kern_sx.c

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/*-
 * Copyright (c) 2007 Attilio Rao <attilio@freebsd.org>
 * Copyright (c) 2001 Jason Evans <jasone@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(s), this list of conditions and the following disclaimer as
 *    the first lines of this file unmodified other than the possible
 *    addition of one or more copyright notices.
 * 2. Redistributions in binary form must reproduce the above copyright
 *    notice(s), 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 COPYRIGHT HOLDER(S) ``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 COPYRIGHT HOLDER(S) 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.
 */

/*
 * Shared/exclusive locks.  This implementation attempts to ensure
 * deterministic lock granting behavior, so that slocks and xlocks are
 * interleaved.
 *
 * Priority propagation will not generally raise the priority of lock holders,
 * so should not be relied upon in combination with sx locks.
 */

#include "opt_ddb.h"
#include "opt_hwpmc_hooks.h"
#include "opt_kdtrace.h"
#include "opt_no_adaptive_sx.h"

#include <sys/cdefs.h>
__FBSDID("$BSDSUniX$");

#include <sys/param.h>
#include <sys/systm.h>
#include <sys/kdb.h>
#include <sys/ktr.h>
#include <sys/lock.h>
#include <sys/mutex.h>
#include <sys/proc.h>
#include <sys/sleepqueue.h>
#include <sys/sx.h>
#include <sys/sysctl.h>

#if defined(SMP) && !defined(NO_ADAPTIVE_SX)
#include <machine/cpu.h>
#endif

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

#if defined(SMP) && !defined(NO_ADAPTIVE_SX)
#define ADAPTIVE_SX
#endif

CTASSERT((SX_NOADAPTIVE & LO_CLASSFLAGS) == SX_NOADAPTIVE);

#ifdef HWPMC_HOOKS
#include <sys/pmckern.h>
PMC_SOFT_DECLARE( , , lock, failed);
#endif

/* Handy macros for sleep queues. */
#define SQ_EXCLUSIVE_QUEUE      0
#define SQ_SHARED_QUEUE         1

#ifdef ADAPTIVE_SX
#define ASX_RETRIES             10
#define ASX_LOOPS               10000
#endif

/*
 * Variations on DROP_GIANT()/PICKUP_GIANT() for use in this file.  We
 * drop Giant anytime we have to sleep or if we adaptively spin.
 */
#define GIANT_DECLARE                                                   \
        int _giantcnt = 0;                                              \
        WITNESS_SAVE_DECL(Giant)                                        \

#define GIANT_SAVE() do {                                               \
        if (mtx_owned(&Giant)) {                                        \
                WITNESS_SAVE(&Giant.lock_object, Giant);                \
                while (mtx_owned(&Giant)) {                             \
                        _giantcnt++;                                    \
                        mtx_unlock(&Giant);                             \
                }                                                       \
        }                                                               \
} while (0)

#define GIANT_RESTORE() do {                                            \
        if (_giantcnt > 0) {                                            \
                mtx_assert(&Giant, MA_NOTOWNED);                        \
                while (_giantcnt--)                                     \
                        mtx_lock(&Giant);                               \
                WITNESS_RESTORE(&Giant.lock_object, Giant);             \
        }                                                               \
} while (0)

/*
 * Returns true if an exclusive lock is recursed.  It assumes
 * curthread currently has an exclusive lock.
 */
#define sx_recurse              lock_object.lo_data
#define sx_recursed(sx)         ((sx)->sx_recurse != 0)

static void     assert_sx(struct lock_object *lock, int what);
#ifdef DDB
static void     db_show_sx(struct lock_object *lock);
#endif
static void     lock_sx(struct lock_object *lock, int how);
#ifdef KDTRACE_HOOKS
static int      owner_sx(struct lock_object *lock, struct thread **owner);
#endif
static int      unlock_sx(struct lock_object *lock);

struct lock_class lock_class_sx = {
        .lc_name = "sx",
        .lc_flags = LC_SLEEPLOCK | LC_SLEEPABLE | LC_RECURSABLE | LC_UPGRADABLE,
        .lc_assert = assert_sx,
#ifdef DDB
        .lc_ddb_show = db_show_sx,
#endif
        .lc_lock = lock_sx,
        .lc_unlock = unlock_sx,
#ifdef KDTRACE_HOOKS
        .lc_owner = owner_sx,
#endif
};

#ifndef INVARIANTS
#define _sx_assert(sx, what, file, line)
#endif

void
assert_sx(struct lock_object *lock, int what)
{

        sx_assert((struct sx *)lock, what);
}

void
lock_sx(struct lock_object *lock, int how)
{
        struct sx *sx;

        sx = (struct sx *)lock;
        if (how)
                sx_xlock(sx);
        else
                sx_slock(sx);
}

int
unlock_sx(struct lock_object *lock)
{
        struct sx *sx;

        sx = (struct sx *)lock;
        sx_assert(sx, SA_LOCKED | SA_NOTRECURSED);
        if (sx_xlocked(sx)) {
                sx_xunlock(sx);
                return (1);
        } else {
                sx_sunlock(sx);
                return (0);
        }
}

#ifdef KDTRACE_HOOKS
int
owner_sx(struct lock_object *lock, struct thread **owner)
{
        struct sx *sx = (struct sx *)lock;
        uintptr_t x = sx->sx_lock;

        *owner = (struct thread *)SX_OWNER(x);
        return ((x & SX_LOCK_SHARED) != 0 ? (SX_SHARERS(x) != 0) :
            (*owner != NULL));
}
#endif

void
sx_sysinit(void *arg)
{
        struct sx_args *sargs = arg;

        sx_init_flags(sargs->sa_sx, sargs->sa_desc, sargs->sa_flags);
}

void
sx_init_flags(struct sx *sx, const char *description, int opts)
{
        int flags;

        MPASS((opts & ~(SX_QUIET | SX_RECURSE | SX_NOWITNESS | SX_DUPOK |
            SX_NOPROFILE | SX_NOADAPTIVE)) == 0);
        ASSERT_ATOMIC_LOAD_PTR(sx->sx_lock,
            ("%s: sx_lock not aligned for %s: %p", __func__, description,
            &sx->sx_lock));

        flags = LO_SLEEPABLE | LO_UPGRADABLE;
        if (opts & SX_DUPOK)
                flags |= LO_DUPOK;
        if (opts & SX_NOPROFILE)
                flags |= LO_NOPROFILE;
        if (!(opts & SX_NOWITNESS))
                flags |= LO_WITNESS;
        if (opts & SX_RECURSE)
                flags |= LO_RECURSABLE;
        if (opts & SX_QUIET)
                flags |= LO_QUIET;

        flags |= opts & SX_NOADAPTIVE;
        sx->sx_lock = SX_LOCK_UNLOCKED;
        sx->sx_recurse = 0;
        lock_init(&sx->lock_object, &lock_class_sx, description, NULL, flags);
}

void
sx_destroy(struct sx *sx)
{

        KASSERT(sx->sx_lock == SX_LOCK_UNLOCKED, ("sx lock still held"));
        KASSERT(sx->sx_recurse == 0, ("sx lock still recursed"));
        sx->sx_lock = SX_LOCK_DESTROYED;
        lock_destroy(&sx->lock_object);
}

int
_sx_slock(struct sx *sx, int opts, const char *file, int line)
{
        int error = 0;

        if (SCHEDULER_STOPPED())
                return (0);
        KASSERT(kdb_active != 0 || !TD_IS_IDLETHREAD(curthread),
            ("sx_slock() by idle thread %p on sx %s @ %s:%d",
            curthread, sx->lock_object.lo_name, file, line));
        KASSERT(sx->sx_lock != SX_LOCK_DESTROYED,
            ("sx_slock() of destroyed sx @ %s:%d", file, line));
        WITNESS_CHECKORDER(&sx->lock_object, LOP_NEWORDER, file, line, NULL);
        error = __sx_slock(sx, opts, file, line);
        if (!error) {
                LOCK_LOG_LOCK("SLOCK", &sx->lock_object, 0, 0, file, line);
                WITNESS_LOCK(&sx->lock_object, 0, file, line);
                curthread->td_locks++;
        }

        return (error);
}

int
_sx_try_slock(struct sx *sx, const char *file, int line)
{
        uintptr_t x;

        if (SCHEDULER_STOPPED())
                return (1);

        KASSERT(kdb_active != 0 || !TD_IS_IDLETHREAD(curthread),
            ("sx_try_slock() by idle thread %p on sx %s @ %s:%d",
            curthread, sx->lock_object.lo_name, file, line));

        for (;;) {
                x = sx->sx_lock;
                KASSERT(x != SX_LOCK_DESTROYED,
                    ("sx_try_slock() of destroyed sx @ %s:%d", file, line));
                if (!(x & SX_LOCK_SHARED))
                        break;
                if (atomic_cmpset_acq_ptr(&sx->sx_lock, x, x + SX_ONE_SHARER)) {
                        LOCK_LOG_TRY("SLOCK", &sx->lock_object, 0, 1, file, line);
                        WITNESS_LOCK(&sx->lock_object, LOP_TRYLOCK, file, line);
                        LOCKSTAT_PROFILE_OBTAIN_LOCK_SUCCESS(LS_SX_SLOCK_ACQUIRE,
                            sx, 0, 0, file, line);
                        curthread->td_locks++;
                        return (1);
                }
        }

        LOCK_LOG_TRY("SLOCK", &sx->lock_object, 0, 0, file, line);
        return (0);
}

int
_sx_xlock(struct sx *sx, int opts, const char *file, int line)
{
        int error = 0;

        if (SCHEDULER_STOPPED())
                return (0);
        KASSERT(kdb_active != 0 || !TD_IS_IDLETHREAD(curthread),
            ("sx_xlock() by idle thread %p on sx %s @ %s:%d",
            curthread, sx->lock_object.lo_name, file, line));
        KASSERT(sx->sx_lock != SX_LOCK_DESTROYED,
            ("sx_xlock() of destroyed sx @ %s:%d", file, line));
        WITNESS_CHECKORDER(&sx->lock_object, LOP_NEWORDER | LOP_EXCLUSIVE, file,
            line, NULL);
        error = __sx_xlock(sx, curthread, opts, file, line);
        if (!error) {
                LOCK_LOG_LOCK("XLOCK", &sx->lock_object, 0, sx->sx_recurse,
                    file, line);
                WITNESS_LOCK(&sx->lock_object, LOP_EXCLUSIVE, file, line);
                curthread->td_locks++;
        }

        return (error);
}

int
_sx_try_xlock(struct sx *sx, const char *file, int line)
{
        int rval;

        if (SCHEDULER_STOPPED())
                return (1);

        KASSERT(kdb_active != 0 || !TD_IS_IDLETHREAD(curthread),
            ("sx_try_xlock() by idle thread %p on sx %s @ %s:%d",
            curthread, sx->lock_object.lo_name, file, line));
        KASSERT(sx->sx_lock != SX_LOCK_DESTROYED,
            ("sx_try_xlock() of destroyed sx @ %s:%d", file, line));

        if (sx_xlocked(sx) &&
            (sx->lock_object.lo_flags & LO_RECURSABLE) != 0) {
                sx->sx_recurse++;
                atomic_set_ptr(&sx->sx_lock, SX_LOCK_RECURSED);
                rval = 1;
        } else
                rval = atomic_cmpset_acq_ptr(&sx->sx_lock, SX_LOCK_UNLOCKED,
                    (uintptr_t)curthread);
        LOCK_LOG_TRY("XLOCK", &sx->lock_object, 0, rval, file, line);
        if (rval) {
                WITNESS_LOCK(&sx->lock_object, LOP_EXCLUSIVE | LOP_TRYLOCK,
                    file, line);
                if (!sx_recursed(sx))
                        LOCKSTAT_PROFILE_OBTAIN_LOCK_SUCCESS(LS_SX_XLOCK_ACQUIRE,
                            sx, 0, 0, file, line);
                curthread->td_locks++;
        }

        return (rval);
}

void
_sx_sunlock(struct sx *sx, const char *file, int line)
{

        if (SCHEDULER_STOPPED())
                return;
        KASSERT(sx->sx_lock != SX_LOCK_DESTROYED,
            ("sx_sunlock() of destroyed sx @ %s:%d", file, line));
        _sx_assert(sx, SA_SLOCKED, file, line);
        curthread->td_locks--;
        WITNESS_UNLOCK(&sx->lock_object, 0, file, line);
        LOCK_LOG_LOCK("SUNLOCK", &sx->lock_object, 0, 0, file, line);
        __sx_sunlock(sx, file, line);
        LOCKSTAT_PROFILE_RELEASE_LOCK(LS_SX_SUNLOCK_RELEASE, sx);
}

void
_sx_xunlock(struct sx *sx, const char *file, int line)
{

        if (SCHEDULER_STOPPED())
                return;
        KASSERT(sx->sx_lock != SX_LOCK_DESTROYED,
            ("sx_xunlock() of destroyed sx @ %s:%d", file, line));
        _sx_assert(sx, SA_XLOCKED, file, line);
        curthread->td_locks--;
        WITNESS_UNLOCK(&sx->lock_object, LOP_EXCLUSIVE, file, line);
        LOCK_LOG_LOCK("XUNLOCK", &sx->lock_object, 0, sx->sx_recurse, file,
            line);
        if (!sx_recursed(sx))
                LOCKSTAT_PROFILE_RELEASE_LOCK(LS_SX_XUNLOCK_RELEASE, sx);
        __sx_xunlock(sx, curthread, file, line);
}

/*
 * Try to do a non-blocking upgrade from a shared lock to an exclusive lock.
 * This will only succeed if this thread holds a single shared lock.
 * Return 1 if if the upgrade succeed, 0 otherwise.
 */
int
_sx_try_upgrade(struct sx *sx, const char *file, int line)
{
        uintptr_t x;
        int success;

        if (SCHEDULER_STOPPED())
                return (1);

        KASSERT(sx->sx_lock != SX_LOCK_DESTROYED,
            ("sx_try_upgrade() of destroyed sx @ %s:%d", file, line));
        _sx_assert(sx, SA_SLOCKED, file, line);

        /*
         * Try to switch from one shared lock to an exclusive lock.  We need
         * to maintain the SX_LOCK_EXCLUSIVE_WAITERS flag if set so that
         * we will wake up the exclusive waiters when we drop the lock.
         */
        x = sx->sx_lock & SX_LOCK_EXCLUSIVE_WAITERS;
        success = atomic_cmpset_ptr(&sx->sx_lock, SX_SHARERS_LOCK(1) | x,
            (uintptr_t)curthread | x);
        LOCK_LOG_TRY("XUPGRADE", &sx->lock_object, 0, success, file, line);
        if (success) {
                WITNESS_UPGRADE(&sx->lock_object, LOP_EXCLUSIVE | LOP_TRYLOCK,
                    file, line);
                LOCKSTAT_RECORD0(LS_SX_TRYUPGRADE_UPGRADE, sx);
        }
        return (success);
}

/*
 * Downgrade an unrecursed exclusive lock into a single shared lock.
 */
void
_sx_downgrade(struct sx *sx, const char *file, int line)
{
        uintptr_t x;
        int wakeup_swapper;

        if (SCHEDULER_STOPPED())
                return;

        KASSERT(sx->sx_lock != SX_LOCK_DESTROYED,
            ("sx_downgrade() of destroyed sx @ %s:%d", file, line));
        _sx_assert(sx, SA_XLOCKED | SA_NOTRECURSED, file, line);
#ifndef INVARIANTS
        if (sx_recursed(sx))
                panic("downgrade of a recursed lock");
#endif

        WITNESS_DOWNGRADE(&sx->lock_object, 0, file, line);

        /*
         * Try to switch from an exclusive lock with no shared waiters
         * to one sharer with no shared waiters.  If there are
         * exclusive waiters, we don't need to lock the sleep queue so
         * long as we preserve the flag.  We do one quick try and if
         * that fails we grab the sleepq lock to keep the flags from
         * changing and do it the slow way.
         *
         * We have to lock the sleep queue if there are shared waiters
         * so we can wake them up.
         */
        x = sx->sx_lock;
        if (!(x & SX_LOCK_SHARED_WAITERS) &&
            atomic_cmpset_rel_ptr(&sx->sx_lock, x, SX_SHARERS_LOCK(1) |
            (x & SX_LOCK_EXCLUSIVE_WAITERS))) {
                LOCK_LOG_LOCK("XDOWNGRADE", &sx->lock_object, 0, 0, file, line);
                return;
        }

        /*
         * Lock the sleep queue so we can read the waiters bits
         * without any races and wakeup any shared waiters.
         */
        sleepq_lock(&sx->lock_object);

        /*
         * Preserve SX_LOCK_EXCLUSIVE_WAITERS while downgraded to a single
         * shared lock.  If there are any shared waiters, wake them up.
         */
        wakeup_swapper = 0;
        x = sx->sx_lock;
        atomic_store_rel_ptr(&sx->sx_lock, SX_SHARERS_LOCK(1) |
            (x & SX_LOCK_EXCLUSIVE_WAITERS));
        if (x & SX_LOCK_SHARED_WAITERS)
                wakeup_swapper = sleepq_broadcast(&sx->lock_object, SLEEPQ_SX,
                    0, SQ_SHARED_QUEUE);
        sleepq_release(&sx->lock_object);

        LOCK_LOG_LOCK("XDOWNGRADE", &sx->lock_object, 0, 0, file, line);
        LOCKSTAT_RECORD0(LS_SX_DOWNGRADE_DOWNGRADE, sx);

        if (wakeup_swapper)
                kick_proc0();
}

/*
 * This function represents the so-called 'hard case' for sx_xlock
 * operation.  All 'easy case' failures are redirected to this.  Note
 * that ideally this would be a static function, but it needs to be
 * accessible from at least sx.h.
 */
int
_sx_xlock_hard(struct sx *sx, uintptr_t tid, int opts, const char *file,
    int line)
{
        GIANT_DECLARE;
#ifdef ADAPTIVE_SX
        volatile struct thread *owner;
        u_int i, spintries = 0;
#endif
        uintptr_t x;
#ifdef LOCK_PROFILING
        uint64_t waittime = 0;
        int contested = 0;
#endif
        int error = 0;
#ifdef  KDTRACE_HOOKS
        uintptr_t state;
        uint64_t spin_cnt = 0;
        uint64_t sleep_cnt = 0;
        int64_t sleep_time = 0;
        int64_t all_time = 0;
#endif

        if (SCHEDULER_STOPPED())
                return (0);

        /* If we already hold an exclusive lock, then recurse. */
        if (sx_xlocked(sx)) {
                KASSERT((sx->lock_object.lo_flags & LO_RECURSABLE) != 0,
            ("_sx_xlock_hard: recursed on non-recursive sx %s @ %s:%d\n",
                    sx->lock_object.lo_name, file, line));
                sx->sx_recurse++;
                atomic_set_ptr(&sx->sx_lock, SX_LOCK_RECURSED);
                if (LOCK_LOG_TEST(&sx->lock_object, 0))
                        CTR2(KTR_LOCK, "%s: %p recursing", __func__, sx);
                return (0);
        }

        if (LOCK_LOG_TEST(&sx->lock_object, 0))
                CTR5(KTR_LOCK, "%s: %s contested (lock=%p) at %s:%d", __func__,
                    sx->lock_object.lo_name, (void *)sx->sx_lock, file, line);

#ifdef KDTRACE_HOOKS
        all_time -= lockstat_nsecs(&sx->lock_object);
        state = sx->sx_lock;
#endif
        while (!atomic_cmpset_acq_ptr(&sx->sx_lock, SX_LOCK_UNLOCKED, tid)) {
#ifdef KDTRACE_HOOKS
                spin_cnt++;
#endif
#ifdef HWPMC_HOOKS
                PMC_SOFT_CALL( , , lock, failed);
#endif
                lock_profile_obtain_lock_failed(&sx->lock_object, &contested,
                    &waittime);
#ifdef ADAPTIVE_SX
                /*
                 * If the lock is write locked and the owner is
                 * running on another CPU, spin until the owner stops
                 * running or the state of the lock changes.
                 */
                x = sx->sx_lock;
                if ((sx->lock_object.lo_flags & SX_NOADAPTIVE) == 0) {
                        if ((x & SX_LOCK_SHARED) == 0) {
                                x = SX_OWNER(x);
                                owner = (struct thread *)x;
                                if (TD_IS_RUNNING(owner)) {
                                        if (LOCK_LOG_TEST(&sx->lock_object, 0))
                                                CTR3(KTR_LOCK,
                                            "%s: spinning on %p held by %p",
                                                    __func__, sx, owner);
                                        GIANT_SAVE();
                                        while (SX_OWNER(sx->sx_lock) == x &&
                                            TD_IS_RUNNING(owner)) {
                                                cpu_spinwait();
#ifdef KDTRACE_HOOKS
                                                spin_cnt++;
#endif
                                        }
                                        continue;
                                }
                        } else if (SX_SHARERS(x) && spintries < ASX_RETRIES) {
                                GIANT_SAVE();
                                spintries++;
                                for (i = 0; i < ASX_LOOPS; i++) {
                                        if (LOCK_LOG_TEST(&sx->lock_object, 0))
                                                CTR4(KTR_LOCK,
                                    "%s: shared spinning on %p with %u and %u",
                                                    __func__, sx, spintries, i);
                                        x = sx->sx_lock;
                                        if ((x & SX_LOCK_SHARED) == 0 ||
                                            SX_SHARERS(x) == 0)
                                                break;
                                        cpu_spinwait();
#ifdef KDTRACE_HOOKS
                                        spin_cnt++;
#endif
                                }
                                if (i != ASX_LOOPS)
                                        continue;
                        }
                }
#endif

                sleepq_lock(&sx->lock_object);
                x = sx->sx_lock;

                /*
                 * If the lock was released while spinning on the
                 * sleep queue chain lock, try again.
                 */
                if (x == SX_LOCK_UNLOCKED) {
                        sleepq_release(&sx->lock_object);
                        continue;
                }

#ifdef ADAPTIVE_SX
                /*
                 * The current lock owner might have started executing
                 * on another CPU (or the lock could have changed
                 * owners) while we were waiting on the sleep queue
                 * chain lock.  If so, drop the sleep queue lock and try
                 * again.
                 */
                if (!(x & SX_LOCK_SHARED) &&
                    (sx->lock_object.lo_flags & SX_NOADAPTIVE) == 0) {
                        owner = (struct thread *)SX_OWNER(x);
                        if (TD_IS_RUNNING(owner)) {
                                sleepq_release(&sx->lock_object);
                                continue;
                        }
                }
#endif

                /*
                 * If an exclusive lock was released with both shared
                 * and exclusive waiters and a shared waiter hasn't
                 * woken up and acquired the lock yet, sx_lock will be
                 * set to SX_LOCK_UNLOCKED | SX_LOCK_EXCLUSIVE_WAITERS.
                 * If we see that value, try to acquire it once.  Note
                 * that we have to preserve SX_LOCK_EXCLUSIVE_WAITERS
                 * as there are other exclusive waiters still.  If we
                 * fail, restart the loop.
                 */
                if (x == (SX_LOCK_UNLOCKED | SX_LOCK_EXCLUSIVE_WAITERS)) {
                        if (atomic_cmpset_acq_ptr(&sx->sx_lock,
                            SX_LOCK_UNLOCKED | SX_LOCK_EXCLUSIVE_WAITERS,
                            tid | SX_LOCK_EXCLUSIVE_WAITERS)) {
                                sleepq_release(&sx->lock_object);
                                CTR2(KTR_LOCK, "%s: %p claimed by new writer",
                                    __func__, sx);
                                break;
                        }
                        sleepq_release(&sx->lock_object);
                        continue;
                }

                /*
                 * Try to set the SX_LOCK_EXCLUSIVE_WAITERS.  If we fail,
                 * than loop back and retry.
                 */
                if (!(x & SX_LOCK_EXCLUSIVE_WAITERS)) {
                        if (!atomic_cmpset_ptr(&sx->sx_lock, x,
                            x | SX_LOCK_EXCLUSIVE_WAITERS)) {
                                sleepq_release(&sx->lock_object);
                                continue;
                        }
                        if (LOCK_LOG_TEST(&sx->lock_object, 0))
                                CTR2(KTR_LOCK, "%s: %p set excl waiters flag",
                                    __func__, sx);
                }

                /*
                 * Since we have been unable to acquire the exclusive
                 * lock and the exclusive waiters flag is set, we have
                 * to sleep.
                 */
                if (LOCK_LOG_TEST(&sx->lock_object, 0))
                        CTR2(KTR_LOCK, "%s: %p blocking on sleep queue",
                            __func__, sx);

#ifdef KDTRACE_HOOKS
                sleep_time -= lockstat_nsecs(&sx->lock_object);
#endif
                GIANT_SAVE();
                sleepq_add(&sx->lock_object, NULL, sx->lock_object.lo_name,
                    SLEEPQ_SX | ((opts & SX_INTERRUPTIBLE) ?
                    SLEEPQ_INTERRUPTIBLE : 0), SQ_EXCLUSIVE_QUEUE);
                if (!(opts & SX_INTERRUPTIBLE))
                        sleepq_wait(&sx->lock_object, 0);
                else
                        error = sleepq_wait_sig(&sx->lock_object, 0);
#ifdef KDTRACE_HOOKS
                sleep_time += lockstat_nsecs(&sx->lock_object);
                sleep_cnt++;
#endif
                if (error) {
                        if (LOCK_LOG_TEST(&sx->lock_object, 0))
                                CTR2(KTR_LOCK,
                        "%s: interruptible sleep by %p suspended by signal",
                                    __func__, sx);
                        break;
                }
                if (LOCK_LOG_TEST(&sx->lock_object, 0))
                        CTR2(KTR_LOCK, "%s: %p resuming from sleep queue",
                            __func__, sx);
        }
#ifdef KDTRACE_HOOKS
        all_time += lockstat_nsecs(&sx->lock_object);
        if (sleep_time)
                LOCKSTAT_RECORD4(LS_SX_XLOCK_BLOCK, sx, sleep_time,
                    LOCKSTAT_WRITER, (state & SX_LOCK_SHARED) == 0,
                    (state & SX_LOCK_SHARED) == 0 ? 0 : SX_SHARERS(state));
        if (spin_cnt > sleep_cnt)
                LOCKSTAT_RECORD4(LS_SX_XLOCK_SPIN, sx, all_time - sleep_time,
                    LOCKSTAT_WRITER, (state & SX_LOCK_SHARED) == 0,
                    (state & SX_LOCK_SHARED) == 0 ? 0 : SX_SHARERS(state));
#endif
        if (!error)
                LOCKSTAT_PROFILE_OBTAIN_LOCK_SUCCESS(LS_SX_XLOCK_ACQUIRE, sx,
                    contested, waittime, file, line);
        GIANT_RESTORE();
        return (error);
}

/*
 * This function represents the so-called 'hard case' for sx_xunlock
 * operation.  All 'easy case' failures are redirected to this.  Note
 * that ideally this would be a static function, but it needs to be
 * accessible from at least sx.h.
 */
void
_sx_xunlock_hard(struct sx *sx, uintptr_t tid, const char *file, int line)
{
        uintptr_t x;
        int queue, wakeup_swapper;

        if (SCHEDULER_STOPPED())
                return;

        MPASS(!(sx->sx_lock & SX_LOCK_SHARED));

        /* If the lock is recursed, then unrecurse one level. */
        if (sx_xlocked(sx) && sx_recursed(sx)) {
                if ((--sx->sx_recurse) == 0)
                        atomic_clear_ptr(&sx->sx_lock, SX_LOCK_RECURSED);
                if (LOCK_LOG_TEST(&sx->lock_object, 0))
                        CTR2(KTR_LOCK, "%s: %p unrecursing", __func__, sx);
                return;
        }
        MPASS(sx->sx_lock & (SX_LOCK_SHARED_WAITERS |
            SX_LOCK_EXCLUSIVE_WAITERS));
        if (LOCK_LOG_TEST(&sx->lock_object, 0))
                CTR2(KTR_LOCK, "%s: %p contested", __func__, sx);

        sleepq_lock(&sx->lock_object);
        x = SX_LOCK_UNLOCKED;

        /*
         * The wake up algorithm here is quite simple and probably not
         * ideal.  It gives precedence to shared waiters if they are
         * present.  For this condition, we have to preserve the
         * state of the exclusive waiters flag.
         * If interruptible sleeps left the shared queue empty avoid a
         * starvation for the threads sleeping on the exclusive queue by giving
         * them precedence and cleaning up the shared waiters bit anyway.
         */
        if ((sx->sx_lock & SX_LOCK_SHARED_WAITERS) != 0 &&
            sleepq_sleepcnt(&sx->lock_object, SQ_SHARED_QUEUE) != 0) {
                queue = SQ_SHARED_QUEUE;
                x |= (sx->sx_lock & SX_LOCK_EXCLUSIVE_WAITERS);
        } else
                queue = SQ_EXCLUSIVE_QUEUE;

        /* Wake up all the waiters for the specific queue. */
        if (LOCK_LOG_TEST(&sx->lock_object, 0))
                CTR3(KTR_LOCK, "%s: %p waking up all threads on %s queue",
                    __func__, sx, queue == SQ_SHARED_QUEUE ? "shared" :
                    "exclusive");
        atomic_store_rel_ptr(&sx->sx_lock, x);
        wakeup_swapper = sleepq_broadcast(&sx->lock_object, SLEEPQ_SX, 0,
            queue);
        sleepq_release(&sx->lock_object);
        if (wakeup_swapper)
                kick_proc0();
}

/*
 * This function represents the so-called 'hard case' for sx_slock
 * operation.  All 'easy case' failures are redirected to this.  Note
 * that ideally this would be a static function, but it needs to be
 * accessible from at least sx.h.
 */
int
_sx_slock_hard(struct sx *sx, int opts, const char *file, int line)
{
        GIANT_DECLARE;
#ifdef ADAPTIVE_SX
        volatile struct thread *owner;
#endif
#ifdef LOCK_PROFILING
        uint64_t waittime = 0;
        int contested = 0;
#endif
        uintptr_t x;
        int error = 0;
#ifdef KDTRACE_HOOKS
        uintptr_t state;
        uint64_t spin_cnt = 0;
        uint64_t sleep_cnt = 0;
        int64_t sleep_time = 0;
        int64_t all_time = 0;
#endif

        if (SCHEDULER_STOPPED())
                return (0);

#ifdef KDTRACE_HOOKS
        state = sx->sx_lock;
        all_time -= lockstat_nsecs(&sx->lock_object);
#endif

        /*
         * As with rwlocks, we don't make any attempt to try to block
         * shared locks once there is an exclusive waiter.
         */
        for (;;) {
#ifdef KDTRACE_HOOKS
                spin_cnt++;
#endif
                x = sx->sx_lock;

                /*
                 * If no other thread has an exclusive lock then try to bump up
                 * the count of sharers.  Since we have to preserve the state
                 * of SX_LOCK_EXCLUSIVE_WAITERS, if we fail to acquire the
                 * shared lock loop back and retry.
                 */
                if (x & SX_LOCK_SHARED) {
                        MPASS(!(x & SX_LOCK_SHARED_WAITERS));
                        if (atomic_cmpset_acq_ptr(&sx->sx_lock, x,
                            x + SX_ONE_SHARER)) {
                                if (LOCK_LOG_TEST(&sx->lock_object, 0))
                                        CTR4(KTR_LOCK,
                                            "%s: %p succeed %p -> %p", __func__,
                                            sx, (void *)x,
                                            (void *)(x + SX_ONE_SHARER));
                                break;
                        }
                        continue;
                }
#ifdef HWPMC_HOOKS
                PMC_SOFT_CALL( , , lock, failed);
#endif
                lock_profile_obtain_lock_failed(&sx->lock_object, &contested,
                    &waittime);

#ifdef ADAPTIVE_SX
                /*
                 * If the owner is running on another CPU, spin until
                 * the owner stops running or the state of the lock
                 * changes.
                 */
                if ((sx->lock_object.lo_flags & SX_NOADAPTIVE) == 0) {
                        x = SX_OWNER(x);
                        owner = (struct thread *)x;
                        if (TD_IS_RUNNING(owner)) {
                                if (LOCK_LOG_TEST(&sx->lock_object, 0))
                                        CTR3(KTR_LOCK,
                                            "%s: spinning on %p held by %p",
                                            __func__, sx, owner);
                                GIANT_SAVE();
                                while (SX_OWNER(sx->sx_lock) == x &&
                                    TD_IS_RUNNING(owner)) {
#ifdef KDTRACE_HOOKS
                                        spin_cnt++;
#endif
                                        cpu_spinwait();
                                }
                                continue;
                        }
                }
#endif

                /*
                 * Some other thread already has an exclusive lock, so
                 * start the process of blocking.
                 */
                sleepq_lock(&sx->lock_object);
                x = sx->sx_lock;

                /*
                 * The lock could have been released while we spun.
                 * In this case loop back and retry.
                 */
                if (x & SX_LOCK_SHARED) {
                        sleepq_release(&sx->lock_object);
                        continue;
                }

#ifdef ADAPTIVE_SX
                /*
                 * If the owner is running on another CPU, spin until
                 * the owner stops running or the state of the lock
                 * changes.
                 */
                if (!(x & SX_LOCK_SHARED) &&
                    (sx->lock_object.lo_flags & SX_NOADAPTIVE) == 0) {
                        owner = (struct thread *)SX_OWNER(x);
                        if (TD_IS_RUNNING(owner)) {
                                sleepq_release(&sx->lock_object);
                                continue;
                        }
                }
#endif

                /*
                 * Try to set the SX_LOCK_SHARED_WAITERS flag.  If we
                 * fail to set it drop the sleep queue lock and loop
                 * back.
                 */
                if (!(x & SX_LOCK_SHARED_WAITERS)) {
                        if (!atomic_cmpset_ptr(&sx->sx_lock, x,
                            x | SX_LOCK_SHARED_WAITERS)) {
                                sleepq_release(&sx->lock_object);
                                continue;
                        }
                        if (LOCK_LOG_TEST(&sx->lock_object, 0))
                                CTR2(KTR_LOCK, "%s: %p set shared waiters flag",
                                    __func__, sx);
                }

                /*
                 * Since we have been unable to acquire the shared lock,
                 * we have to sleep.
                 */
                if (LOCK_LOG_TEST(&sx->lock_object, 0))
                        CTR2(KTR_LOCK, "%s: %p blocking on sleep queue",
                            __func__, sx);

#ifdef KDTRACE_HOOKS
                sleep_time -= lockstat_nsecs(&sx->lock_object);
#endif
                GIANT_SAVE();
                sleepq_add(&sx->lock_object, NULL, sx->lock_object.lo_name,
                    SLEEPQ_SX | ((opts & SX_INTERRUPTIBLE) ?
                    SLEEPQ_INTERRUPTIBLE : 0), SQ_SHARED_QUEUE);
                if (!(opts & SX_INTERRUPTIBLE))
                        sleepq_wait(&sx->lock_object, 0);
                else
                        error = sleepq_wait_sig(&sx->lock_object, 0);
#ifdef KDTRACE_HOOKS
                sleep_time += lockstat_nsecs(&sx->lock_object);
                sleep_cnt++;
#endif
                if (error) {
                        if (LOCK_LOG_TEST(&sx->lock_object, 0))
                                CTR2(KTR_LOCK,
                        "%s: interruptible sleep by %p suspended by signal",
                                    __func__, sx);
                        break;
                }
                if (LOCK_LOG_TEST(&sx->lock_object, 0))
                        CTR2(KTR_LOCK, "%s: %p resuming from sleep queue",
                            __func__, sx);
        }
#ifdef KDTRACE_HOOKS
        all_time += lockstat_nsecs(&sx->lock_object);
        if (sleep_time)
                LOCKSTAT_RECORD4(LS_SX_SLOCK_BLOCK, sx, sleep_time,
                    LOCKSTAT_READER, (state & SX_LOCK_SHARED) == 0,
                    (state & SX_LOCK_SHARED) == 0 ? 0 : SX_SHARERS(state));
        if (spin_cnt > sleep_cnt)
                LOCKSTAT_RECORD4(LS_SX_SLOCK_SPIN, sx, all_time - sleep_time,
                    LOCKSTAT_READER, (state & SX_LOCK_SHARED) == 0,
                    (state & SX_LOCK_SHARED) == 0 ? 0 : SX_SHARERS(state));
#endif
        if (error == 0)
                LOCKSTAT_PROFILE_OBTAIN_LOCK_SUCCESS(LS_SX_SLOCK_ACQUIRE, sx,
                    contested, waittime, file, line);
        GIANT_RESTORE();
        return (error);
}

/*
 * This function represents the so-called 'hard case' for sx_sunlock
 * operation.  All 'easy case' failures are redirected to this.  Note
 * that ideally this would be a static function, but it needs to be
 * accessible from at least sx.h.
 */
void
_sx_sunlock_hard(struct sx *sx, const char *file, int line)
{
        uintptr_t x;
        int wakeup_swapper;

        if (SCHEDULER_STOPPED())
                return;

        for (;;) {
                x = sx->sx_lock;

                /*
                 * We should never have sharers while at least one thread
                 * holds a shared lock.
                 */
                KASSERT(!(x & SX_LOCK_SHARED_WAITERS),
                    ("%s: waiting sharers", __func__));

                /*
                 * See if there is more than one shared lock held.  If
                 * so, just drop one and return.
                 */
                if (SX_SHARERS(x) > 1) {
                        if (atomic_cmpset_rel_ptr(&sx->sx_lock, x,
                            x - SX_ONE_SHARER)) {
                                if (LOCK_LOG_TEST(&sx->lock_object, 0))
                                        CTR4(KTR_LOCK,
                                            "%s: %p succeeded %p -> %p",
                                            __func__, sx, (void *)x,
                                            (void *)(x - SX_ONE_SHARER));
                                break;
                        }
                        continue;
                }

                /*
                 * If there aren't any waiters for an exclusive lock,
                 * then try to drop it quickly.
                 */
                if (!(x & SX_LOCK_EXCLUSIVE_WAITERS)) {
                        MPASS(x == SX_SHARERS_LOCK(1));
                        if (atomic_cmpset_rel_ptr(&sx->sx_lock,
                            SX_SHARERS_LOCK(1), SX_LOCK_UNLOCKED)) {
                                if (LOCK_LOG_TEST(&sx->lock_object, 0))
                                        CTR2(KTR_LOCK, "%s: %p last succeeded",
                                            __func__, sx);
                                break;
                        }
                        continue;
                }

                /*
                 * At this point, there should just be one sharer with
                 * exclusive waiters.
                 */
                MPASS(x == (SX_SHARERS_LOCK(1) | SX_LOCK_EXCLUSIVE_WAITERS));

                sleepq_lock(&sx->lock_object);

                /*
                 * Wake up semantic here is quite simple:
                 * Just wake up all the exclusive waiters.
                 * Note that the state of the lock could have changed,
                 * so if it fails loop back and retry.
                 */
                if (!atomic_cmpset_rel_ptr(&sx->sx_lock,
                    SX_SHARERS_LOCK(1) | SX_LOCK_EXCLUSIVE_WAITERS,
                    SX_LOCK_UNLOCKED)) {
                        sleepq_release(&sx->lock_object);
                        continue;
                }
                if (LOCK_LOG_TEST(&sx->lock_object, 0))
                        CTR2(KTR_LOCK, "%s: %p waking up all thread on"
                            "exclusive queue", __func__, sx);
                wakeup_swapper = sleepq_broadcast(&sx->lock_object, SLEEPQ_SX,
                    0, SQ_EXCLUSIVE_QUEUE);
                sleepq_release(&sx->lock_object);
                if (wakeup_swapper)
                        kick_proc0();
                break;
        }
}

#ifdef INVARIANT_SUPPORT
#ifndef INVARIANTS
#undef  _sx_assert
#endif

/*
 * In the non-WITNESS case, sx_assert() can only detect that at least
 * *some* thread owns an slock, but it cannot guarantee that *this*
 * thread owns an slock.
 */
void
_sx_assert(struct sx *sx, int what, const char *file, int line)
{
#ifndef WITNESS
        int slocked = 0;
#endif

        if (panicstr != NULL)
                return;
        switch (what) {
        case SA_SLOCKED:
        case SA_SLOCKED | SA_NOTRECURSED:
        case SA_SLOCKED | SA_RECURSED:
#ifndef WITNESS
                slocked = 1;
                /* FALLTHROUGH */
#endif
        case SA_LOCKED:
        case SA_LOCKED | SA_NOTRECURSED:
        case SA_LOCKED | SA_RECURSED:
#ifdef WITNESS
                witness_assert(&sx->lock_object, what, file, line);
#else
                /*
                 * If some other thread has an exclusive lock or we
                 * have one and are asserting a shared lock, fail.
                 * Also, if no one has a lock at all, fail.
                 */
                if (sx->sx_lock == SX_LOCK_UNLOCKED ||
                    (!(sx->sx_lock & SX_LOCK_SHARED) && (slocked ||
                    sx_xholder(sx) != curthread)))
                        panic("Lock %s not %slocked @ %s:%d\n",
                            sx->lock_object.lo_name, slocked ? "share " : "",
                            file, line);

                if (!(sx->sx_lock & SX_LOCK_SHARED)) {
                        if (sx_recursed(sx)) {
                                if (what & SA_NOTRECURSED)
                                        panic("Lock %s recursed @ %s:%d\n",
                                            sx->lock_object.lo_name, file,
                                            line);
                        } else if (what & SA_RECURSED)
                                panic("Lock %s not recursed @ %s:%d\n",
                                    sx->lock_object.lo_name, file, line);
                }
#endif
                break;
        case SA_XLOCKED:
        case SA_XLOCKED | SA_NOTRECURSED:
        case SA_XLOCKED | SA_RECURSED:
                if (sx_xholder(sx) != curthread)
                        panic("Lock %s not exclusively locked @ %s:%d\n",
                            sx->lock_object.lo_name, file, line);
                if (sx_recursed(sx)) {
                        if (what & SA_NOTRECURSED)
                                panic("Lock %s recursed @ %s:%d\n",
                                    sx->lock_object.lo_name, file, line);
                } else if (what & SA_RECURSED)
                        panic("Lock %s not recursed @ %s:%d\n",
                            sx->lock_object.lo_name, file, line);
                break;
        case SA_UNLOCKED:
#ifdef WITNESS
                witness_assert(&sx->lock_object, what, file, line);
#else
                /*
                 * If we hold an exclusve lock fail.  We can't
                 * reliably check to see if we hold a shared lock or
                 * not.
                 */
                if (sx_xholder(sx) == curthread)
                        panic("Lock %s exclusively locked @ %s:%d\n",
                            sx->lock_object.lo_name, file, line);
#endif
                break;
        default:
                panic("Unknown sx lock assertion: %d @ %s:%d", what, file,
                    line);
        }
}
#endif  /* INVARIANT_SUPPORT */

#ifdef DDB
static void
db_show_sx(struct lock_object *lock)
{
        struct thread *td;
        struct sx *sx;

        sx = (struct sx *)lock;

        db_printf(" state: ");
        if (sx->sx_lock == SX_LOCK_UNLOCKED)
                db_printf("UNLOCKED\n");
        else if (sx->sx_lock == SX_LOCK_DESTROYED) {
                db_printf("DESTROYED\n");
                return;
        } else if (sx->sx_lock & SX_LOCK_SHARED)
                db_printf("SLOCK: %ju\n", (uintmax_t)SX_SHARERS(sx->sx_lock));
        else {
                td = sx_xholder(sx);
                db_printf("XLOCK: %p (tid %d, pid %d, \"%s\")\n", td,
                    td->td_tid, td->td_proc->p_pid, td->td_name);
                if (sx_recursed(sx))
                        db_printf(" recursed: %d\n", sx->sx_recurse);
        }

        db_printf(" waiters: ");
        switch(sx->sx_lock &
            (SX_LOCK_SHARED_WAITERS | SX_LOCK_EXCLUSIVE_WAITERS)) {
        case SX_LOCK_SHARED_WAITERS:
                db_printf("shared\n");
                break;
        case SX_LOCK_EXCLUSIVE_WAITERS:
                db_printf("exclusive\n");
                break;
        case SX_LOCK_SHARED_WAITERS | SX_LOCK_EXCLUSIVE_WAITERS:
                db_printf("exclusive and shared\n");
                break;
        default:
                db_printf("none\n");
        }
}

/*
 * Check to see if a thread that is blocked on a sleep queue is actually
 * blocked on an sx lock.  If so, output some details and return true.
 * If the lock has an exclusive owner, return that in *ownerp.
 */
int
sx_chain(struct thread *td, struct thread **ownerp)
{
        struct sx *sx;

        /*
         * Check to see if this thread is blocked on an sx lock.
         * First, we check the lock class.  If that is ok, then we
         * compare the lock name against the wait message.
         */
        sx = td->td_wchan;
        if (LOCK_CLASS(&sx->lock_object) != &lock_class_sx ||
            sx->lock_object.lo_name != td->td_wmesg)
                return (0);

        /* We think we have an sx lock, so output some details. */
        db_printf("blocked on sx \"%s\" ", td->td_wmesg);
        *ownerp = sx_xholder(sx);
        if (sx->sx_lock & SX_LOCK_SHARED)
                db_printf("SLOCK (count %ju)\n",
                    (uintmax_t)SX_SHARERS(sx->sx_lock));
        else
                db_printf("XLOCK\n");
        return (1);
}
#endif