kernel - Refactor smp collision statistics

[dragonfly.git] / sys / kern / kern_mutex.c

/*
 * Copyright (c) 2009 The DragonFly Project.  All rights reserved.
 *
 * This code is derived from software contributed to The DragonFly Project
 * by Matthew Dillon <dillon@backplane.com>
 *
 * 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.
 * 3. Neither the name of The DragonFly Project nor the names of its
 *    contributors may be used to endorse or promote products derived
 *    from this software without specific, prior written permission.
 *
 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS 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
 * COPYRIGHT HOLDERS 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.
 */
/*
 * Implement fast persistent locks based on atomic_cmpset_int() with
 * semantics similar to lockmgr locks but faster and taking up much less
 * space.  Taken from HAMMER's lock implementation.
 *
 * These are meant to complement our LWKT tokens.  Tokens are only held
 * while the thread is running.  Mutexes can be held across blocking
 * conditions.
 *
 * - Exclusive priority over shared to prevent SMP starvation.
 * - locks can be aborted (async callback, if any, will be made w/ENOLCK).
 * - locks can be asynchronous.
 * - synchronous fast path if no blocking occurs (async callback is not
 *   made in this case).
 *
 * Generally speaking any caller-supplied link state must be properly
 * initialized before use.
 *
 * Most of the support is in sys/mutex[2].h.  We mostly provide backoff
 * functions here.
 */

#include <sys/param.h>
#include <sys/systm.h>
#include <sys/kernel.h>
#include <sys/sysctl.h>
#include <sys/indefinite.h>
#include <sys/thread.h>

#include <machine/cpufunc.h>

#include <sys/thread2.h>
#include <sys/mutex2.h>
#include <sys/indefinite2.h>

static int mtx_chain_link_ex(mtx_t *mtx, u_int olock);
static int mtx_chain_link_sh(mtx_t *mtx, u_int olock);
static void mtx_delete_link(mtx_t *mtx, mtx_link_t *link);

/*
 * Exclusive-lock a mutex, block until acquired unless link is async.
 * Recursion is allowed.
 *
 * Returns 0 on success, the tsleep() return code on failure, EINPROGRESS
 * if async.  If immediately successful an async exclusive lock will return 0
 * and not issue the async callback or link the link structure.  The caller
 * must handle this case (typically this is an optimal code path).
 *
 * A tsleep() error can only be returned if PCATCH is specified in the flags.
 */
static __inline int
__mtx_lock_ex(mtx_t *mtx, mtx_link_t *link, int flags, int to)
{
        thread_t td;
        u_int   lock;
        u_int   nlock;
        int     error;
        int     isasync;

        for (;;) {
                lock = mtx->mtx_lock;
                cpu_ccfence();

                if (lock == 0) {
                        nlock = MTX_EXCLUSIVE | 1;
                        if (atomic_cmpset_int(&mtx->mtx_lock, 0, nlock)) {
                                mtx->mtx_owner = curthread;
                                cpu_sfence();
                                link->state = MTX_LINK_ACQUIRED;
                                error = 0;
                                break;
                        }
                        continue;
                }
                if ((lock & MTX_EXCLUSIVE) && mtx->mtx_owner == curthread) {
                        KKASSERT((lock & MTX_MASK) != MTX_MASK);
                        nlock = lock + 1;
                        if (atomic_cmpset_int(&mtx->mtx_lock, lock, nlock)) {
                                cpu_sfence();
                                link->state = MTX_LINK_ACQUIRED;
                                error = 0;
                                break;
                        }
                        continue;
                }

                /*
                 * We need MTX_LINKSPIN to manipulate exlink or
                 * shlink.
                 *
                 * We must set MTX_EXWANTED with MTX_LINKSPIN to indicate
                 * pending exclusive requests.  It cannot be set as a separate
                 * operation prior to acquiring MTX_LINKSPIN.
                 *
                 * To avoid unnecessary cpu cache traffic we poll
                 * for collisions.  It is also possible that EXWANTED
                 * state failing the above test was spurious, so all the
                 * tests must be repeated if we cannot obtain LINKSPIN
                 * with the prior state tests intact (i.e. don't reload
                 * the (lock) variable here, for heaven's sake!).
                 */
                if (lock & MTX_LINKSPIN) {
                        cpu_pause();
                        continue;
                }
                td = curthread;
                nlock = lock | MTX_EXWANTED | MTX_LINKSPIN;
                crit_enter_raw(td);
                if (atomic_cmpset_int(&mtx->mtx_lock, lock, nlock) == 0) {
                        crit_exit_raw(td);
                        continue;
                }

                /*
                 * Check for early abort.
                 */
                if (link->state == MTX_LINK_ABORTED) {
                        if (mtx->mtx_exlink == NULL) {
                                atomic_clear_int(&mtx->mtx_lock,
                                                 MTX_LINKSPIN |
                                                 MTX_EXWANTED);
                        } else {
                                atomic_clear_int(&mtx->mtx_lock,
                                                 MTX_LINKSPIN);
                        }
                        crit_exit_raw(td);
                        link->state = MTX_LINK_IDLE;
                        error = ENOLCK;
                        break;
                }

                /*
                 * Add our link to the exlink list and release LINKSPIN.
                 */
                link->owner = td;
                link->state = MTX_LINK_LINKED_EX;
                if (mtx->mtx_exlink) {
                        link->next = mtx->mtx_exlink;
                        link->prev = link->next->prev;
                        link->next->prev = link;
                        link->prev->next = link;
                } else {
                        link->next = link;
                        link->prev = link;
                        mtx->mtx_exlink = link;
                }
                isasync = (link->callback != NULL);
                atomic_clear_int(&mtx->mtx_lock, MTX_LINKSPIN);
                crit_exit_raw(td);

                /* 
                 * If asynchronous lock request return without
                 * blocking, leave link structure linked.
                 */
                if (isasync) {
                        error = EINPROGRESS;
                        break;
                }

                /*
                 * Wait for lock
                 */
                error = mtx_wait_link(mtx, link, flags, to);
                break;
        }
        return (error);
}

int
_mtx_lock_ex_link(mtx_t *mtx, mtx_link_t *link, int flags, int to)
{
        return(__mtx_lock_ex(mtx, link, flags, to));
}

int
_mtx_lock_ex(mtx_t *mtx, int flags, int to)
{
        mtx_link_t link;

        mtx_link_init(&link);
        return(__mtx_lock_ex(mtx, &link, flags, to));
}

int
_mtx_lock_ex_quick(mtx_t *mtx)
{
        mtx_link_t link;

        mtx_link_init(&link);
        return(__mtx_lock_ex(mtx, &link, 0, 0));
}

/*
 * Share-lock a mutex, block until acquired.  Recursion is allowed.
 *
 * Returns 0 on success, or the tsleep() return code on failure.
 * An error can only be returned if PCATCH is specified in the flags.
 *
 * NOTE: Shared locks get a mass-wakeup so if the tsleep fails we
 *       do not have to chain the wakeup().
 */
static __inline int
__mtx_lock_sh(mtx_t *mtx, mtx_link_t *link, int flags, int to)
{
        thread_t td;
        u_int   lock;
        u_int   nlock;
        int     error;
        int     isasync;

        for (;;) {
                lock = mtx->mtx_lock;
                cpu_ccfence();

                if (lock == 0) {
                        nlock = 1;
                        if (atomic_cmpset_int(&mtx->mtx_lock, 0, nlock)) {
                                error = 0;
                                cpu_sfence();
                                link->state = MTX_LINK_ACQUIRED;
                                break;
                        }
                        continue;
                }
                if ((lock & (MTX_EXCLUSIVE | MTX_EXWANTED)) == 0) {
                        KKASSERT((lock & MTX_MASK) != MTX_MASK);
                        nlock = lock + 1;
                        if (atomic_cmpset_int(&mtx->mtx_lock, lock, nlock)) {
                                error = 0;
                                cpu_sfence();
                                link->state = MTX_LINK_ACQUIRED;
                                break;
                        }
                        continue;
                }

                /*
                 * We need MTX_LINKSPIN to manipulate exlink or
                 * shlink.
                 *
                 * We must set MTX_SHWANTED with MTX_LINKSPIN to indicate
                 * pending shared requests.  It cannot be set as a separate
                 * operation prior to acquiring MTX_LINKSPIN.
                 *
                 * To avoid unnecessary cpu cache traffic we poll
                 * for collisions.  It is also possible that EXWANTED
                 * state failing the above test was spurious, so all the
                 * tests must be repeated if we cannot obtain LINKSPIN
                 * with the prior state tests intact (i.e. don't reload
                 * the (lock) variable here, for heaven's sake!).
                 */
                if (lock & MTX_LINKSPIN) {
                        cpu_pause();
                        continue;
                }
                td = curthread;
                nlock = lock | MTX_SHWANTED | MTX_LINKSPIN;
                crit_enter_raw(td);
                if (atomic_cmpset_int(&mtx->mtx_lock, lock, nlock) == 0) {
                        crit_exit_raw(td);
                        continue;
                }

                /*
                 * Check for early abort.  Other shared lock requestors
                 * could have sneaked in before we set LINKSPIN so make
                 * sure we undo the state properly.
                 */
                if (link->state == MTX_LINK_ABORTED) {
                        if (mtx->mtx_shlink) {
                                atomic_clear_int(&mtx->mtx_lock,
                                                 MTX_LINKSPIN);
                        } else {
                                atomic_clear_int(&mtx->mtx_lock,
                                                 MTX_LINKSPIN |
                                                 MTX_SHWANTED);
                        }
                        crit_exit_raw(td);
                        link->state = MTX_LINK_IDLE;
                        error = ENOLCK;
                        break;
                }

                /*
                 * Add our link to the shlink list and release LINKSPIN.
                 */
                link->owner = td;
                link->state = MTX_LINK_LINKED_SH;
                if (mtx->mtx_shlink) {
                        link->next = mtx->mtx_shlink;
                        link->prev = link->next->prev;
                        link->next->prev = link;
                        link->prev->next = link;
                } else {
                        link->next = link;
                        link->prev = link;
                        mtx->mtx_shlink = link;
                }
                isasync = (link->callback != NULL);
                atomic_clear_int(&mtx->mtx_lock, MTX_LINKSPIN);
                crit_exit_raw(td);

                /* 
                 * If asynchronous lock request return without
                 * blocking, leave link structure linked.
                 */
                if (isasync) {
                        error = EINPROGRESS;
                        break;
                }

                /*
                 * Wait for lock
                 */
                error = mtx_wait_link(mtx, link, flags, to);
                break;
        }
        return (error);
}

int
_mtx_lock_sh_link(mtx_t *mtx, mtx_link_t *link, int flags, int to)
{
        return(__mtx_lock_sh(mtx, link, flags, to));
}

int
_mtx_lock_sh(mtx_t *mtx, int flags, int to)
{
        mtx_link_t link;

        mtx_link_init(&link);
        return(__mtx_lock_sh(mtx, &link, flags, to));
}

int
_mtx_lock_sh_quick(mtx_t *mtx)
{
        mtx_link_t link;

        mtx_link_init(&link);
        return(__mtx_lock_sh(mtx, &link, 0, 0));
}

/*
 * Get an exclusive spinlock the hard way.
 */
void
_mtx_spinlock(mtx_t *mtx)
{
        u_int   lock;
        u_int   nlock;
        int     bb = 1;
        int     bo;

        for (;;) {
                lock = mtx->mtx_lock;
                if (lock == 0) {
                        nlock = MTX_EXCLUSIVE | 1;
                        if (atomic_cmpset_int(&mtx->mtx_lock, 0, nlock)) {
                                mtx->mtx_owner = curthread;
                                break;
                        }
                } else if ((lock & MTX_EXCLUSIVE) &&
                           mtx->mtx_owner == curthread) {
                        KKASSERT((lock & MTX_MASK) != MTX_MASK);
                        nlock = lock + 1;
                        if (atomic_cmpset_int(&mtx->mtx_lock, lock, nlock))
                                break;
                } else {
                        /* MWAIT here */
                        if (bb < 1000)
                                ++bb;
                        cpu_pause();
                        for (bo = 0; bo < bb; ++bo)
                                ;
                }
                cpu_pause();
        }
}

/*
 * Attempt to acquire a spinlock, if we fail we must undo the
 * gd->gd_spinlocks/gd->gd_curthead->td_critcount predisposition.
 *
 * Returns 0 on success, EAGAIN on failure.
 */
int
_mtx_spinlock_try(mtx_t *mtx)
{
        globaldata_t gd = mycpu;
        u_int   lock;
        u_int   nlock;
        int     res = 0;

        for (;;) {
                lock = mtx->mtx_lock;
                if (lock == 0) {
                        nlock = MTX_EXCLUSIVE | 1;
                        if (atomic_cmpset_int(&mtx->mtx_lock, 0, nlock)) {
                                mtx->mtx_owner = gd->gd_curthread;
                                break;
                        }
                } else if ((lock & MTX_EXCLUSIVE) &&
                           mtx->mtx_owner == gd->gd_curthread) {
                        KKASSERT((lock & MTX_MASK) != MTX_MASK);
                        nlock = lock + 1;
                        if (atomic_cmpset_int(&mtx->mtx_lock, lock, nlock))
                                break;
                } else {
                        --gd->gd_spinlocks;
                        cpu_ccfence();
                        crit_exit_raw(gd->gd_curthread);
                        res = EAGAIN;
                        break;
                }
                cpu_pause();
        }
        return res;
}

#if 0

void
_mtx_spinlock_sh(mtx_t *mtx)
{
        u_int   lock;
        u_int   nlock;
        int     bb = 1;
        int     bo;

        for (;;) {
                lock = mtx->mtx_lock;
                if ((lock & MTX_EXCLUSIVE) == 0) {
                        KKASSERT((lock & MTX_MASK) != MTX_MASK);
                        nlock = lock + 1;
                        if (atomic_cmpset_int(&mtx->mtx_lock, lock, nlock))
                                break;
                } else {
                        /* MWAIT here */
                        if (bb < 1000)
                                ++bb;
                        cpu_pause();
                        for (bo = 0; bo < bb; ++bo)
                                ;
                }
                cpu_pause();
        }
}

#endif

int
_mtx_lock_ex_try(mtx_t *mtx)
{
        u_int   lock;
        u_int   nlock;
        int     error;

        for (;;) {
                lock = mtx->mtx_lock;
                if (lock == 0) {
                        nlock = MTX_EXCLUSIVE | 1;
                        if (atomic_cmpset_int(&mtx->mtx_lock, 0, nlock)) {
                                mtx->mtx_owner = curthread;
                                error = 0;
                                break;
                        }
                } else if ((lock & MTX_EXCLUSIVE) &&
                           mtx->mtx_owner == curthread) {
                        KKASSERT((lock & MTX_MASK) != MTX_MASK);
                        nlock = lock + 1;
                        if (atomic_cmpset_int(&mtx->mtx_lock, lock, nlock)) {
                                error = 0;
                                break;
                        }
                } else {
                        error = EAGAIN;
                        break;
                }
                cpu_pause();
        }
        return (error);
}

int
_mtx_lock_sh_try(mtx_t *mtx)
{
        u_int   lock;
        u_int   nlock;
        int     error = 0;

        for (;;) {
                lock = mtx->mtx_lock;
                if ((lock & MTX_EXCLUSIVE) == 0) {
                        KKASSERT((lock & MTX_MASK) != MTX_MASK);
                        nlock = lock + 1;
                        if (atomic_cmpset_int(&mtx->mtx_lock, lock, nlock))
                                break;
                } else {
                        error = EAGAIN;
                        break;
                }
                cpu_pause();
        }
        return (error);
}

/*
 * If the lock is held exclusively it must be owned by the caller.  If the
 * lock is already a shared lock this operation is a NOP.  A panic will
 * occur if the lock is not held either shared or exclusive.
 *
 * The exclusive count is converted to a shared count.
 */
void
_mtx_downgrade(mtx_t *mtx)
{
        u_int   lock;
        u_int   nlock;

        for (;;) {
                lock = mtx->mtx_lock;
                cpu_ccfence();

                /*
                 * NOP if already shared.
                 */
                if ((lock & MTX_EXCLUSIVE) == 0) {
                        KKASSERT((lock & MTX_MASK) > 0);
                        break;
                }

                /*
                 * Transfer count to shared.  Any additional pending shared
                 * waiters must be woken up.
                 */
                if (lock & MTX_SHWANTED) {
                        if (mtx_chain_link_sh(mtx, lock))
                                break;
                        /* retry */
                } else {
                        nlock = lock & ~MTX_EXCLUSIVE;
                        if (atomic_cmpset_int(&mtx->mtx_lock, lock, nlock))
                                break;
                        /* retry */
                }
                cpu_pause();
        }
}

/*
 * Upgrade a shared lock to an exclusive lock.  The upgrade will fail if
 * the shared lock has a count other then 1.  Optimize the most likely case
 * but note that a single cmpset can fail due to WANTED races.
 *
 * If the lock is held exclusively it must be owned by the caller and
 * this function will simply return without doing anything.   A panic will
 * occur if the lock is held exclusively by someone other then the caller.
 *
 * Returns 0 on success, EDEADLK on failure.
 */
int
_mtx_upgrade_try(mtx_t *mtx)
{
        u_int   lock;
        u_int   nlock;
        int     error = 0;

        for (;;) {
                lock = mtx->mtx_lock;
                cpu_ccfence();

                if ((lock & ~MTX_EXWANTED) == 1) {
                        nlock = lock | MTX_EXCLUSIVE;
                        if (atomic_cmpset_int(&mtx->mtx_lock, lock, nlock)) {
                                mtx->mtx_owner = curthread;
                                break;
                        }
                } else if (lock & MTX_EXCLUSIVE) {
                        KKASSERT(mtx->mtx_owner == curthread);
                        break;
                } else {
                        error = EDEADLK;
                        break;
                }
                cpu_pause();
        }
        return (error);
}

/*
 * Unlock a lock.  The caller must hold the lock either shared or exclusive.
 *
 * On the last release we handle any pending chains.
 */
void
_mtx_unlock(mtx_t *mtx)
{
        u_int   lock;
        u_int   nlock;

        for (;;) {
                lock = mtx->mtx_lock;
                cpu_ccfence();

                switch(lock) {
                case MTX_EXCLUSIVE | 1:
                        /*
                         * Last release, exclusive lock.
                         * No exclusive or shared requests pending.
                         */
                        mtx->mtx_owner = NULL;
                        nlock = 0;
                        if (atomic_cmpset_int(&mtx->mtx_lock, lock, nlock))
                                goto done;
                        break;
                case MTX_EXCLUSIVE | MTX_EXWANTED | 1:
                case MTX_EXCLUSIVE | MTX_EXWANTED | MTX_SHWANTED | 1:
                        /*
                         * Last release, exclusive lock.
                         * Exclusive requests pending.
                         * Exclusive requests have priority over shared reqs.
                         */
                        if (mtx_chain_link_ex(mtx, lock))
                                goto done;
                        break;
                case MTX_EXCLUSIVE | MTX_SHWANTED | 1:
                        /*
                         * Last release, exclusive lock.
                         *
                         * Shared requests are pending.  Transfer our count (1)
                         * to the first shared request, wakeup all shared reqs.
                         */
                        if (mtx_chain_link_sh(mtx, lock))
                                goto done;
                        break;
                case 1:
                        /*
                         * Last release, shared lock.
                         * No exclusive or shared requests pending.
                         */
                        nlock = 0;
                        if (atomic_cmpset_int(&mtx->mtx_lock, lock, nlock))
                                goto done;
                        break;
                case MTX_EXWANTED | 1:
                case MTX_EXWANTED | MTX_SHWANTED | 1:
                        /*
                         * Last release, shared lock.
                         *
                         * Exclusive requests are pending.  Upgrade this
                         * final shared lock to exclusive and transfer our
                         * count (1) to the next exclusive request.
                         *
                         * Exclusive requests have priority over shared reqs.
                         */
                        if (mtx_chain_link_ex(mtx, lock))
                                goto done;
                        break;
                case MTX_SHWANTED | 1:
                        /*
                         * Last release, shared lock.
                         * Shared requests pending.
                         */
                        if (mtx_chain_link_sh(mtx, lock))
                                goto done;
                        break;
                default:
                        /*
                         * We have to loop if this is the last release but
                         * someone is fiddling with LINKSPIN.
                         */
                        if ((lock & MTX_MASK) == 1) {
                                KKASSERT(lock & MTX_LINKSPIN);
                                break;
                        }

                        /*
                         * Not the last release (shared or exclusive)
                         */
                        nlock = lock - 1;
                        KKASSERT((nlock & MTX_MASK) != MTX_MASK);
                        if (atomic_cmpset_int(&mtx->mtx_lock, lock, nlock))
                                goto done;
                        break;
                }
                /* loop try again */
                cpu_pause();
        }
done:
        ;
}

/*
 * Chain pending links.  Called on the last release of an exclusive or
 * shared lock when the appropriate WANTED bit is set.  mtx_lock old state
 * is passed in with the count left at 1, which we can inherit, and other
 * bits which we must adjust in a single atomic operation.
 *
 * Return non-zero on success, 0 if caller needs to retry.
 *
 * NOTE: It's ok if MTX_EXWANTED is in an indeterminant state while we are
 *       acquiring LINKSPIN as all other cases will also need to acquire
 *       LINKSPIN when handling the EXWANTED case.
 */
static int
mtx_chain_link_ex(mtx_t *mtx, u_int olock)
{
        thread_t td = curthread;
        mtx_link_t *link;
        u_int   nlock;

        olock &= ~MTX_LINKSPIN;
        nlock = olock | MTX_LINKSPIN | MTX_EXCLUSIVE;   /* upgrade if necc */
        crit_enter_raw(td);
        if (atomic_cmpset_int(&mtx->mtx_lock, olock, nlock)) {
                link = mtx->mtx_exlink;
                KKASSERT(link != NULL);
                if (link->next == link) {
                        mtx->mtx_exlink = NULL;
                        nlock = MTX_LINKSPIN | MTX_EXWANTED;    /* to clear */
                } else {
                        mtx->mtx_exlink = link->next;
                        link->next->prev = link->prev;
                        link->prev->next = link->next;
                        nlock = MTX_LINKSPIN;                   /* to clear */
                }
                KKASSERT(link->state == MTX_LINK_LINKED_EX);
                mtx->mtx_owner = link->owner;
                cpu_sfence();

                /*
                 * WARNING! The callback can only be safely
                 *          made with LINKSPIN still held
                 *          and in a critical section.
                 *
                 * WARNING! The link can go away after the
                 *          state is set, or after the
                 *          callback.
                 */
                if (link->callback) {
                        link->state = MTX_LINK_CALLEDBACK;
                        link->callback(link, link->arg, 0);
                } else {
                        link->state = MTX_LINK_ACQUIRED;
                        wakeup(link);
                }
                atomic_clear_int(&mtx->mtx_lock, nlock);
                crit_exit_raw(td);
                return 1;
        }
        /* retry */
        crit_exit_raw(td);

        return 0;
}

/*
 * Flush waiting shared locks.  The lock's prior state is passed in and must
 * be adjusted atomically only if it matches and LINKSPIN is not set.
 *
 * IMPORTANT! The caller has left one active count on the lock for us to
 *            consume.  We will apply this to the first link, but must add
 *            additional counts for any other links.
 */
static int
mtx_chain_link_sh(mtx_t *mtx, u_int olock)
{
        thread_t td = curthread;
        mtx_link_t *link;
        u_int   addcount;
        u_int   nlock;

        olock &= ~MTX_LINKSPIN;
        nlock = olock | MTX_LINKSPIN;
        nlock &= ~MTX_EXCLUSIVE;
        crit_enter_raw(td);
        if (atomic_cmpset_int(&mtx->mtx_lock, olock, nlock)) {
                /*
                 * It should not be possible for SHWANTED to be set without
                 * any links pending.
                 */
                KKASSERT(mtx->mtx_shlink != NULL);

                /*
                 * We have to process the count for all shared locks before
                 * we process any of the links.  Count the additional shared
                 * locks beyond the first link (which is already accounted
                 * for) and associate the full count with the lock
                 * immediately.
                 */
                addcount = 0;
                for (link = mtx->mtx_shlink->next; link != mtx->mtx_shlink;
                     link = link->next) {
                        ++addcount;
                }
                if (addcount > 0)
                        atomic_add_int(&mtx->mtx_lock, addcount);

                /*
                 * We can wakeup all waiting shared locks.
                 */
                while ((link = mtx->mtx_shlink) != NULL) {
                        KKASSERT(link->state == MTX_LINK_LINKED_SH);
                        if (link->next == link) {
                                mtx->mtx_shlink = NULL;
                        } else {
                                mtx->mtx_shlink = link->next;
                                link->next->prev = link->prev;
                                link->prev->next = link->next;
                        }
                        link->next = NULL;
                        link->prev = NULL;
                        cpu_sfence();
                        if (link->callback) {
                                link->state = MTX_LINK_CALLEDBACK;
                                link->callback(link, link->arg, 0);
                        } else {
                                cpu_sfence();
                                link->state = MTX_LINK_ACQUIRED;
                                wakeup(link);
                        }
                }
                atomic_clear_int(&mtx->mtx_lock, MTX_LINKSPIN |
                                                 MTX_SHWANTED);
                crit_exit_raw(td);
                return 1;
        }
        /* retry */
        crit_exit_raw(td);

        return 0;
}

/*
 * Delete a link structure after tsleep has failed.  This code is not
 * in the critical path as most exclusive waits are chained.
 */
static
void
mtx_delete_link(mtx_t *mtx, mtx_link_t *link)
{
        thread_t td = curthread;
        u_int   lock;
        u_int   nlock;

        /*
         * Acquire MTX_LINKSPIN.
         *
         * Do not use cmpxchg to wait for LINKSPIN to clear as this might
         * result in too much cpu cache traffic.
         */
        crit_enter_raw(td);
        for (;;) {
                lock = mtx->mtx_lock;
                if (lock & MTX_LINKSPIN) {
                        cpu_pause();
                        continue;
                }
                nlock = lock | MTX_LINKSPIN;
                if (atomic_cmpset_int(&mtx->mtx_lock, lock, nlock))
                        break;
                cpu_pause();
        }

        /*
         * Delete the link and release LINKSPIN.
         */
        nlock = MTX_LINKSPIN;   /* to clear */

        switch(link->state) {
        case MTX_LINK_LINKED_EX:
                if (link->next == link) {
                        mtx->mtx_exlink = NULL;
                        nlock |= MTX_EXWANTED;  /* to clear */
                } else {
                        mtx->mtx_exlink = link->next;
                        link->next->prev = link->prev;
                        link->prev->next = link->next;
                }
                break;
        case MTX_LINK_LINKED_SH:
                if (link->next == link) {
                        mtx->mtx_shlink = NULL;
                        nlock |= MTX_SHWANTED;  /* to clear */
                } else {
                        mtx->mtx_shlink = link->next;
                        link->next->prev = link->prev;
                        link->prev->next = link->next;
                }
                break;
        default:
                /* no change */
                break;
        }
        atomic_clear_int(&mtx->mtx_lock, nlock);
        crit_exit_raw(td);
}

/*
 * Wait for async lock completion or abort.  Returns ENOLCK if an abort
 * occurred.
 */
int
mtx_wait_link(mtx_t *mtx, mtx_link_t *link, int flags, int to)
{
        thread_t td = curthread;
        int error;

        if ((mtx->mtx_flags & MTXF_NOCOLLSTATS) == 0) {
                indefinite_init(&td->td_indefinite, mtx->mtx_ident, 1,
                        ((link->state & MTX_LINK_LINKED_SH) ? 'm' : 'M'));
        }

        /*
         * Sleep.  Handle false wakeups, interruptions, etc.
         * The link may also have been aborted.  The LINKED
         * bit was set by this cpu so we can test it without
         * fences.
         */
        error = 0;
        while (link->state & MTX_LINK_LINKED) {
                tsleep_interlock(link, 0);
                cpu_lfence();
                if (link->state & MTX_LINK_LINKED) {
                        error = tsleep(link, flags | PINTERLOCKED,
                                       mtx->mtx_ident, to);
                        if (error)
                                break;
                }
                if ((mtx->mtx_flags & MTXF_NOCOLLSTATS) == 0)
                        indefinite_check(&td->td_indefinite);
        }

        /*
         * We need at least a lfence (load fence) to ensure our cpu does not
         * reorder loads (of data outside the lock structure) prior to the
         * remote cpu's release, since the above test may have run without
         * any atomic interactions.
         *
         * If we do not do this then state updated by the other cpu before
         * releasing its lock may not be read cleanly by our cpu when this
         * function returns.  Even though the other cpu ordered its stores,
         * our loads can still be out of order.
         */
        cpu_mfence();

        /*
         * We are done, make sure the link structure is unlinked.
         * It may still be on the list due to e.g. EINTR or
         * EWOULDBLOCK.
         *
         * It is possible for the tsleep to race an ABORT and cause
         * error to be 0.
         *
         * The tsleep() can be woken up for numerous reasons and error
         * might be zero in situations where we intend to return an error.
         *
         * (This is the synchronous case so state cannot be CALLEDBACK)
         */
        switch(link->state) {
        case MTX_LINK_ACQUIRED:
        case MTX_LINK_CALLEDBACK:
                error = 0;
                break;
        case MTX_LINK_ABORTED:
                error = ENOLCK;
                break;
        case MTX_LINK_LINKED_EX:
        case MTX_LINK_LINKED_SH:
                mtx_delete_link(mtx, link);
                /* fall through */
        default:
                if (error == 0)
                        error = EWOULDBLOCK;
                break;
        }

        /*
         * Clear state on status returned.
         */
        link->state = MTX_LINK_IDLE;

        if ((mtx->mtx_flags & MTXF_NOCOLLSTATS) == 0)
                indefinite_done(&td->td_indefinite);

        return error;
}

/*
 * Abort a mutex locking operation, causing mtx_lock_ex_link() to
 * return ENOLCK.  This may be called at any time after the mtx_link
 * is initialized or the status from a previous lock has been
 * returned.  If called prior to the next (non-try) lock attempt, the
 * next lock attempt using this link structure will abort instantly.
 *
 * Caller must still wait for the operation to complete, either from a
 * blocking call that is still in progress or by calling mtx_wait_link().
 *
 * If an asynchronous lock request is possibly in-progress, the caller
 * should call mtx_wait_link() synchronously.  Note that the asynchronous
 * lock callback will NOT be called if a successful abort occurred. XXX
 */
void
mtx_abort_link(mtx_t *mtx, mtx_link_t *link)
{
        thread_t td = curthread;
        u_int   lock;
        u_int   nlock;

        /*
         * Acquire MTX_LINKSPIN
         */
        crit_enter_raw(td);
        for (;;) {
                lock = mtx->mtx_lock;
                if (lock & MTX_LINKSPIN) {
                        cpu_pause();
                        continue;
                }
                nlock = lock | MTX_LINKSPIN;
                if (atomic_cmpset_int(&mtx->mtx_lock, lock, nlock))
                        break;
                cpu_pause();
        }

        /*
         * Do the abort.
         *
         * WARNING! Link structure can disappear once link->state is set.
         */
        nlock = MTX_LINKSPIN;   /* to clear */

        switch(link->state) {
        case MTX_LINK_IDLE:
                /*
                 * Link not started yet
                 */
                link->state = MTX_LINK_ABORTED;
                break;
        case MTX_LINK_LINKED_EX:
                /*
                 * de-link, mark aborted, and potentially wakeup the thread
                 * or issue the callback.
                 */
                if (link->next == link) {
                        if (mtx->mtx_exlink == link) {
                                mtx->mtx_exlink = NULL;
                                nlock |= MTX_EXWANTED;  /* to clear */
                        }
                } else {
                        if (mtx->mtx_exlink == link)
                                mtx->mtx_exlink = link->next;
                        link->next->prev = link->prev;
                        link->prev->next = link->next;
                }

                /*
                 * When aborting the async callback is still made.  We must
                 * not set the link status to ABORTED in the callback case
                 * since there is nothing else to clear its status if the
                 * link is reused.
                 */
                if (link->callback) {
                        link->state = MTX_LINK_CALLEDBACK;
                        link->callback(link, link->arg, ENOLCK);
                } else {
                        link->state = MTX_LINK_ABORTED;
                        wakeup(link);
                }
                break;
        case MTX_LINK_LINKED_SH:
                /*
                 * de-link, mark aborted, and potentially wakeup the thread
                 * or issue the callback.
                 */
                if (link->next == link) {
                        if (mtx->mtx_shlink == link) {
                                mtx->mtx_shlink = NULL;
                                nlock |= MTX_SHWANTED;  /* to clear */
                        }
                } else {
                        if (mtx->mtx_shlink == link)
                                mtx->mtx_shlink = link->next;
                        link->next->prev = link->prev;
                        link->prev->next = link->next;
                }

                /*
                 * When aborting the async callback is still made.  We must
                 * not set the link status to ABORTED in the callback case
                 * since there is nothing else to clear its status if the
                 * link is reused.
                 */
                if (link->callback) {
                        link->state = MTX_LINK_CALLEDBACK;
                        link->callback(link, link->arg, ENOLCK);
                } else {
                        link->state = MTX_LINK_ABORTED;
                        wakeup(link);
                }
                break;
        case MTX_LINK_ACQUIRED:
        case MTX_LINK_CALLEDBACK:
                /*
                 * Too late, the lock was acquired.  Let it complete.
                 */
                break;
        default:
                /*
                 * link already aborted, do nothing.
                 */
                break;
        }
        atomic_clear_int(&mtx->mtx_lock, nlock);
        crit_exit_raw(td);
}