xref: /freebsd-11-stable/sys/kern/subr_sleepqueue.c

/*-
 * Copyright (c) 2004 John Baldwin <jhb@FreeBSD.org>
 * All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 * 1. Redistributions of source code must retain the above copyright
 *    notice, this list of conditions and the following disclaimer.
 * 2. Redistributions in binary form must reproduce the above copyright
 *    notice, this list of conditions and the following disclaimer in the
 *    documentation and/or other materials provided with the distribution.
 *
 * THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
 * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
 * ARE DISCLAIMED.  IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
 * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
 * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
 * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
 * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
 * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
 * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 * SUCH DAMAGE.
 */

/*
 * Implementation of sleep queues used to hold queue of threads blocked on
 * a wait channel.  Sleep queues are different from turnstiles in that wait
 * channels are not owned by anyone, so there is no priority propagation.
 * Sleep queues can also provide a timeout and can also be interrupted by
 * signals.  That said, there are several similarities between the turnstile
 * and sleep queue implementations.  (Note: turnstiles were implemented
 * first.)  For example, both use a hash table of the same size where each
 * bucket is referred to as a "chain" that contains both a spin lock and
 * a linked list of queues.  An individual queue is located by using a hash
 * to pick a chain, locking the chain, and then walking the chain searching
 * for the queue.  This means that a wait channel object does not need to
 * embed its queue head just as locks do not embed their turnstile queue
 * head.  Threads also carry around a sleep queue that they lend to the
 * wait channel when blocking.  Just as in turnstiles, the queue includes
 * a free list of the sleep queues of other threads blocked on the same
 * wait channel in the case of multiple waiters.
 *
 * Some additional functionality provided by sleep queues include the
 * ability to set a timeout.  The timeout is managed using a per-thread
 * callout that resumes a thread if it is asleep.  A thread may also
 * catch signals while it is asleep (aka an interruptible sleep).  The
 * signal code uses sleepq_abort() to interrupt a sleeping thread.  Finally,
 * sleep queues also provide some extra assertions.  One is not allowed to
 * mix the sleep/wakeup and cv APIs for a given wait channel.  Also, one
 * must consistently use the same lock to synchronize with a wait channel,
 * though this check is currently only a warning for sleep/wakeup due to
 * pre-existing abuse of that API.  The same lock must also be held when
 * awakening threads, though that is currently only enforced for condition
 * variables.
 */

#include <sys/cdefs.h>
__FBSDID("$FreeBSD: stable/11/sys/kern/subr_sleepqueue.c 350357 2019-07-26 10:43:07Z kib $");

#include "opt_sleepqueue_profiling.h"
#include "opt_ddb.h"
#include "opt_sched.h"
#include "opt_stack.h"

#include <sys/param.h>
#include <sys/systm.h>
#include <sys/lock.h>
#include <sys/kernel.h>
#include <sys/ktr.h>
#include <sys/mutex.h>
#include <sys/proc.h>
#include <sys/sbuf.h>
#include <sys/sched.h>
#include <sys/sdt.h>
#include <sys/signalvar.h>
#include <sys/sleepqueue.h>
#include <sys/stack.h>
#include <sys/sysctl.h>
#include <sys/time.h>

#include <machine/atomic.h>

#include <vm/uma.h>

#ifdef DDB
#include <ddb/ddb.h>
#endif


/*
 * Constants for the hash table of sleep queue chains.
 * SC_TABLESIZE must be a power of two for SC_MASK to work properly.
 */
#define	SC_TABLESIZE	256			/* Must be power of 2. */
#define	SC_MASK		(SC_TABLESIZE - 1)
#define	SC_SHIFT	8
#define	SC_HASH(wc)	((((uintptr_t)(wc) >> SC_SHIFT) ^ (uintptr_t)(wc)) & \
			    SC_MASK)
#define	SC_LOOKUP(wc)	&sleepq_chains[SC_HASH(wc)]
#define NR_SLEEPQS      2
/*
 * There are two different lists of sleep queues.  Both lists are connected
 * via the sq_hash entries.  The first list is the sleep queue chain list
 * that a sleep queue is on when it is attached to a wait channel.  The
 * second list is the free list hung off of a sleep queue that is attached
 * to a wait channel.
 *
 * Each sleep queue also contains the wait channel it is attached to, the
 * list of threads blocked on that wait channel, flags specific to the
 * wait channel, and the lock used to synchronize with a wait channel.
 * The flags are used to catch mismatches between the various consumers
 * of the sleep queue API (e.g. sleep/wakeup and condition variables).
 * The lock pointer is only used when invariants are enabled for various
 * debugging checks.
 *
 * Locking key:
 *  c - sleep queue chain lock
 */
struct sleepqueue {
	TAILQ_HEAD(, thread) sq_blocked[NR_SLEEPQS];	/* (c) Blocked threads. */
	u_int sq_blockedcnt[NR_SLEEPQS];	/* (c) N. of blocked threads. */
	LIST_ENTRY(sleepqueue) sq_hash;		/* (c) Chain and free list. */
	LIST_HEAD(, sleepqueue) sq_free;	/* (c) Free queues. */
	void	*sq_wchan;			/* (c) Wait channel. */
	int	sq_type;			/* (c) Queue type. */
#ifdef INVARIANTS
	struct lock_object *sq_lock;		/* (c) Associated lock. */
#endif
};

struct sleepqueue_chain {
	LIST_HEAD(, sleepqueue) sc_queues;	/* List of sleep queues. */
	struct mtx sc_lock;			/* Spin lock for this chain. */
#ifdef SLEEPQUEUE_PROFILING
	u_int	sc_depth;			/* Length of sc_queues. */
	u_int	sc_max_depth;			/* Max length of sc_queues. */
#endif
};

#ifdef SLEEPQUEUE_PROFILING
u_int sleepq_max_depth;
static SYSCTL_NODE(_debug, OID_AUTO, sleepq, CTLFLAG_RD, 0, "sleepq profiling");
static SYSCTL_NODE(_debug_sleepq, OID_AUTO, chains, CTLFLAG_RD, 0,
    "sleepq chain stats");
SYSCTL_UINT(_debug_sleepq, OID_AUTO, max_depth, CTLFLAG_RD, &sleepq_max_depth,
    0, "maxmimum depth achieved of a single chain");

static void	sleepq_profile(const char *wmesg);
static int	prof_enabled;
#endif
static struct sleepqueue_chain sleepq_chains[SC_TABLESIZE];
static uma_zone_t sleepq_zone;

/*
 * Prototypes for non-exported routines.
 */
static int	sleepq_catch_signals(void *wchan, int pri);
static int	sleepq_check_signals(void);
static int	sleepq_check_timeout(void);
#ifdef INVARIANTS
static void	sleepq_dtor(void *mem, int size, void *arg);
#endif
static int	sleepq_init(void *mem, int size, int flags);
static int	sleepq_resume_thread(struct sleepqueue *sq, struct thread *td,
		    int pri);
static void	sleepq_switch(void *wchan, int pri);
static void	sleepq_timeout(void *arg);

SDT_PROBE_DECLARE(sched, , , sleep);
SDT_PROBE_DECLARE(sched, , , wakeup);

/*
 * Initialize SLEEPQUEUE_PROFILING specific sysctl nodes.
 * Note that it must happen after sleepinit() has been fully executed, so
 * it must happen after SI_SUB_KMEM SYSINIT() subsystem setup.
 */
#ifdef SLEEPQUEUE_PROFILING
static void
init_sleepqueue_profiling(void)
{
	char chain_name[10];
	struct sysctl_oid *chain_oid;
	u_int i;

	for (i = 0; i < SC_TABLESIZE; i++) {
		snprintf(chain_name, sizeof(chain_name), "%u", i);
		chain_oid = SYSCTL_ADD_NODE(NULL,
		    SYSCTL_STATIC_CHILDREN(_debug_sleepq_chains), OID_AUTO,
		    chain_name, CTLFLAG_RD, NULL, "sleepq chain stats");
		SYSCTL_ADD_UINT(NULL, SYSCTL_CHILDREN(chain_oid), OID_AUTO,
		    "depth", CTLFLAG_RD, &sleepq_chains[i].sc_depth, 0, NULL);
		SYSCTL_ADD_UINT(NULL, SYSCTL_CHILDREN(chain_oid), OID_AUTO,
		    "max_depth", CTLFLAG_RD, &sleepq_chains[i].sc_max_depth, 0,
		    NULL);
	}
}

SYSINIT(sleepqueue_profiling, SI_SUB_LOCK, SI_ORDER_ANY,
    init_sleepqueue_profiling, NULL);
#endif

/*
 * Early initialization of sleep queues that is called from the sleepinit()
 * SYSINIT.
 */
void
init_sleepqueues(void)
{
	int i;

	for (i = 0; i < SC_TABLESIZE; i++) {
		LIST_INIT(&sleepq_chains[i].sc_queues);
		mtx_init(&sleepq_chains[i].sc_lock, "sleepq chain", NULL,
		    MTX_SPIN | MTX_RECURSE);
	}
	sleepq_zone = uma_zcreate("SLEEPQUEUE", sizeof(struct sleepqueue),
#ifdef INVARIANTS
	    NULL, sleepq_dtor, sleepq_init, NULL, UMA_ALIGN_CACHE, 0);
#else
	    NULL, NULL, sleepq_init, NULL, UMA_ALIGN_CACHE, 0);
#endif

	thread0.td_sleepqueue = sleepq_alloc();
}

/*
 * Get a sleep queue for a new thread.
 */
struct sleepqueue *
sleepq_alloc(void)
{

	return (uma_zalloc(sleepq_zone, M_WAITOK));
}

/*
 * Free a sleep queue when a thread is destroyed.
 */
void
sleepq_free(struct sleepqueue *sq)
{

	uma_zfree(sleepq_zone, sq);
}

/*
 * Lock the sleep queue chain associated with the specified wait channel.
 */
void
sleepq_lock(void *wchan)
{
	struct sleepqueue_chain *sc;

	sc = SC_LOOKUP(wchan);
	mtx_lock_spin(&sc->sc_lock);
}

/*
 * Look up the sleep queue associated with a given wait channel in the hash
 * table locking the associated sleep queue chain.  If no queue is found in
 * the table, NULL is returned.
 */
struct sleepqueue *
sleepq_lookup(void *wchan)
{
	struct sleepqueue_chain *sc;
	struct sleepqueue *sq;

	KASSERT(wchan != NULL, ("%s: invalid NULL wait channel", __func__));
	sc = SC_LOOKUP(wchan);
	mtx_assert(&sc->sc_lock, MA_OWNED);
	LIST_FOREACH(sq, &sc->sc_queues, sq_hash)
		if (sq->sq_wchan == wchan)
			return (sq);
	return (NULL);
}

/*
 * Unlock the sleep queue chain associated with a given wait channel.
 */
void
sleepq_release(void *wchan)
{
	struct sleepqueue_chain *sc;

	sc = SC_LOOKUP(wchan);
	mtx_unlock_spin(&sc->sc_lock);
}

/*
 * Places the current thread on the sleep queue for the specified wait
 * channel.  If INVARIANTS is enabled, then it associates the passed in
 * lock with the sleepq to make sure it is held when that sleep queue is
 * woken up.
 */
void
sleepq_add(void *wchan, struct lock_object *lock, const char *wmesg, int flags,
    int queue)
{
	struct sleepqueue_chain *sc;
	struct sleepqueue *sq;
	struct thread *td;

	td = curthread;
	sc = SC_LOOKUP(wchan);
	mtx_assert(&sc->sc_lock, MA_OWNED);
	MPASS(td->td_sleepqueue != NULL);
	MPASS(wchan != NULL);
	MPASS((queue >= 0) && (queue < NR_SLEEPQS));

	/* If this thread is not allowed to sleep, die a horrible death. */
	KASSERT(td->td_no_sleeping == 0,
	    ("%s: td %p to sleep on wchan %p with sleeping prohibited",
	    __func__, td, wchan));

	/* Look up the sleep queue associated with the wait channel 'wchan'. */
	sq = sleepq_lookup(wchan);

	/*
	 * If the wait channel does not already have a sleep queue, use
	 * this thread's sleep queue.  Otherwise, insert the current thread
	 * into the sleep queue already in use by this wait channel.
	 */
	if (sq == NULL) {
#ifdef INVARIANTS
		int i;

		sq = td->td_sleepqueue;
		for (i = 0; i < NR_SLEEPQS; i++) {
			KASSERT(TAILQ_EMPTY(&sq->sq_blocked[i]),
			    ("thread's sleep queue %d is not empty", i));
			KASSERT(sq->sq_blockedcnt[i] == 0,
			    ("thread's sleep queue %d count mismatches", i));
		}
		KASSERT(LIST_EMPTY(&sq->sq_free),
		    ("thread's sleep queue has a non-empty free list"));
		KASSERT(sq->sq_wchan == NULL, ("stale sq_wchan pointer"));
		sq->sq_lock = lock;
#endif
#ifdef SLEEPQUEUE_PROFILING
		sc->sc_depth++;
		if (sc->sc_depth > sc->sc_max_depth) {
			sc->sc_max_depth = sc->sc_depth;
			if (sc->sc_max_depth > sleepq_max_depth)
				sleepq_max_depth = sc->sc_max_depth;
		}
#endif
		sq = td->td_sleepqueue;
		LIST_INSERT_HEAD(&sc->sc_queues, sq, sq_hash);
		sq->sq_wchan = wchan;
		sq->sq_type = flags & SLEEPQ_TYPE;
	} else {
		MPASS(wchan == sq->sq_wchan);
		MPASS(lock == sq->sq_lock);
		MPASS((flags & SLEEPQ_TYPE) == sq->sq_type);
		LIST_INSERT_HEAD(&sq->sq_free, td->td_sleepqueue, sq_hash);
	}
	thread_lock(td);
	TAILQ_INSERT_TAIL(&sq->sq_blocked[queue], td, td_slpq);
	sq->sq_blockedcnt[queue]++;
	td->td_sleepqueue = NULL;
	td->td_sqqueue = queue;
	td->td_wchan = wchan;
	td->td_wmesg = wmesg;
	if (flags & SLEEPQ_INTERRUPTIBLE) {
		td->td_flags |= TDF_SINTR;
		td->td_flags &= ~TDF_SLEEPABORT;
	}
	thread_unlock(td);
}

/*
 * Sets a timeout that will remove the current thread from the specified
 * sleep queue after timo ticks if the thread has not already been awakened.
 */
void
sleepq_set_timeout_sbt(void *wchan, sbintime_t sbt, sbintime_t pr,
    int flags)
{
	struct sleepqueue_chain *sc;
	struct thread *td;
	sbintime_t pr1;

	td = curthread;
	sc = SC_LOOKUP(wchan);
	mtx_assert(&sc->sc_lock, MA_OWNED);
	MPASS(TD_ON_SLEEPQ(td));
	MPASS(td->td_sleepqueue == NULL);
	MPASS(wchan != NULL);
	if (cold && td == &thread0)
		panic("timed sleep before timers are working");
	KASSERT(td->td_sleeptimo == 0, ("td %d %p td_sleeptimo %jx",
	    td->td_tid, td, (uintmax_t)td->td_sleeptimo));
	thread_lock(td);
	callout_when(sbt, pr, flags, &td->td_sleeptimo, &pr1);
	thread_unlock(td);
	callout_reset_sbt_on(&td->td_slpcallout, td->td_sleeptimo, pr1,
	    sleepq_timeout, td, PCPU_GET(cpuid), flags | C_PRECALC |
	    C_DIRECT_EXEC);
}

/*
 * Return the number of actual sleepers for the specified queue.
 */
u_int
sleepq_sleepcnt(void *wchan, int queue)
{
	struct sleepqueue *sq;

	KASSERT(wchan != NULL, ("%s: invalid NULL wait channel", __func__));
	MPASS((queue >= 0) && (queue < NR_SLEEPQS));
	sq = sleepq_lookup(wchan);
	if (sq == NULL)
		return (0);
	return (sq->sq_blockedcnt[queue]);
}

/*
 * Marks the pending sleep of the current thread as interruptible and
 * makes an initial check for pending signals before putting a thread
 * to sleep. Enters and exits with the thread lock held.  Thread lock
 * may have transitioned from the sleepq lock to a run lock.
 */
static int
sleepq_catch_signals(void *wchan, int pri)
{
	struct sleepqueue_chain *sc;
	struct sleepqueue *sq;
	struct thread *td;
	struct proc *p;
	struct sigacts *ps;
	int sig, ret;

	ret = 0;
	td = curthread;
	p = curproc;
	sc = SC_LOOKUP(wchan);
	mtx_assert(&sc->sc_lock, MA_OWNED);
	MPASS(wchan != NULL);
	if ((td->td_pflags & TDP_WAKEUP) != 0) {
		td->td_pflags &= ~TDP_WAKEUP;
		ret = EINTR;
		thread_lock(td);
		goto out;
	}

	/*
	 * See if there are any pending signals or suspension requests for this
	 * thread.  If not, we can switch immediately.
	 */
	thread_lock(td);
	if ((td->td_flags & (TDF_NEEDSIGCHK | TDF_NEEDSUSPCHK)) != 0) {
		thread_unlock(td);
		mtx_unlock_spin(&sc->sc_lock);
		CTR3(KTR_PROC, "sleepq catching signals: thread %p (pid %ld, %s)",
			(void *)td, (long)p->p_pid, td->td_name);
		PROC_LOCK(p);
		/*
		 * Check for suspension first. Checking for signals and then
		 * suspending could result in a missed signal, since a signal
		 * can be delivered while this thread is suspended.
		 */
		if ((td->td_flags & TDF_NEEDSUSPCHK) != 0) {
			ret = thread_suspend_check(1);
			MPASS(ret == 0 || ret == EINTR || ret == ERESTART);
			if (ret != 0) {
				PROC_UNLOCK(p);
				mtx_lock_spin(&sc->sc_lock);
				thread_lock(td);
				goto out;
			}
		}
		if ((td->td_flags & TDF_NEEDSIGCHK) != 0) {
			ps = p->p_sigacts;
			mtx_lock(&ps->ps_mtx);
			sig = cursig(td);
			if (sig == -1) {
				mtx_unlock(&ps->ps_mtx);
				KASSERT((td->td_flags & TDF_SBDRY) != 0,
				    ("lost TDF_SBDRY"));
				KASSERT(TD_SBDRY_INTR(td),
				    ("lost TDF_SERESTART of TDF_SEINTR"));
				KASSERT((td->td_flags &
				    (TDF_SEINTR | TDF_SERESTART)) !=
				    (TDF_SEINTR | TDF_SERESTART),
				    ("both TDF_SEINTR and TDF_SERESTART"));
				ret = TD_SBDRY_ERRNO(td);
			} else if (sig != 0) {
				ret = SIGISMEMBER(ps->ps_sigintr, sig) ?
				    EINTR : ERESTART;
				mtx_unlock(&ps->ps_mtx);
			} else {
				mtx_unlock(&ps->ps_mtx);
			}

			/*
			 * Do not go into sleep if this thread was the
			 * ptrace(2) attach leader.  cursig() consumed
			 * SIGSTOP from PT_ATTACH, but we usually act
			 * on the signal by interrupting sleep, and
			 * should do that here as well.
			 */
			if ((td->td_dbgflags & TDB_FSTP) != 0) {
				if (ret == 0)
					ret = EINTR;
				td->td_dbgflags &= ~TDB_FSTP;
			}
		}
		/*
		 * Lock the per-process spinlock prior to dropping the PROC_LOCK
		 * to avoid a signal delivery race.  PROC_LOCK, PROC_SLOCK, and
		 * thread_lock() are currently held in tdsendsignal().
		 */
		PROC_SLOCK(p);
		mtx_lock_spin(&sc->sc_lock);
		PROC_UNLOCK(p);
		thread_lock(td);
		PROC_SUNLOCK(p);
	}
	if (ret == 0) {
		sleepq_switch(wchan, pri);
		return (0);
	}
out:
	/*
	 * There were pending signals and this thread is still
	 * on the sleep queue, remove it from the sleep queue.
	 */
	if (TD_ON_SLEEPQ(td)) {
		sq = sleepq_lookup(wchan);
		if (sleepq_resume_thread(sq, td, 0)) {
#ifdef INVARIANTS
			/*
			 * This thread hasn't gone to sleep yet, so it
			 * should not be swapped out.
			 */
			panic("not waking up swapper");
#endif
		}
	}
	mtx_unlock_spin(&sc->sc_lock);
	MPASS(td->td_lock != &sc->sc_lock);
	return (ret);
}

/*
 * Switches to another thread if we are still asleep on a sleep queue.
 * Returns with thread lock.
 */
static void
sleepq_switch(void *wchan, int pri)
{
	struct sleepqueue_chain *sc;
	struct sleepqueue *sq;
	struct thread *td;
	bool rtc_changed;

	td = curthread;
	sc = SC_LOOKUP(wchan);
	mtx_assert(&sc->sc_lock, MA_OWNED);
	THREAD_LOCK_ASSERT(td, MA_OWNED);

	/*
	 * If we have a sleep queue, then we've already been woken up, so
	 * just return.
	 */
	if (td->td_sleepqueue != NULL) {
		mtx_unlock_spin(&sc->sc_lock);
		return;
	}

	/*
	 * If TDF_TIMEOUT is set, then our sleep has been timed out
	 * already but we are still on the sleep queue, so dequeue the
	 * thread and return.
	 *
	 * Do the same if the real-time clock has been adjusted since this
	 * thread calculated its timeout based on that clock.  This handles
	 * the following race:
	 * - The Ts thread needs to sleep until an absolute real-clock time.
	 *   It copies the global rtc_generation into curthread->td_rtcgen,
	 *   reads the RTC, and calculates a sleep duration based on that time.
	 *   See umtxq_sleep() for an example.
	 * - The Tc thread adjusts the RTC, bumps rtc_generation, and wakes
	 *   threads that are sleeping until an absolute real-clock time.
	 *   See tc_setclock() and the POSIX specification of clock_settime().
	 * - Ts reaches the code below.  It holds the sleepqueue chain lock,
	 *   so Tc has finished waking, so this thread must test td_rtcgen.
	 * (The declaration of td_rtcgen refers to this comment.)
	 */
	rtc_changed = td->td_rtcgen != 0 && td->td_rtcgen != rtc_generation;
	if ((td->td_flags & TDF_TIMEOUT) || rtc_changed) {
		if (rtc_changed) {
			td->td_rtcgen = 0;
		}
		MPASS(TD_ON_SLEEPQ(td));
		sq = sleepq_lookup(wchan);
		if (sleepq_resume_thread(sq, td, 0)) {
#ifdef INVARIANTS
			/*
			 * This thread hasn't gone to sleep yet, so it
			 * should not be swapped out.
			 */
			panic("not waking up swapper");
#endif
		}
		mtx_unlock_spin(&sc->sc_lock);
		return;
	}
#ifdef SLEEPQUEUE_PROFILING
	if (prof_enabled)
		sleepq_profile(td->td_wmesg);
#endif
	MPASS(td->td_sleepqueue == NULL);
	sched_sleep(td, pri);
	thread_lock_set(td, &sc->sc_lock);
	SDT_PROBE0(sched, , , sleep);
	TD_SET_SLEEPING(td);
	mi_switch(SW_VOL | SWT_SLEEPQ, NULL);
	KASSERT(TD_IS_RUNNING(td), ("running but not TDS_RUNNING"));
	CTR3(KTR_PROC, "sleepq resume: thread %p (pid %ld, %s)",
	    (void *)td, (long)td->td_proc->p_pid, (void *)td->td_name);
}

/*
 * Check to see if we timed out.
 */
static int
sleepq_check_timeout(void)
{
	struct thread *td;
	int res;

	td = curthread;
	THREAD_LOCK_ASSERT(td, MA_OWNED);

	/*
	 * If TDF_TIMEOUT is set, we timed out.  But recheck
	 * td_sleeptimo anyway.
	 */
	res = 0;
	if (td->td_sleeptimo != 0) {
		if (td->td_sleeptimo <= sbinuptime())
			res = EWOULDBLOCK;
		td->td_sleeptimo = 0;
	}
	if (td->td_flags & TDF_TIMEOUT)
		td->td_flags &= ~TDF_TIMEOUT;
	else
		/*
		 * We ignore the situation where timeout subsystem was
		 * unable to stop our callout.  The struct thread is
		 * type-stable, the callout will use the correct
		 * memory when running.  The checks of the
		 * td_sleeptimo value in this function and in
		 * sleepq_timeout() ensure that the thread does not
		 * get spurious wakeups, even if the callout was reset
		 * or thread reused.
		 */
		callout_stop(&td->td_slpcallout);
	return (res);
}

/*
 * Check to see if we were awoken by a signal.
 */
static int
sleepq_check_signals(void)
{
	struct thread *td;

	td = curthread;
	THREAD_LOCK_ASSERT(td, MA_OWNED);

	/* We are no longer in an interruptible sleep. */
	if (td->td_flags & TDF_SINTR)
		td->td_flags &= ~TDF_SINTR;

	if (td->td_flags & TDF_SLEEPABORT) {
		td->td_flags &= ~TDF_SLEEPABORT;
		return (td->td_intrval);
	}

	return (0);
}

/*
 * Block the current thread until it is awakened from its sleep queue.
 */
void
sleepq_wait(void *wchan, int pri)
{
	struct thread *td;

	td = curthread;
	MPASS(!(td->td_flags & TDF_SINTR));
	thread_lock(td);
	sleepq_switch(wchan, pri);
	thread_unlock(td);
}

/*
 * Block the current thread until it is awakened from its sleep queue
 * or it is interrupted by a signal.
 */
int
sleepq_wait_sig(void *wchan, int pri)
{
	int rcatch;
	int rval;

	rcatch = sleepq_catch_signals(wchan, pri);
	rval = sleepq_check_signals();
	thread_unlock(curthread);
	if (rcatch)
		return (rcatch);
	return (rval);
}

/*
 * Block the current thread until it is awakened from its sleep queue
 * or it times out while waiting.
 */
int
sleepq_timedwait(void *wchan, int pri)
{
	struct thread *td;
	int rval;

	td = curthread;
	MPASS(!(td->td_flags & TDF_SINTR));
	thread_lock(td);
	sleepq_switch(wchan, pri);
	rval = sleepq_check_timeout();
	thread_unlock(td);

	return (rval);
}

/*
 * Block the current thread until it is awakened from its sleep queue,
 * it is interrupted by a signal, or it times out waiting to be awakened.
 */
int
sleepq_timedwait_sig(void *wchan, int pri)
{
	int rcatch, rvalt, rvals;

	rcatch = sleepq_catch_signals(wchan, pri);
	rvalt = sleepq_check_timeout();
	rvals = sleepq_check_signals();
	thread_unlock(curthread);
	if (rcatch)
		return (rcatch);
	if (rvals)
		return (rvals);
	return (rvalt);
}

/*
 * Returns the type of sleepqueue given a waitchannel.
 */
int
sleepq_type(void *wchan)
{
	struct sleepqueue *sq;
	int type;

	MPASS(wchan != NULL);

	sleepq_lock(wchan);
	sq = sleepq_lookup(wchan);
	if (sq == NULL) {
		sleepq_release(wchan);
		return (-1);
	}
	type = sq->sq_type;
	sleepq_release(wchan);
	return (type);
}

/*
 * Removes a thread from a sleep queue and makes it
 * runnable.
 */
static int
sleepq_resume_thread(struct sleepqueue *sq, struct thread *td, int pri)
{
	struct sleepqueue_chain *sc;

	MPASS(td != NULL);
	MPASS(sq->sq_wchan != NULL);
	MPASS(td->td_wchan == sq->sq_wchan);
	MPASS(td->td_sqqueue < NR_SLEEPQS && td->td_sqqueue >= 0);
	THREAD_LOCK_ASSERT(td, MA_OWNED);
	sc = SC_LOOKUP(sq->sq_wchan);
	mtx_assert(&sc->sc_lock, MA_OWNED);

	SDT_PROBE2(sched, , , wakeup, td, td->td_proc);

	/* Remove the thread from the queue. */
	sq->sq_blockedcnt[td->td_sqqueue]--;
	TAILQ_REMOVE(&sq->sq_blocked[td->td_sqqueue], td, td_slpq);

	/*
	 * Get a sleep queue for this thread.  If this is the last waiter,
	 * use the queue itself and take it out of the chain, otherwise,
	 * remove a queue from the free list.
	 */
	if (LIST_EMPTY(&sq->sq_free)) {
		td->td_sleepqueue = sq;
#ifdef INVARIANTS
		sq->sq_wchan = NULL;
#endif
#ifdef SLEEPQUEUE_PROFILING
		sc->sc_depth--;
#endif
	} else
		td->td_sleepqueue = LIST_FIRST(&sq->sq_free);
	LIST_REMOVE(td->td_sleepqueue, sq_hash);

	td->td_wmesg = NULL;
	td->td_wchan = NULL;
	td->td_flags &= ~TDF_SINTR;

	CTR3(KTR_PROC, "sleepq_wakeup: thread %p (pid %ld, %s)",
	    (void *)td, (long)td->td_proc->p_pid, td->td_name);

	/* Adjust priority if requested. */
	MPASS(pri == 0 || (pri >= PRI_MIN && pri <= PRI_MAX));
	if (pri != 0 && td->td_priority > pri &&
	    PRI_BASE(td->td_pri_class) == PRI_TIMESHARE)
		sched_prio(td, pri);

	/*
	 * Note that thread td might not be sleeping if it is running
	 * sleepq_catch_signals() on another CPU or is blocked on its
	 * proc lock to check signals.  There's no need to mark the
	 * thread runnable in that case.
	 */
	if (TD_IS_SLEEPING(td)) {
		TD_CLR_SLEEPING(td);
		return (setrunnable(td));
	}
	return (0);
}

#ifdef INVARIANTS
/*
 * UMA zone item deallocator.
 */
static void
sleepq_dtor(void *mem, int size, void *arg)
{
	struct sleepqueue *sq;
	int i;

	sq = mem;
	for (i = 0; i < NR_SLEEPQS; i++) {
		MPASS(TAILQ_EMPTY(&sq->sq_blocked[i]));
		MPASS(sq->sq_blockedcnt[i] == 0);
	}
}
#endif

/*
 * UMA zone item initializer.
 */
static int
sleepq_init(void *mem, int size, int flags)
{
	struct sleepqueue *sq;
	int i;

	bzero(mem, size);
	sq = mem;
	for (i = 0; i < NR_SLEEPQS; i++) {
		TAILQ_INIT(&sq->sq_blocked[i]);
		sq->sq_blockedcnt[i] = 0;
	}
	LIST_INIT(&sq->sq_free);
	return (0);
}

/*
 * Find the highest priority thread sleeping on a wait channel and resume it.
 */
int
sleepq_signal(void *wchan, int flags, int pri, int queue)
{
	struct sleepqueue *sq;
	struct thread *td, *besttd;
	int wakeup_swapper;

	CTR2(KTR_PROC, "sleepq_signal(%p, %d)", wchan, flags);
	KASSERT(wchan != NULL, ("%s: invalid NULL wait channel", __func__));
	MPASS((queue >= 0) && (queue < NR_SLEEPQS));
	sq = sleepq_lookup(wchan);
	if (sq == NULL)
		return (0);
	KASSERT(sq->sq_type == (flags & SLEEPQ_TYPE),
	    ("%s: mismatch between sleep/wakeup and cv_*", __func__));

	/*
	 * Find the highest priority thread on the queue.  If there is a
	 * tie, use the thread that first appears in the queue as it has
	 * been sleeping the longest since threads are always added to
	 * the tail of sleep queues.
	 */
	besttd = NULL;
	TAILQ_FOREACH(td, &sq->sq_blocked[queue], td_slpq) {
		if (besttd == NULL || td->td_priority < besttd->td_priority)
			besttd = td;
	}
	MPASS(besttd != NULL);
	thread_lock(besttd);
	wakeup_swapper = sleepq_resume_thread(sq, besttd, pri);
	thread_unlock(besttd);
	return (wakeup_swapper);
}

static bool
match_any(struct thread *td __unused)
{

	return (true);
}

/*
 * Resume all threads sleeping on a specified wait channel.
 */
int
sleepq_broadcast(void *wchan, int flags, int pri, int queue)
{
	struct sleepqueue *sq;

	CTR2(KTR_PROC, "sleepq_broadcast(%p, %d)", wchan, flags);
	KASSERT(wchan != NULL, ("%s: invalid NULL wait channel", __func__));
	MPASS((queue >= 0) && (queue < NR_SLEEPQS));
	sq = sleepq_lookup(wchan);
	if (sq == NULL)
		return (0);
	KASSERT(sq->sq_type == (flags & SLEEPQ_TYPE),
	    ("%s: mismatch between sleep/wakeup and cv_*", __func__));

	return (sleepq_remove_matching(sq, queue, match_any, pri));
}

/*
 * Resume threads on the sleep queue that match the given predicate.
 */
int
sleepq_remove_matching(struct sleepqueue *sq, int queue,
    bool (*matches)(struct thread *), int pri)
{
	struct thread *td, *tdn;
	int wakeup_swapper;

	/*
	 * The last thread will be given ownership of sq and may
	 * re-enqueue itself before sleepq_resume_thread() returns,
	 * so we must cache the "next" queue item at the beginning
	 * of the final iteration.
	 */
	wakeup_swapper = 0;
	TAILQ_FOREACH_SAFE(td, &sq->sq_blocked[queue], td_slpq, tdn) {
		thread_lock(td);
		if (matches(td))
			wakeup_swapper |= sleepq_resume_thread(sq, td, pri);
		thread_unlock(td);
	}

	return (wakeup_swapper);
}

/*
 * Time sleeping threads out.  When the timeout expires, the thread is
 * removed from the sleep queue and made runnable if it is still asleep.
 */
static void
sleepq_timeout(void *arg)
{
	struct sleepqueue_chain *sc;
	struct sleepqueue *sq;
	struct thread *td;
	void *wchan;
	int wakeup_swapper;

	td = arg;
	wakeup_swapper = 0;
	CTR3(KTR_PROC, "sleepq_timeout: thread %p (pid %ld, %s)",
	    (void *)td, (long)td->td_proc->p_pid, (void *)td->td_name);

	thread_lock(td);

	if (td->td_sleeptimo > sbinuptime() || td->td_sleeptimo == 0) {
		/*
		 * The thread does not want a timeout (yet).
		 */
	} else if (TD_IS_SLEEPING(td) && TD_ON_SLEEPQ(td)) {
		/*
		 * See if the thread is asleep and get the wait
		 * channel if it is.
		 */
		wchan = td->td_wchan;
		sc = SC_LOOKUP(wchan);
		THREAD_LOCKPTR_ASSERT(td, &sc->sc_lock);
		sq = sleepq_lookup(wchan);
		MPASS(sq != NULL);
		td->td_flags |= TDF_TIMEOUT;
		wakeup_swapper = sleepq_resume_thread(sq, td, 0);
	} else if (TD_ON_SLEEPQ(td)) {
		/*
		 * If the thread is on the SLEEPQ but isn't sleeping
		 * yet, it can either be on another CPU in between
		 * sleepq_add() and one of the sleepq_*wait*()
		 * routines or it can be in sleepq_catch_signals().
		 */
		td->td_flags |= TDF_TIMEOUT;
	}

	thread_unlock(td);
	if (wakeup_swapper)
		kick_proc0();
}

/*
 * Resumes a specific thread from the sleep queue associated with a specific
 * wait channel if it is on that queue.
 */
void
sleepq_remove(struct thread *td, void *wchan)
{
	struct sleepqueue *sq;
	int wakeup_swapper;

	/*
	 * Look up the sleep queue for this wait channel, then re-check
	 * that the thread is asleep on that channel, if it is not, then
	 * bail.
	 */
	MPASS(wchan != NULL);
	sleepq_lock(wchan);
	sq = sleepq_lookup(wchan);
	/*
	 * We can not lock the thread here as it may be sleeping on a
	 * different sleepq.  However, holding the sleepq lock for this
	 * wchan can guarantee that we do not miss a wakeup for this
	 * channel.  The asserts below will catch any false positives.
	 */
	if (!TD_ON_SLEEPQ(td) || td->td_wchan != wchan) {
		sleepq_release(wchan);
		return;
	}
	/* Thread is asleep on sleep queue sq, so wake it up. */
	thread_lock(td);
	MPASS(sq != NULL);
	MPASS(td->td_wchan == wchan);
	wakeup_swapper = sleepq_resume_thread(sq, td, 0);
	thread_unlock(td);
	sleepq_release(wchan);
	if (wakeup_swapper)
		kick_proc0();
}

/*
 * Abort a thread as if an interrupt had occurred.  Only abort
 * interruptible waits (unfortunately it isn't safe to abort others).
 */
int
sleepq_abort(struct thread *td, int intrval)
{
	struct sleepqueue *sq;
	void *wchan;

	THREAD_LOCK_ASSERT(td, MA_OWNED);
	MPASS(TD_ON_SLEEPQ(td));
	MPASS(td->td_flags & TDF_SINTR);
	MPASS(intrval == EINTR || intrval == ERESTART);

	/*
	 * If the TDF_TIMEOUT flag is set, just leave. A
	 * timeout is scheduled anyhow.
	 */
	if (td->td_flags & TDF_TIMEOUT)
		return (0);

	CTR3(KTR_PROC, "sleepq_abort: thread %p (pid %ld, %s)",
	    (void *)td, (long)td->td_proc->p_pid, (void *)td->td_name);
	td->td_intrval = intrval;
	td->td_flags |= TDF_SLEEPABORT;
	/*
	 * If the thread has not slept yet it will find the signal in
	 * sleepq_catch_signals() and call sleepq_resume_thread.  Otherwise
	 * we have to do it here.
	 */
	if (!TD_IS_SLEEPING(td))
		return (0);
	wchan = td->td_wchan;
	MPASS(wchan != NULL);
	sq = sleepq_lookup(wchan);
	MPASS(sq != NULL);

	/* Thread is asleep on sleep queue sq, so wake it up. */
	return (sleepq_resume_thread(sq, td, 0));
}

void
sleepq_chains_remove_matching(bool (*matches)(struct thread *))
{
	struct sleepqueue_chain *sc;
	struct sleepqueue *sq, *sq1;
	int i, wakeup_swapper;

	wakeup_swapper = 0;
	for (sc = &sleepq_chains[0]; sc < sleepq_chains + SC_TABLESIZE; ++sc) {
		if (LIST_EMPTY(&sc->sc_queues)) {
			continue;
		}
		mtx_lock_spin(&sc->sc_lock);
		LIST_FOREACH_SAFE(sq, &sc->sc_queues, sq_hash, sq1) {
			for (i = 0; i < NR_SLEEPQS; ++i) {
				wakeup_swapper |= sleepq_remove_matching(sq, i,
				    matches, 0);
			}
		}
		mtx_unlock_spin(&sc->sc_lock);
	}
	if (wakeup_swapper) {
		kick_proc0();
	}
}

/*
 * Prints the stacks of all threads presently sleeping on wchan/queue to
 * the sbuf sb.  Sets count_stacks_printed to the number of stacks actually
 * printed.  Typically, this will equal the number of threads sleeping on the
 * queue, but may be less if sb overflowed before all stacks were printed.
 */
#ifdef STACK
int
sleepq_sbuf_print_stacks(struct sbuf *sb, void *wchan, int queue,
    int *count_stacks_printed)
{
	struct thread *td, *td_next;
	struct sleepqueue *sq;
	struct stack **st;
	struct sbuf **td_infos;
	int i, stack_idx, error, stacks_to_allocate;
	bool finished, partial_print;

	error = 0;
	finished = false;
	partial_print = false;

	KASSERT(wchan != NULL, ("%s: invalid NULL wait channel", __func__));
	MPASS((queue >= 0) && (queue < NR_SLEEPQS));

	stacks_to_allocate = 10;
	for (i = 0; i < 3 && !finished ; i++) {
		/* We cannot malloc while holding the queue's spinlock, so
		 * we do our mallocs now, and hope it is enough.  If it
		 * isn't, we will free these, drop the lock, malloc more,
		 * and try again, up to a point.  After that point we will
		 * give up and report ENOMEM. We also cannot write to sb
		 * during this time since the client may have set the
		 * SBUF_AUTOEXTEND flag on their sbuf, which could cause a
		 * malloc as we print to it.  So we defer actually printing
		 * to sb until after we drop the spinlock.
		 */

		/* Where we will store the stacks. */
		st = malloc(sizeof(struct stack *) * stacks_to_allocate,
		    M_TEMP, M_WAITOK);
		for (stack_idx = 0; stack_idx < stacks_to_allocate;
		    stack_idx++)
			st[stack_idx] = stack_create();

		/* Where we will store the td name, tid, etc. */
		td_infos = malloc(sizeof(struct sbuf *) * stacks_to_allocate,
		    M_TEMP, M_WAITOK);
		for (stack_idx = 0; stack_idx < stacks_to_allocate;
		    stack_idx++)
			td_infos[stack_idx] = sbuf_new(NULL, NULL,
			    MAXCOMLEN + sizeof(struct thread *) * 2 + 40,
			    SBUF_FIXEDLEN);

		sleepq_lock(wchan);
		sq = sleepq_lookup(wchan);
		if (sq == NULL) {
			/* This sleepq does not exist; exit and return ENOENT. */
			error = ENOENT;
			finished = true;
			sleepq_release(wchan);
			goto loop_end;
		}

		stack_idx = 0;
		/* Save thread info */
		TAILQ_FOREACH_SAFE(td, &sq->sq_blocked[queue], td_slpq,
		    td_next) {
			if (stack_idx >= stacks_to_allocate)
				goto loop_end;

			/* Note the td_lock is equal to the sleepq_lock here. */
			stack_save_td(st[stack_idx], td);

			sbuf_printf(td_infos[stack_idx], "%d: %s %p",
			    td->td_tid, td->td_name, td);

			++stack_idx;
		}

		finished = true;
		sleepq_release(wchan);

		/* Print the stacks */
		for (i = 0; i < stack_idx; i++) {
			sbuf_finish(td_infos[i]);
			sbuf_printf(sb, "--- thread %s: ---\n", sbuf_data(td_infos[i]));
			stack_sbuf_print(sb, st[i]);
			sbuf_printf(sb, "\n");

			error = sbuf_error(sb);
			if (error == 0)
				*count_stacks_printed = stack_idx;
		}

loop_end:
		if (!finished)
			sleepq_release(wchan);
		for (stack_idx = 0; stack_idx < stacks_to_allocate;
		    stack_idx++)
			stack_destroy(st[stack_idx]);
		for (stack_idx = 0; stack_idx < stacks_to_allocate;
		    stack_idx++)
			sbuf_delete(td_infos[stack_idx]);
		free(st, M_TEMP);
		free(td_infos, M_TEMP);
		stacks_to_allocate *= 10;
	}

	if (!finished && error == 0)
		error = ENOMEM;

	return (error);
}
#endif

#ifdef SLEEPQUEUE_PROFILING
#define	SLEEPQ_PROF_LOCATIONS	1024
#define	SLEEPQ_SBUFSIZE		512
struct sleepq_prof {
	LIST_ENTRY(sleepq_prof) sp_link;
	const char	*sp_wmesg;
	long		sp_count;
};

LIST_HEAD(sqphead, sleepq_prof);

struct sqphead sleepq_prof_free;
struct sqphead sleepq_hash[SC_TABLESIZE];
static struct sleepq_prof sleepq_profent[SLEEPQ_PROF_LOCATIONS];
static struct mtx sleepq_prof_lock;
MTX_SYSINIT(sleepq_prof_lock, &sleepq_prof_lock, "sleepq_prof", MTX_SPIN);

static void
sleepq_profile(const char *wmesg)
{
	struct sleepq_prof *sp;

	mtx_lock_spin(&sleepq_prof_lock);
	if (prof_enabled == 0)
		goto unlock;
	LIST_FOREACH(sp, &sleepq_hash[SC_HASH(wmesg)], sp_link)
		if (sp->sp_wmesg == wmesg)
			goto done;
	sp = LIST_FIRST(&sleepq_prof_free);
	if (sp == NULL)
		goto unlock;
	sp->sp_wmesg = wmesg;
	LIST_REMOVE(sp, sp_link);
	LIST_INSERT_HEAD(&sleepq_hash[SC_HASH(wmesg)], sp, sp_link);
done:
	sp->sp_count++;
unlock:
	mtx_unlock_spin(&sleepq_prof_lock);
	return;
}

static void
sleepq_prof_reset(void)
{
	struct sleepq_prof *sp;
	int enabled;
	int i;

	mtx_lock_spin(&sleepq_prof_lock);
	enabled = prof_enabled;
	prof_enabled = 0;
	for (i = 0; i < SC_TABLESIZE; i++)
		LIST_INIT(&sleepq_hash[i]);
	LIST_INIT(&sleepq_prof_free);
	for (i = 0; i < SLEEPQ_PROF_LOCATIONS; i++) {
		sp = &sleepq_profent[i];
		sp->sp_wmesg = NULL;
		sp->sp_count = 0;
		LIST_INSERT_HEAD(&sleepq_prof_free, sp, sp_link);
	}
	prof_enabled = enabled;
	mtx_unlock_spin(&sleepq_prof_lock);
}

static int
enable_sleepq_prof(SYSCTL_HANDLER_ARGS)
{
	int error, v;

	v = prof_enabled;
	error = sysctl_handle_int(oidp, &v, v, req);
	if (error)
		return (error);
	if (req->newptr == NULL)
		return (error);
	if (v == prof_enabled)
		return (0);
	if (v == 1)
		sleepq_prof_reset();
	mtx_lock_spin(&sleepq_prof_lock);
	prof_enabled = !!v;
	mtx_unlock_spin(&sleepq_prof_lock);

	return (0);
}

static int
reset_sleepq_prof_stats(SYSCTL_HANDLER_ARGS)
{
	int error, v;

	v = 0;
	error = sysctl_handle_int(oidp, &v, 0, req);
	if (error)
		return (error);
	if (req->newptr == NULL)
		return (error);
	if (v == 0)
		return (0);
	sleepq_prof_reset();

	return (0);
}

static int
dump_sleepq_prof_stats(SYSCTL_HANDLER_ARGS)
{
	struct sleepq_prof *sp;
	struct sbuf *sb;
	int enabled;
	int error;
	int i;

	error = sysctl_wire_old_buffer(req, 0);
	if (error != 0)
		return (error);
	sb = sbuf_new_for_sysctl(NULL, NULL, SLEEPQ_SBUFSIZE, req);
	sbuf_printf(sb, "\nwmesg\tcount\n");
	enabled = prof_enabled;
	mtx_lock_spin(&sleepq_prof_lock);
	prof_enabled = 0;
	mtx_unlock_spin(&sleepq_prof_lock);
	for (i = 0; i < SC_TABLESIZE; i++) {
		LIST_FOREACH(sp, &sleepq_hash[i], sp_link) {
			sbuf_printf(sb, "%s\t%ld\n",
			    sp->sp_wmesg, sp->sp_count);
		}
	}
	mtx_lock_spin(&sleepq_prof_lock);
	prof_enabled = enabled;
	mtx_unlock_spin(&sleepq_prof_lock);

	error = sbuf_finish(sb);
	sbuf_delete(sb);
	return (error);
}

SYSCTL_PROC(_debug_sleepq, OID_AUTO, stats, CTLTYPE_STRING | CTLFLAG_RD,
    NULL, 0, dump_sleepq_prof_stats, "A", "Sleepqueue profiling statistics");
SYSCTL_PROC(_debug_sleepq, OID_AUTO, reset, CTLTYPE_INT | CTLFLAG_RW,
    NULL, 0, reset_sleepq_prof_stats, "I",
    "Reset sleepqueue profiling statistics");
SYSCTL_PROC(_debug_sleepq, OID_AUTO, enable, CTLTYPE_INT | CTLFLAG_RW,
    NULL, 0, enable_sleepq_prof, "I", "Enable sleepqueue profiling");
#endif

#ifdef DDB
DB_SHOW_COMMAND(sleepq, db_show_sleepqueue)
{
	struct sleepqueue_chain *sc;
	struct sleepqueue *sq;
#ifdef INVARIANTS
	struct lock_object *lock;
#endif
	struct thread *td;
	void *wchan;
	int i;

	if (!have_addr)
		return;

	/*
	 * First, see if there is an active sleep queue for the wait channel
	 * indicated by the address.
	 */
	wchan = (void *)addr;
	sc = SC_LOOKUP(wchan);
	LIST_FOREACH(sq, &sc->sc_queues, sq_hash)
		if (sq->sq_wchan == wchan)
			goto found;

	/*
	 * Second, see if there is an active sleep queue at the address
	 * indicated.
	 */
	for (i = 0; i < SC_TABLESIZE; i++)
		LIST_FOREACH(sq, &sleepq_chains[i].sc_queues, sq_hash) {
			if (sq == (struct sleepqueue *)addr)
				goto found;
		}

	db_printf("Unable to locate a sleep queue via %p\n", (void *)addr);
	return;
found:
	db_printf("Wait channel: %p\n", sq->sq_wchan);
	db_printf("Queue type: %d\n", sq->sq_type);
#ifdef INVARIANTS
	if (sq->sq_lock) {
		lock = sq->sq_lock;
		db_printf("Associated Interlock: %p - (%s) %s\n", lock,
		    LOCK_CLASS(lock)->lc_name, lock->lo_name);
	}
#endif
	db_printf("Blocked threads:\n");
	for (i = 0; i < NR_SLEEPQS; i++) {
		db_printf("\nQueue[%d]:\n", i);
		if (TAILQ_EMPTY(&sq->sq_blocked[i]))
			db_printf("\tempty\n");
		else
			TAILQ_FOREACH(td, &sq->sq_blocked[i],
				      td_slpq) {
				db_printf("\t%p (tid %d, pid %d, \"%s\")\n", td,
					  td->td_tid, td->td_proc->p_pid,
					  td->td_name);
			}
		db_printf("(expected: %u)\n", sq->sq_blockedcnt[i]);
	}
}

/* Alias 'show sleepqueue' to 'show sleepq'. */
DB_SHOW_ALIAS(sleepqueue, db_show_sleepqueue);
#endif