
315 		WITNESS_LOCK(&sx->lock_object, LOP_EXCLUSIVE, file, line);
316 		curthread->td_locks++;
317 	}
318 
319 	return (error);
320 }
321 
322 int
323 sx_try_xlock_(struct sx *sx, const char *file, int line)
324 {
325 	int rval;
326 
327 	if (SCHEDULER_STOPPED())
328 		return (1);
329 
330 	KASSERT(kdb_active != 0 || !TD_IS_IDLETHREAD(curthread),
331 	    ("sx_try_xlock() by idle thread %p on sx %s @ %s:%d",
332 	    curthread, sx->lock_object.lo_name, file, line));
333 	KASSERT(sx->sx_lock != SX_LOCK_DESTROYED,
334 	    ("sx_try_xlock() of destroyed sx @ %s:%d", file, line));
335 
336 	if (sx_xlocked(sx) &&
337 	    (sx->lock_object.lo_flags & LO_RECURSABLE) != 0) {
338 		sx->sx_recurse++;
339 		atomic_set_ptr(&sx->sx_lock, SX_LOCK_RECURSED);
340 		rval = 1;
341 	} else
342 		rval = atomic_cmpset_acq_ptr(&sx->sx_lock, SX_LOCK_UNLOCKED,
343 		    (uintptr_t)curthread);
344 	LOCK_LOG_TRY("XLOCK", &sx->lock_object, 0, rval, file, line);
345 	if (rval) {
346 		WITNESS_LOCK(&sx->lock_object, LOP_EXCLUSIVE | LOP_TRYLOCK,
347 		    file, line);
348 		curthread->td_locks++;
349 	}
350 
351 	return (rval);
352 }
353 
354 void
355 _sx_sunlock(struct sx *sx, const char *file, int line)
356 {
357 
358 	if (SCHEDULER_STOPPED())
359 		return;
360 	KASSERT(sx->sx_lock != SX_LOCK_DESTROYED,
361 	    ("sx_sunlock() of destroyed sx @ %s:%d", file, line));
362 	_sx_assert(sx, SA_SLOCKED, file, line);
363 	WITNESS_UNLOCK(&sx->lock_object, 0, file, line);
364 	LOCK_LOG_LOCK("SUNLOCK", &sx->lock_object, 0, 0, file, line);
365 	__sx_sunlock(sx, file, line);
366 	curthread->td_locks--;
367 }
368 
369 void
370 _sx_xunlock(struct sx *sx, const char *file, int line)
371 {
372 
373 	if (SCHEDULER_STOPPED())
374 		return;
375 	KASSERT(sx->sx_lock != SX_LOCK_DESTROYED,
376 	    ("sx_xunlock() of destroyed sx @ %s:%d", file, line));
377 	_sx_assert(sx, SA_XLOCKED, file, line);
378 	WITNESS_UNLOCK(&sx->lock_object, LOP_EXCLUSIVE, file, line);
379 	LOCK_LOG_LOCK("XUNLOCK", &sx->lock_object, 0, sx->sx_recurse, file,
380 	    line);
381 	__sx_xunlock(sx, curthread, file, line);
382 	curthread->td_locks--;
383 }
384 
385 /*
386  * Try to do a non-blocking upgrade from a shared lock to an exclusive lock.
387  * This will only succeed if this thread holds a single shared lock.
388  * Return 1 if if the upgrade succeed, 0 otherwise.
389  */
390 int
391 sx_try_upgrade_(struct sx *sx, const char *file, int line)
392 {
393 	uintptr_t x;
394 	int success;
395 
396 	if (SCHEDULER_STOPPED())
397 		return (1);
398 
399 	KASSERT(sx->sx_lock != SX_LOCK_DESTROYED,
400 	    ("sx_try_upgrade() of destroyed sx @ %s:%d", file, line));
401 	_sx_assert(sx, SA_SLOCKED, file, line);
402 
403 	/*
404 	 * Try to switch from one shared lock to an exclusive lock.  We need
405 	 * to maintain the SX_LOCK_EXCLUSIVE_WAITERS flag if set so that
406 	 * we will wake up the exclusive waiters when we drop the lock.
407 	 */
408 	x = sx->sx_lock & SX_LOCK_EXCLUSIVE_WAITERS;
409 	success = atomic_cmpset_ptr(&sx->sx_lock, SX_SHARERS_LOCK(1) | x,
410 	    (uintptr_t)curthread | x);
411 	LOCK_LOG_TRY("XUPGRADE", &sx->lock_object, 0, success, file, line);
412 	if (success) {
413 		WITNESS_UPGRADE(&sx->lock_object, LOP_EXCLUSIVE | LOP_TRYLOCK,
414 		    file, line);
415 		LOCKSTAT_RECORD0(LS_SX_TRYUPGRADE_UPGRADE, sx);
416 	}
417 	return (success);
418 }
419 
420 /*
421  * Downgrade an unrecursed exclusive lock into a single shared lock.
422  */
423 void
424 sx_downgrade_(struct sx *sx, const char *file, int line)
425 {
426 	uintptr_t x;
427 	int wakeup_swapper;
428 
429 	if (SCHEDULER_STOPPED())
430 		return;
431 
432 	KASSERT(sx->sx_lock != SX_LOCK_DESTROYED,
433 	    ("sx_downgrade() of destroyed sx @ %s:%d", file, line));
434 	_sx_assert(sx, SA_XLOCKED | SA_NOTRECURSED, file, line);
435 #ifndef INVARIANTS
436 	if (sx_recursed(sx))
437 		panic("downgrade of a recursed lock");
438 #endif
439 
440 	WITNESS_DOWNGRADE(&sx->lock_object, 0, file, line);
441 
442 	/*
443 	 * Try to switch from an exclusive lock with no shared waiters
444 	 * to one sharer with no shared waiters.  If there are
445 	 * exclusive waiters, we don't need to lock the sleep queue so
446 	 * long as we preserve the flag.  We do one quick try and if
447 	 * that fails we grab the sleepq lock to keep the flags from
448 	 * changing and do it the slow way.
449 	 *
450 	 * We have to lock the sleep queue if there are shared waiters
451 	 * so we can wake them up.
452 	 */
453 	x = sx->sx_lock;
454 	if (!(x & SX_LOCK_SHARED_WAITERS) &&
455 	    atomic_cmpset_rel_ptr(&sx->sx_lock, x, SX_SHARERS_LOCK(1) |
456 	    (x & SX_LOCK_EXCLUSIVE_WAITERS))) {
457 		LOCK_LOG_LOCK("XDOWNGRADE", &sx->lock_object, 0, 0, file, line);
458 		return;
459 	}
460 
461 	/*
462 	 * Lock the sleep queue so we can read the waiters bits
463 	 * without any races and wakeup any shared waiters.
464 	 */
465 	sleepq_lock(&sx->lock_object);
466 
467 	/*
468 	 * Preserve SX_LOCK_EXCLUSIVE_WAITERS while downgraded to a single
469 	 * shared lock.  If there are any shared waiters, wake them up.
470 	 */
471 	wakeup_swapper = 0;
472 	x = sx->sx_lock;
473 	atomic_store_rel_ptr(&sx->sx_lock, SX_SHARERS_LOCK(1) |
474 	    (x & SX_LOCK_EXCLUSIVE_WAITERS));
475 	if (x & SX_LOCK_SHARED_WAITERS)
476 		wakeup_swapper = sleepq_broadcast(&sx->lock_object, SLEEPQ_SX,
477 		    0, SQ_SHARED_QUEUE);
478 	sleepq_release(&sx->lock_object);
479 
480 	LOCK_LOG_LOCK("XDOWNGRADE", &sx->lock_object, 0, 0, file, line);
481 	LOCKSTAT_RECORD0(LS_SX_DOWNGRADE_DOWNGRADE, sx);
482 
483 	if (wakeup_swapper)
484 		kick_proc0();
485 }
486 
487 /*
488  * This function represents the so-called 'hard case' for sx_xlock
489  * operation.  All 'easy case' failures are redirected to this.  Note
490  * that ideally this would be a static function, but it needs to be
491  * accessible from at least sx.h.
492  */
493 int
494 _sx_xlock_hard(struct sx *sx, uintptr_t tid, int opts, const char *file,
495     int line)
496 {
497 	GIANT_DECLARE;
498 #ifdef ADAPTIVE_SX
499 	volatile struct thread *owner;
500 	u_int i, spintries = 0;
501 #endif
502 	uintptr_t x;
503 #ifdef LOCK_PROFILING
504 	uint64_t waittime = 0;
505 	int contested = 0;
506 #endif
507 	int error = 0;
508 #ifdef	KDTRACE_HOOKS
509 	uint64_t spin_cnt = 0;
510 	uint64_t sleep_cnt = 0;
511 	int64_t sleep_time = 0;
512 #endif
513 
514 	if (SCHEDULER_STOPPED())
515 		return (0);
516 
517 	/* If we already hold an exclusive lock, then recurse. */
518 	if (sx_xlocked(sx)) {
519 		KASSERT((sx->lock_object.lo_flags & LO_RECURSABLE) != 0,
520 	    ("_sx_xlock_hard: recursed on non-recursive sx %s @ %s:%d\n",
521 		    sx->lock_object.lo_name, file, line));
522 		sx->sx_recurse++;
523 		atomic_set_ptr(&sx->sx_lock, SX_LOCK_RECURSED);
524 		if (LOCK_LOG_TEST(&sx->lock_object, 0))
525 			CTR2(KTR_LOCK, "%s: %p recursing", __func__, sx);
526 		return (0);
527 	}
528 
529 	if (LOCK_LOG_TEST(&sx->lock_object, 0))
530 		CTR5(KTR_LOCK, "%s: %s contested (lock=%p) at %s:%d", __func__,
531 		    sx->lock_object.lo_name, (void *)sx->sx_lock, file, line);
532 
533 	while (!atomic_cmpset_acq_ptr(&sx->sx_lock, SX_LOCK_UNLOCKED, tid)) {
534 #ifdef KDTRACE_HOOKS
535 		spin_cnt++;
536 #endif
537 #ifdef HWPMC_HOOKS
538 		PMC_SOFT_CALL( , , lock, failed);
539 #endif
540 		lock_profile_obtain_lock_failed(&sx->lock_object, &contested,
541 		    &waittime);
542 #ifdef ADAPTIVE_SX
543 		/*
544 		 * If the lock is write locked and the owner is
545 		 * running on another CPU, spin until the owner stops
546 		 * running or the state of the lock changes.
547 		 */
548 		x = sx->sx_lock;
549 		if ((sx->lock_object.lo_flags & SX_NOADAPTIVE) == 0) {
550 			if ((x & SX_LOCK_SHARED) == 0) {
551 				x = SX_OWNER(x);
552 				owner = (struct thread *)x;
553 				if (TD_IS_RUNNING(owner)) {
554 					if (LOCK_LOG_TEST(&sx->lock_object, 0))
555 						CTR3(KTR_LOCK,
556 					    "%s: spinning on %p held by %p",
557 						    __func__, sx, owner);
558 					GIANT_SAVE();
559 					while (SX_OWNER(sx->sx_lock) == x &&
560 					    TD_IS_RUNNING(owner)) {
561 						cpu_spinwait();
562 #ifdef KDTRACE_HOOKS
563 						spin_cnt++;
564 #endif
565 					}
566 					continue;
567 				}
568 			} else if (SX_SHARERS(x) && spintries < asx_retries) {
569 				GIANT_SAVE();
570 				spintries++;
571 				for (i = 0; i < asx_loops; i++) {
572 					if (LOCK_LOG_TEST(&sx->lock_object, 0))
573 						CTR4(KTR_LOCK,
574 				    "%s: shared spinning on %p with %u and %u",
575 						    __func__, sx, spintries, i);
576 					x = sx->sx_lock;
577 					if ((x & SX_LOCK_SHARED) == 0 ||
578 					    SX_SHARERS(x) == 0)
579 						break;
580 					cpu_spinwait();
581 #ifdef KDTRACE_HOOKS
582 					spin_cnt++;
583 #endif
584 				}
585 				if (i != asx_loops)
586 					continue;
587 			}
588 		}
589 #endif
590 
591 		sleepq_lock(&sx->lock_object);
592 		x = sx->sx_lock;
593 
594 		/*
595 		 * If the lock was released while spinning on the
596 		 * sleep queue chain lock, try again.
597 		 */
598 		if (x == SX_LOCK_UNLOCKED) {
599 			sleepq_release(&sx->lock_object);
600 			continue;
601 		}
602 
603 #ifdef ADAPTIVE_SX
604 		/*
605 		 * The current lock owner might have started executing
606 		 * on another CPU (or the lock could have changed
607 		 * owners) while we were waiting on the sleep queue
608 		 * chain lock.  If so, drop the sleep queue lock and try
609 		 * again.
610 		 */
611 		if (!(x & SX_LOCK_SHARED) &&
612 		    (sx->lock_object.lo_flags & SX_NOADAPTIVE) == 0) {
613 			owner = (struct thread *)SX_OWNER(x);
614 			if (TD_IS_RUNNING(owner)) {
615 				sleepq_release(&sx->lock_object);
616 				continue;
617 			}
618 		}
619 #endif
620 
621 		/*
622 		 * If an exclusive lock was released with both shared
623 		 * and exclusive waiters and a shared waiter hasn't
624 		 * woken up and acquired the lock yet, sx_lock will be
625 		 * set to SX_LOCK_UNLOCKED | SX_LOCK_EXCLUSIVE_WAITERS.
626 		 * If we see that value, try to acquire it once.  Note
627 		 * that we have to preserve SX_LOCK_EXCLUSIVE_WAITERS
628 		 * as there are other exclusive waiters still.  If we
629 		 * fail, restart the loop.
630 		 */
631 		if (x == (SX_LOCK_UNLOCKED | SX_LOCK_EXCLUSIVE_WAITERS)) {
632 			if (atomic_cmpset_acq_ptr(&sx->sx_lock,
633 			    SX_LOCK_UNLOCKED | SX_LOCK_EXCLUSIVE_WAITERS,
634 			    tid | SX_LOCK_EXCLUSIVE_WAITERS)) {
635 				sleepq_release(&sx->lock_object);
636 				CTR2(KTR_LOCK, "%s: %p claimed by new writer",
637 				    __func__, sx);
638 				break;
639 			}
640 			sleepq_release(&sx->lock_object);
641 			continue;
642 		}
643 
644 		/*
645 		 * Try to set the SX_LOCK_EXCLUSIVE_WAITERS.  If we fail,
646 		 * than loop back and retry.
647 		 */
648 		if (!(x & SX_LOCK_EXCLUSIVE_WAITERS)) {
649 			if (!atomic_cmpset_ptr(&sx->sx_lock, x,
650 			    x | SX_LOCK_EXCLUSIVE_WAITERS)) {
651 				sleepq_release(&sx->lock_object);
652 				continue;
653 			}
654 			if (LOCK_LOG_TEST(&sx->lock_object, 0))
655 				CTR2(KTR_LOCK, "%s: %p set excl waiters flag",
656 				    __func__, sx);
657 		}
658 
659 		/*
660 		 * Since we have been unable to acquire the exclusive
661 		 * lock and the exclusive waiters flag is set, we have
662 		 * to sleep.
663 		 */
664 		if (LOCK_LOG_TEST(&sx->lock_object, 0))
665 			CTR2(KTR_LOCK, "%s: %p blocking on sleep queue",
666 			    __func__, sx);
667 
668 #ifdef KDTRACE_HOOKS
669 		sleep_time -= lockstat_nsecs();
670 #endif
671 		GIANT_SAVE();
672 		sleepq_add(&sx->lock_object, NULL, sx->lock_object.lo_name,
673 		    SLEEPQ_SX | ((opts & SX_INTERRUPTIBLE) ?
674 		    SLEEPQ_INTERRUPTIBLE : 0), SQ_EXCLUSIVE_QUEUE);
675 		if (!(opts & SX_INTERRUPTIBLE))
676 			sleepq_wait(&sx->lock_object, 0);
677 		else
678 			error = sleepq_wait_sig(&sx->lock_object, 0);
679 #ifdef KDTRACE_HOOKS
680 		sleep_time += lockstat_nsecs();
681 		sleep_cnt++;
682 #endif
683 		if (error) {
684 			if (LOCK_LOG_TEST(&sx->lock_object, 0))
685 				CTR2(KTR_LOCK,
686 			"%s: interruptible sleep by %p suspended by signal",
687 				    __func__, sx);
688 			break;
689 		}
690 		if (LOCK_LOG_TEST(&sx->lock_object, 0))
691 			CTR2(KTR_LOCK, "%s: %p resuming from sleep queue",
692 			    __func__, sx);
693 	}
694 
695 	GIANT_RESTORE();
696 	if (!error)
697 		LOCKSTAT_PROFILE_OBTAIN_LOCK_SUCCESS(LS_SX_XLOCK_ACQUIRE, sx,
698 		    contested, waittime, file, line);
699 #ifdef KDTRACE_HOOKS
700 	if (sleep_time)
701 		LOCKSTAT_RECORD1(LS_SX_XLOCK_BLOCK, sx, sleep_time);
702 	if (spin_cnt > sleep_cnt)
703 		LOCKSTAT_RECORD1(LS_SX_XLOCK_SPIN, sx, (spin_cnt - sleep_cnt));
704 #endif
705 	return (error);
706 }
707 
708 /*
709  * This function represents the so-called 'hard case' for sx_xunlock
710  * operation.  All 'easy case' failures are redirected to this.  Note
711  * that ideally this would be a static function, but it needs to be
712  * accessible from at least sx.h.
713  */
714 void
715 _sx_xunlock_hard(struct sx *sx, uintptr_t tid, const char *file, int line)
716 {
717 	uintptr_t x;
718 	int queue, wakeup_swapper;
719 
720 	if (SCHEDULER_STOPPED())
721 		return;
722 
723 	MPASS(!(sx->sx_lock & SX_LOCK_SHARED));
724 
725 	/* If the lock is recursed, then unrecurse one level. */
726 	if (sx_xlocked(sx) && sx_recursed(sx)) {
727 		if ((--sx->sx_recurse) == 0)
728 			atomic_clear_ptr(&sx->sx_lock, SX_LOCK_RECURSED);
729 		if (LOCK_LOG_TEST(&sx->lock_object, 0))
730 			CTR2(KTR_LOCK, "%s: %p unrecursing", __func__, sx);
731 		return;
732 	}
733 	MPASS(sx->sx_lock & (SX_LOCK_SHARED_WAITERS |
734 	    SX_LOCK_EXCLUSIVE_WAITERS));
735 	if (LOCK_LOG_TEST(&sx->lock_object, 0))
736 		CTR2(KTR_LOCK, "%s: %p contested", __func__, sx);
737 
738 	sleepq_lock(&sx->lock_object);
739 	x = SX_LOCK_UNLOCKED;
740 
741 	/*
742 	 * The wake up algorithm here is quite simple and probably not
743 	 * ideal.  It gives precedence to shared waiters if they are
744 	 * present.  For this condition, we have to preserve the
745 	 * state of the exclusive waiters flag.
746 	 * If interruptible sleeps left the shared queue empty avoid a
747 	 * starvation for the threads sleeping on the exclusive queue by giving
748 	 * them precedence and cleaning up the shared waiters bit anyway.
749 	 */
750 	if ((sx->sx_lock & SX_LOCK_SHARED_WAITERS) != 0 &&
751 	    sleepq_sleepcnt(&sx->lock_object, SQ_SHARED_QUEUE) != 0) {
752 		queue = SQ_SHARED_QUEUE;
753 		x |= (sx->sx_lock & SX_LOCK_EXCLUSIVE_WAITERS);
754 	} else
755 		queue = SQ_EXCLUSIVE_QUEUE;
756 
757 	/* Wake up all the waiters for the specific queue. */
758 	if (LOCK_LOG_TEST(&sx->lock_object, 0))
759 		CTR3(KTR_LOCK, "%s: %p waking up all threads on %s queue",
760 		    __func__, sx, queue == SQ_SHARED_QUEUE ? "shared" :
761 		    "exclusive");
762 	atomic_store_rel_ptr(&sx->sx_lock, x);
763 	wakeup_swapper = sleepq_broadcast(&sx->lock_object, SLEEPQ_SX, 0,
764 	    queue);
765 	sleepq_release(&sx->lock_object);
766 	if (wakeup_swapper)
767 		kick_proc0();
768 }
769 
770 /*
771  * This function represents the so-called 'hard case' for sx_slock
772  * operation.  All 'easy case' failures are redirected to this.  Note
773  * that ideally this would be a static function, but it needs to be
774  * accessible from at least sx.h.
775  */
776 int
777 _sx_slock_hard(struct sx *sx, int opts, const char *file, int line)
778 {
779 	GIANT_DECLARE;
780 #ifdef ADAPTIVE_SX
781 	volatile struct thread *owner;
782 #endif
783 #ifdef LOCK_PROFILING
784 	uint64_t waittime = 0;
785 	int contested = 0;
786 #endif
787 	uintptr_t x;
788 	int error = 0;
789 #ifdef KDTRACE_HOOKS
790 	uint64_t spin_cnt = 0;
791 	uint64_t sleep_cnt = 0;
792 	int64_t sleep_time = 0;
793 #endif
794 
795 	if (SCHEDULER_STOPPED())
796 		return (0);
797 
798 	/*
799 	 * As with rwlocks, we don't make any attempt to try to block
800 	 * shared locks once there is an exclusive waiter.
801 	 */
802 	for (;;) {
803 #ifdef KDTRACE_HOOKS
804 		spin_cnt++;
805 #endif
806 		x = sx->sx_lock;
807 
808 		/*
809 		 * If no other thread has an exclusive lock then try to bump up
810 		 * the count of sharers.  Since we have to preserve the state
811 		 * of SX_LOCK_EXCLUSIVE_WAITERS, if we fail to acquire the
812 		 * shared lock loop back and retry.
813 		 */
814 		if (x & SX_LOCK_SHARED) {
815 			MPASS(!(x & SX_LOCK_SHARED_WAITERS));
816 			if (atomic_cmpset_acq_ptr(&sx->sx_lock, x,
817 			    x + SX_ONE_SHARER)) {
818 				if (LOCK_LOG_TEST(&sx->lock_object, 0))
819 					CTR4(KTR_LOCK,
820 					    "%s: %p succeed %p -> %p", __func__,
821 					    sx, (void *)x,
822 					    (void *)(x + SX_ONE_SHARER));
823 				break;
824 			}
825 			continue;
826 		}
827 #ifdef HWPMC_HOOKS
828 		PMC_SOFT_CALL( , , lock, failed);
829 #endif
830 		lock_profile_obtain_lock_failed(&sx->lock_object, &contested,
831 		    &waittime);
832 
833 #ifdef ADAPTIVE_SX
834 		/*
835 		 * If the owner is running on another CPU, spin until
836 		 * the owner stops running or the state of the lock
837 		 * changes.
838 		 */
839 		if ((sx->lock_object.lo_flags & SX_NOADAPTIVE) == 0) {
840 			x = SX_OWNER(x);
841 			owner = (struct thread *)x;
842 			if (TD_IS_RUNNING(owner)) {
843 				if (LOCK_LOG_TEST(&sx->lock_object, 0))
844 					CTR3(KTR_LOCK,
845 					    "%s: spinning on %p held by %p",
846 					    __func__, sx, owner);
847 				GIANT_SAVE();
848 				while (SX_OWNER(sx->sx_lock) == x &&
849 				    TD_IS_RUNNING(owner)) {
850 #ifdef KDTRACE_HOOKS
851 					spin_cnt++;
852 #endif
853 					cpu_spinwait();
854 				}
855 				continue;
856 			}
857 		}
858 #endif
859 
860 		/*
861 		 * Some other thread already has an exclusive lock, so
862 		 * start the process of blocking.
863 		 */
864 		sleepq_lock(&sx->lock_object);
865 		x = sx->sx_lock;
866 
867 		/*
868 		 * The lock could have been released while we spun.
869 		 * In this case loop back and retry.
870 		 */
871 		if (x & SX_LOCK_SHARED) {
872 			sleepq_release(&sx->lock_object);
873 			continue;
874 		}
875 
876 #ifdef ADAPTIVE_SX
877 		/*
878 		 * If the owner is running on another CPU, spin until
879 		 * the owner stops running or the state of the lock
880 		 * changes.
881 		 */
882 		if (!(x & SX_LOCK_SHARED) &&
883 		    (sx->lock_object.lo_flags & SX_NOADAPTIVE) == 0) {
884 			owner = (struct thread *)SX_OWNER(x);
885 			if (TD_IS_RUNNING(owner)) {
886 				sleepq_release(&sx->lock_object);
887 				continue;
888 			}
889 		}
890 #endif
891 
892 		/*
893 		 * Try to set the SX_LOCK_SHARED_WAITERS flag.  If we
894 		 * fail to set it drop the sleep queue lock and loop
895 		 * back.
896 		 */
897 		if (!(x & SX_LOCK_SHARED_WAITERS)) {
898 			if (!atomic_cmpset_ptr(&sx->sx_lock, x,
899 			    x | SX_LOCK_SHARED_WAITERS)) {
900 				sleepq_release(&sx->lock_object);
901 				continue;
902 			}
903 			if (LOCK_LOG_TEST(&sx->lock_object, 0))
904 				CTR2(KTR_LOCK, "%s: %p set shared waiters flag",
905 				    __func__, sx);
906 		}
907 
908 		/*
909 		 * Since we have been unable to acquire the shared lock,
910 		 * we have to sleep.
911 		 */
912 		if (LOCK_LOG_TEST(&sx->lock_object, 0))
913 			CTR2(KTR_LOCK, "%s: %p blocking on sleep queue",
914 			    __func__, sx);
915 
916 #ifdef KDTRACE_HOOKS
917 		sleep_time -= lockstat_nsecs();
918 #endif
919 		GIANT_SAVE();
920 		sleepq_add(&sx->lock_object, NULL, sx->lock_object.lo_name,
921 		    SLEEPQ_SX | ((opts & SX_INTERRUPTIBLE) ?
922 		    SLEEPQ_INTERRUPTIBLE : 0), SQ_SHARED_QUEUE);
923 		if (!(opts & SX_INTERRUPTIBLE))
924 			sleepq_wait(&sx->lock_object, 0);
925 		else
926 			error = sleepq_wait_sig(&sx->lock_object, 0);
927 #ifdef KDTRACE_HOOKS
928 		sleep_time += lockstat_nsecs();
929 		sleep_cnt++;
930 #endif
931 		if (error) {
932 			if (LOCK_LOG_TEST(&sx->lock_object, 0))
933 				CTR2(KTR_LOCK,
934 			"%s: interruptible sleep by %p suspended by signal",
935 				    __func__, sx);
936 			break;
937 		}
938 		if (LOCK_LOG_TEST(&sx->lock_object, 0))
939 			CTR2(KTR_LOCK, "%s: %p resuming from sleep queue",
940 			    __func__, sx);
941 	}
942 	if (error == 0)
943 		LOCKSTAT_PROFILE_OBTAIN_LOCK_SUCCESS(LS_SX_SLOCK_ACQUIRE, sx,
944 		    contested, waittime, file, line);
945 #ifdef KDTRACE_HOOKS
946 	if (sleep_time)
947 		LOCKSTAT_RECORD1(LS_SX_XLOCK_BLOCK, sx, sleep_time);
948 	if (spin_cnt > sleep_cnt)
949 		LOCKSTAT_RECORD1(LS_SX_XLOCK_SPIN, sx, (spin_cnt - sleep_cnt));
950 #endif
951 	GIANT_RESTORE();
952 	return (error);
953 }
954 
955 /*
956  * This function represents the so-called 'hard case' for sx_sunlock
957  * operation.  All 'easy case' failures are redirected to this.  Note
958  * that ideally this would be a static function, but it needs to be
959  * accessible from at least sx.h.
960  */
961 void
962 _sx_sunlock_hard(struct sx *sx, const char *file, int line)
963 {
964 	uintptr_t x;
965 	int wakeup_swapper;
966 
967 	if (SCHEDULER_STOPPED())
968 		return;
969 
970 	for (;;) {
971 		x = sx->sx_lock;
972 
973 		/*
974 		 * We should never have sharers while at least one thread
975 		 * holds a shared lock.
976 		 */
977 		KASSERT(!(x & SX_LOCK_SHARED_WAITERS),
978 		    ("%s: waiting sharers", __func__));
979 
980 		/*
981 		 * See if there is more than one shared lock held.  If
982 		 * so, just drop one and return.
983 		 */
984 		if (SX_SHARERS(x) > 1) {
985 			if (atomic_cmpset_rel_ptr(&sx->sx_lock, x,
986 			    x - SX_ONE_SHARER)) {
987 				if (LOCK_LOG_TEST(&sx->lock_object, 0))
988 					CTR4(KTR_LOCK,
989 					    "%s: %p succeeded %p -> %p",
990 					    __func__, sx, (void *)x,
991 					    (void *)(x - SX_ONE_SHARER));
992 				break;
993 			}
994 			continue;
995 		}
996 
997 		/*
998 		 * If there aren't any waiters for an exclusive lock,
999 		 * then try to drop it quickly.
1000 		 */
1001 		if (!(x & SX_LOCK_EXCLUSIVE_WAITERS)) {
1002 			MPASS(x == SX_SHARERS_LOCK(1));
1003 			if (atomic_cmpset_rel_ptr(&sx->sx_lock,
1004 			    SX_SHARERS_LOCK(1), SX_LOCK_UNLOCKED)) {
1005 				if (LOCK_LOG_TEST(&sx->lock_object, 0))
1006 					CTR2(KTR_LOCK, "%s: %p last succeeded",
1007 					    __func__, sx);
1008 				break;
1009 			}
1010 			continue;
1011 		}
1012 
1013 		/*
1014 		 * At this point, there should just be one sharer with
1015 		 * exclusive waiters.
1016 		 */
1017 		MPASS(x == (SX_SHARERS_LOCK(1) | SX_LOCK_EXCLUSIVE_WAITERS));
1018 
1019 		sleepq_lock(&sx->lock_object);
1020 
1021 		/*
1022 		 * Wake up semantic here is quite simple:
1023 		 * Just wake up all the exclusive waiters.
1024 		 * Note that the state of the lock could have changed,
1025 		 * so if it fails loop back and retry.
1026 		 */
1027 		if (!atomic_cmpset_rel_ptr(&sx->sx_lock,
1028 		    SX_SHARERS_LOCK(1) | SX_LOCK_EXCLUSIVE_WAITERS,
1029 		    SX_LOCK_UNLOCKED)) {
1030 			sleepq_release(&sx->lock_object);
1031 			continue;
1032 		}
1033 		if (LOCK_LOG_TEST(&sx->lock_object, 0))
1034 			CTR2(KTR_LOCK, "%s: %p waking up all thread on"
1035 			    "exclusive queue", __func__, sx);
1036 		wakeup_swapper = sleepq_broadcast(&sx->lock_object, SLEEPQ_SX,
1037 		    0, SQ_EXCLUSIVE_QUEUE);
1038 		sleepq_release(&sx->lock_object);
1039 		if (wakeup_swapper)
1040 			kick_proc0();
1041 		break;
1042 	}
1043 }
1044 
1045 #ifdef INVARIANT_SUPPORT
1046 #ifndef INVARIANTS
1047 #undef	_sx_assert
1048 #endif
1049 
1050 /*
1051  * In the non-WITNESS case, sx_assert() can only detect that at least
1052  * *some* thread owns an slock, but it cannot guarantee that *this*
1053  * thread owns an slock.
1054  */
1055 void
1056 _sx_assert(const struct sx *sx, int what, const char *file, int line)
1057 {
1058 #ifndef WITNESS
1059 	int slocked = 0;
1060 #endif
1061 
1062 	if (panicstr != NULL)
1063 		return;
1064 	switch (what) {
1065 	case SA_SLOCKED:
1066 	case SA_SLOCKED | SA_NOTRECURSED:
1067 	case SA_SLOCKED | SA_RECURSED:
1068 #ifndef WITNESS
1069 		slocked = 1;
1070 		/* FALLTHROUGH */
1071 #endif
1072 	case SA_LOCKED:
1073 	case SA_LOCKED | SA_NOTRECURSED:
1074 	case SA_LOCKED | SA_RECURSED:
1075 #ifdef WITNESS
1076 		witness_assert(&sx->lock_object, what, file, line);
1077 #else
1078 		/*
1079 		 * If some other thread has an exclusive lock or we
1080 		 * have one and are asserting a shared lock, fail.
1081 		 * Also, if no one has a lock at all, fail.
1082 		 */
1083 		if (sx->sx_lock == SX_LOCK_UNLOCKED ||
1084 		    (!(sx->sx_lock & SX_LOCK_SHARED) && (slocked ||
1085 		    sx_xholder(sx) != curthread)))
1086 			panic("Lock %s not %slocked @ %s:%d\n",
1087 			    sx->lock_object.lo_name, slocked ? "share " : "",
1088 			    file, line);
1089 
1090 		if (!(sx->sx_lock & SX_LOCK_SHARED)) {
1091 			if (sx_recursed(sx)) {
1092 				if (what & SA_NOTRECURSED)
1093 					panic("Lock %s recursed @ %s:%d\n",
1094 					    sx->lock_object.lo_name, file,
1095 					    line);
1096 			} else if (what & SA_RECURSED)
1097 				panic("Lock %s not recursed @ %s:%d\n",
1098 				    sx->lock_object.lo_name, file, line);
1099 		}
1100 #endif
1101 		break;
1102 	case SA_XLOCKED:
1103 	case SA_XLOCKED | SA_NOTRECURSED:
1104 	case SA_XLOCKED | SA_RECURSED:
1105 		if (sx_xholder(sx) != curthread)
1106 			panic("Lock %s not exclusively locked @ %s:%d\n",
1107 			    sx->lock_object.lo_name, file, line);
1108 		if (sx_recursed(sx)) {
1109 			if (what & SA_NOTRECURSED)
1110 				panic("Lock %s recursed @ %s:%d\n",
1111 				    sx->lock_object.lo_name, file, line);
1112 		} else if (what & SA_RECURSED)
1113 			panic("Lock %s not recursed @ %s:%d\n",
1114 			    sx->lock_object.lo_name, file, line);
1115 		break;
1116 	case SA_UNLOCKED:
1117 #ifdef WITNESS
1118 		witness_assert(&sx->lock_object, what, file, line);
1119 #else
1120 		/*
1121 		 * If we hold an exclusve lock fail.  We can't
1122 		 * reliably check to see if we hold a shared lock or
1123 		 * not.
1124 		 */
1125 		if (sx_xholder(sx) == curthread)
1126 			panic("Lock %s exclusively locked @ %s:%d\n",
1127 			    sx->lock_object.lo_name, file, line);
1128 #endif
1129 		break;
1130 	default:
1131 		panic("Unknown sx lock assertion: %d @ %s:%d", what, file,
1132 		    line);
1133 	}
1134 }
1135 #endif	/* INVARIANT_SUPPORT */
1136 
1137 #ifdef DDB
1138 static void
1139 db_show_sx(const struct lock_object *lock)
1140 {
1141 	struct thread *td;
1142 	const struct sx *sx;
1143 
1144 	sx = (const struct sx *)lock;
1145 
1146 	db_printf(" state: ");
1147 	if (sx->sx_lock == SX_LOCK_UNLOCKED)
1148 		db_printf("UNLOCKED\n");
1149 	else if (sx->sx_lock == SX_LOCK_DESTROYED) {
1150 		db_printf("DESTROYED\n");
1151 		return;
1152 	} else if (sx->sx_lock & SX_LOCK_SHARED)
1153 		db_printf("SLOCK: %ju\n", (uintmax_t)SX_SHARERS(sx->sx_lock));
1154 	else {
1155 		td = sx_xholder(sx);
1156 		db_printf("XLOCK: %p (tid %d, pid %d, \"%s\")\n", td,
1157 		    td->td_tid, td->td_proc->p_pid, td->td_name);
1158 		if (sx_recursed(sx))
1159 			db_printf(" recursed: %d\n", sx->sx_recurse);
1160 	}
1161 
1162 	db_printf(" waiters: ");
1163 	switch(sx->sx_lock &
1164 	    (SX_LOCK_SHARED_WAITERS | SX_LOCK_EXCLUSIVE_WAITERS)) {
1165 	case SX_LOCK_SHARED_WAITERS:
1166 		db_printf("shared\n");
1167 		break;
1168 	case SX_LOCK_EXCLUSIVE_WAITERS:
1169 		db_printf("exclusive\n");
1170 		break;
1171 	case SX_LOCK_SHARED_WAITERS | SX_LOCK_EXCLUSIVE_WAITERS:
1172 		db_printf("exclusive and shared\n");
1173 		break;
1174 	default:
1175 		db_printf("none\n");
1176 	}
1177 }
1178 
1179 /*
1180  * Check to see if a thread that is blocked on a sleep queue is actually
1181  * blocked on an sx lock.  If so, output some details and return true.
1182  * If the lock has an exclusive owner, return that in *ownerp.
1183  */
1184 int
1185 sx_chain(struct thread *td, struct thread **ownerp)
1186 {
1187 	struct sx *sx;
1188 
1189 	/*
1190 	 * Check to see if this thread is blocked on an sx lock.
1191 	 * First, we check the lock class.  If that is ok, then we
1192 	 * compare the lock name against the wait message.
1193 	 */
1194 	sx = td->td_wchan;
1195 	if (LOCK_CLASS(&sx->lock_object) != &lock_class_sx ||
1196 	    sx->lock_object.lo_name != td->td_wmesg)
1197 		return (0);
1198 
1199 	/* We think we have an sx lock, so output some details. */
1200 	db_printf("blocked on sx \"%s\" ", td->td_wmesg);
1201 	*ownerp = sx_xholder(sx);
1202 	if (sx->sx_lock & SX_LOCK_SHARED)
1203 		db_printf("SLOCK (count %ju)\n",
1204 		    (uintmax_t)SX_SHARERS(sx->sx_lock));
1205 	else
1206 		db_printf("XLOCK\n");
1207 	return (1);
1208 }
1209 #endif
1210