Ignore machine-check MSRs

[freebsd-src/fkvm-freebsd.git] / sys / kern / kern_rwlock.c

/*-
 * Copyright (c) 2006 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.
 * 3. Neither the name of the author nor the names of any co-contributors
 *    may be used to endorse or promote products derived from this software
 *    without specific prior written permission.
 *
 * 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.
 */

/*
 * Machine independent bits of reader/writer lock implementation.
 */

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

#include "opt_ddb.h"
#include "opt_no_adaptive_rwlocks.h"

#include <sys/param.h>
#include <sys/ktr.h>
#include <sys/kernel.h>
#include <sys/lock.h>
#include <sys/mutex.h>
#include <sys/proc.h>
#include <sys/rwlock.h>
#include <sys/sysctl.h>
#include <sys/systm.h>
#include <sys/turnstile.h>

#include <machine/cpu.h>

CTASSERT((RW_RECURSE & LO_CLASSFLAGS) == RW_RECURSE);

#if defined(SMP) && !defined(NO_ADAPTIVE_RWLOCKS)
#define ADAPTIVE_RWLOCKS
#endif

#ifdef ADAPTIVE_RWLOCKS
static int rowner_retries = 10;
static int rowner_loops = 10000;
SYSCTL_NODE(_debug, OID_AUTO, rwlock, CTLFLAG_RD, NULL, "rwlock debugging");
SYSCTL_INT(_debug_rwlock, OID_AUTO, retry, CTLFLAG_RW, &rowner_retries, 0, "");
SYSCTL_INT(_debug_rwlock, OID_AUTO, loops, CTLFLAG_RW, &rowner_loops, 0, "");
#endif

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

static void     db_show_rwlock(struct lock_object *lock);
#endif
static void     assert_rw(struct lock_object *lock, int what);
static void     lock_rw(struct lock_object *lock, int how);
static int      unlock_rw(struct lock_object *lock);

struct lock_class lock_class_rw = {
        .lc_name = "rw",
        .lc_flags = LC_SLEEPLOCK | LC_RECURSABLE | LC_UPGRADABLE,
        .lc_assert = assert_rw,
#ifdef DDB
        .lc_ddb_show = db_show_rwlock,
#endif
        .lc_lock = lock_rw,
        .lc_unlock = unlock_rw,
};

/*
 * Return a pointer to the owning thread if the lock is write-locked or
 * NULL if the lock is unlocked or read-locked.
 */
#define rw_wowner(rw)                                                   \
        ((rw)->rw_lock & RW_LOCK_READ ? NULL :                          \
            (struct thread *)RW_OWNER((rw)->rw_lock))

/*
 * Returns if a write owner is recursed.  Write ownership is not assured
 * here and should be previously checked.
 */
#define rw_recursed(rw)         ((rw)->rw_recurse != 0)

/*
 * Return true if curthread helds the lock.
 */
#define rw_wlocked(rw)          (rw_wowner((rw)) == curthread)

/*
 * Return a pointer to the owning thread for this lock who should receive
 * any priority lent by threads that block on this lock.  Currently this
 * is identical to rw_wowner().
 */
#define rw_owner(rw)            rw_wowner(rw)

#ifndef INVARIANTS
#define _rw_assert(rw, what, file, line)
#endif

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

        rw_assert((struct rwlock *)lock, what);
}

void
lock_rw(struct lock_object *lock, int how)
{
        struct rwlock *rw;

        rw = (struct rwlock *)lock;
        if (how)
                rw_wlock(rw);
        else
                rw_rlock(rw);
}

int
unlock_rw(struct lock_object *lock)
{
        struct rwlock *rw;

        rw = (struct rwlock *)lock;
        rw_assert(rw, RA_LOCKED | LA_NOTRECURSED);
        if (rw->rw_lock & RW_LOCK_READ) {
                rw_runlock(rw);
                return (0);
        } else {
                rw_wunlock(rw);
                return (1);
        }
}

void
rw_init_flags(struct rwlock *rw, const char *name, int opts)
{
        int flags;

        MPASS((opts & ~(RW_DUPOK | RW_NOPROFILE | RW_NOWITNESS | RW_QUIET |
            RW_RECURSE)) == 0);

        flags = LO_UPGRADABLE | LO_RECURSABLE;
        if (opts & RW_DUPOK)
                flags |= LO_DUPOK;
        if (opts & RW_NOPROFILE)
                flags |= LO_NOPROFILE;
        if (!(opts & RW_NOWITNESS))
                flags |= LO_WITNESS;
        if (opts & RW_QUIET)
                flags |= LO_QUIET;
        flags |= opts & RW_RECURSE;

        rw->rw_lock = RW_UNLOCKED;
        rw->rw_recurse = 0;
        lock_init(&rw->lock_object, &lock_class_rw, name, NULL, flags);
}

void
rw_destroy(struct rwlock *rw)
{

        KASSERT(rw->rw_lock == RW_UNLOCKED, ("rw lock not unlocked"));
        KASSERT(rw->rw_recurse == 0, ("rw lock still recursed"));
        rw->rw_lock = RW_DESTROYED;
        lock_destroy(&rw->lock_object);
}

void
rw_sysinit(void *arg)
{
        struct rw_args *args = arg;

        rw_init(args->ra_rw, args->ra_desc);
}

int
rw_wowned(struct rwlock *rw)
{

        return (rw_wowner(rw) == curthread);
}

void
_rw_wlock(struct rwlock *rw, const char *file, int line)
{

        MPASS(curthread != NULL);
        KASSERT(rw->rw_lock != RW_DESTROYED,
            ("rw_wlock() of destroyed rwlock @ %s:%d", file, line));
        WITNESS_CHECKORDER(&rw->lock_object, LOP_NEWORDER | LOP_EXCLUSIVE, file,
            line);
        __rw_wlock(rw, curthread, file, line);
        LOCK_LOG_LOCK("WLOCK", &rw->lock_object, 0, rw->rw_recurse, file, line);
        WITNESS_LOCK(&rw->lock_object, LOP_EXCLUSIVE, file, line);
        curthread->td_locks++;
}

int
_rw_try_wlock(struct rwlock *rw, const char *file, int line)
{
        int rval;

        KASSERT(rw->rw_lock != RW_DESTROYED,
            ("rw_try_wlock() of destroyed rwlock @ %s:%d", file, line));

        if (rw_wlocked(rw) && (rw->lock_object.lo_flags & RW_RECURSE) != 0) {
                rw->rw_recurse++;
                rval = 1;
        } else
                rval = atomic_cmpset_acq_ptr(&rw->rw_lock, RW_UNLOCKED,
                    (uintptr_t)curthread);

        LOCK_LOG_TRY("WLOCK", &rw->lock_object, 0, rval, file, line);
        if (rval) {
                WITNESS_LOCK(&rw->lock_object, LOP_EXCLUSIVE | LOP_TRYLOCK,
                    file, line);
                curthread->td_locks++;
        }
        return (rval);
}

void
_rw_wunlock(struct rwlock *rw, const char *file, int line)
{

        MPASS(curthread != NULL);
        KASSERT(rw->rw_lock != RW_DESTROYED,
            ("rw_wunlock() of destroyed rwlock @ %s:%d", file, line));
        _rw_assert(rw, RA_WLOCKED, file, line);
        curthread->td_locks--;
        WITNESS_UNLOCK(&rw->lock_object, LOP_EXCLUSIVE, file, line);
        LOCK_LOG_LOCK("WUNLOCK", &rw->lock_object, 0, rw->rw_recurse, file,
            line);
        if (!rw_recursed(rw))
                lock_profile_release_lock(&rw->lock_object);
        __rw_wunlock(rw, curthread, file, line);
}
/*
 * Determines whether a new reader can acquire a lock.  Succeeds if the
 * reader already owns a read lock and the lock is locked for read to
 * prevent deadlock from reader recursion.  Also succeeds if the lock
 * is unlocked and has no writer waiters or spinners.  Failing otherwise
 * prioritizes writers before readers.
 */
#define RW_CAN_READ(_rw)                                                \
    ((curthread->td_rw_rlocks && (_rw) & RW_LOCK_READ) || ((_rw) &      \
    (RW_LOCK_READ | RW_LOCK_WRITE_WAITERS | RW_LOCK_WRITE_SPINNER)) ==  \
    RW_LOCK_READ)

void
_rw_rlock(struct rwlock *rw, const char *file, int line)
{
        struct turnstile *ts;
#ifdef ADAPTIVE_RWLOCKS
        volatile struct thread *owner;
        int spintries = 0;
        int i;
#endif
        uint64_t waittime = 0;
        int contested = 0;
        uintptr_t v;

        KASSERT(rw->rw_lock != RW_DESTROYED,
            ("rw_rlock() of destroyed rwlock @ %s:%d", file, line));
        KASSERT(rw_wowner(rw) != curthread,
            ("%s (%s): wlock already held @ %s:%d", __func__,
            rw->lock_object.lo_name, file, line));
        WITNESS_CHECKORDER(&rw->lock_object, LOP_NEWORDER, file, line);

        for (;;) {
                /*
                 * Handle the easy case.  If no other thread has a write
                 * lock, then try to bump up the count of read locks.  Note
                 * that we have to preserve the current state of the
                 * RW_LOCK_WRITE_WAITERS flag.  If we fail to acquire a
                 * read lock, then rw_lock must have changed, so restart
                 * the loop.  Note that this handles the case of a
                 * completely unlocked rwlock since such a lock is encoded
                 * as a read lock with no waiters.
                 */
                v = rw->rw_lock;
                if (RW_CAN_READ(v)) {
                        /*
                         * The RW_LOCK_READ_WAITERS flag should only be set
                         * if the lock has been unlocked and write waiters
                         * were present.
                         */
                        if (atomic_cmpset_acq_ptr(&rw->rw_lock, v,
                            v + RW_ONE_READER)) {
                                if (LOCK_LOG_TEST(&rw->lock_object, 0))
                                        CTR4(KTR_LOCK,
                                            "%s: %p succeed %p -> %p", __func__,
                                            rw, (void *)v,
                                            (void *)(v + RW_ONE_READER));
                                break;
                        }
                        cpu_spinwait();
                        continue;
                }
                lock_profile_obtain_lock_failed(&rw->lock_object,
                    &contested, &waittime);

#ifdef ADAPTIVE_RWLOCKS
                /*
                 * If the owner is running on another CPU, spin until
                 * the owner stops running or the state of the lock
                 * changes.
                 */
                if ((v & RW_LOCK_READ) == 0) {
                        owner = (struct thread *)RW_OWNER(v);
                        if (TD_IS_RUNNING(owner)) {
                                if (LOCK_LOG_TEST(&rw->lock_object, 0))
                                        CTR3(KTR_LOCK,
                                            "%s: spinning on %p held by %p",
                                            __func__, rw, owner);
                                while ((struct thread*)RW_OWNER(rw->rw_lock) ==
                                    owner && TD_IS_RUNNING(owner))
                                        cpu_spinwait();
                                continue;
                        }
                } else if (spintries < rowner_retries) {
                        spintries++;
                        for (i = 0; i < rowner_loops; i++) {
                                v = rw->rw_lock;
                                if ((v & RW_LOCK_READ) == 0 || RW_CAN_READ(v))
                                        break;
                                cpu_spinwait();
                        }
                        if (i != rowner_loops)
                                continue;
                }
#endif

                /*
                 * Okay, now it's the hard case.  Some other thread already
                 * has a write lock or there are write waiters present,
                 * acquire the turnstile lock so we can begin the process
                 * of blocking.
                 */
                ts = turnstile_trywait(&rw->lock_object);

                /*
                 * The lock might have been released while we spun, so
                 * recheck its state and restart the loop if needed.
                 */
                v = rw->rw_lock;
                if (RW_CAN_READ(v)) {
                        turnstile_cancel(ts);
                        cpu_spinwait();
                        continue;
                }

#ifdef ADAPTIVE_RWLOCKS
                /*
                 * If the current owner of the lock is executing on another
                 * CPU quit the hard path and try to spin.
                 */
                if ((v & RW_LOCK_READ) == 0) {
                        owner = (struct thread *)RW_OWNER(v);
                        if (TD_IS_RUNNING(owner)) {
                                turnstile_cancel(ts);
                                cpu_spinwait();
                                continue;
                        }
                }
#endif

                /*
                 * The lock is held in write mode or it already has waiters.
                 */
                MPASS(!RW_CAN_READ(v));

                /*
                 * If the RW_LOCK_READ_WAITERS flag is already set, then
                 * we can go ahead and block.  If it is not set then try
                 * to set it.  If we fail to set it drop the turnstile
                 * lock and restart the loop.
                 */
                if (!(v & RW_LOCK_READ_WAITERS)) {
                        if (!atomic_cmpset_ptr(&rw->rw_lock, v,
                            v | RW_LOCK_READ_WAITERS)) {
                                turnstile_cancel(ts);
                                cpu_spinwait();
                                continue;
                        }
                        if (LOCK_LOG_TEST(&rw->lock_object, 0))
                                CTR2(KTR_LOCK, "%s: %p set read waiters flag",
                                    __func__, rw);
                }

                /*
                 * We were unable to acquire the lock and the read waiters
                 * flag is set, so we must block on the turnstile.
                 */
                if (LOCK_LOG_TEST(&rw->lock_object, 0))
                        CTR2(KTR_LOCK, "%s: %p blocking on turnstile", __func__,
                            rw);
                turnstile_wait(ts, rw_owner(rw), TS_SHARED_QUEUE);
                if (LOCK_LOG_TEST(&rw->lock_object, 0))
                        CTR2(KTR_LOCK, "%s: %p resuming from turnstile",
                            __func__, rw);
        }

        /*
         * TODO: acquire "owner of record" here.  Here be turnstile dragons
         * however.  turnstiles don't like owners changing between calls to
         * turnstile_wait() currently.
         */
        lock_profile_obtain_lock_success( &rw->lock_object, contested,
            waittime, file, line);
        LOCK_LOG_LOCK("RLOCK", &rw->lock_object, 0, 0, file, line);
        WITNESS_LOCK(&rw->lock_object, 0, file, line);
        curthread->td_locks++;
        curthread->td_rw_rlocks++;
}

int
_rw_try_rlock(struct rwlock *rw, const char *file, int line)
{
        uintptr_t x;

        for (;;) {
                x = rw->rw_lock;
                KASSERT(rw->rw_lock != RW_DESTROYED,
                    ("rw_try_rlock() of destroyed rwlock @ %s:%d", file, line));
                if (!(x & RW_LOCK_READ))
                        break;
                if (atomic_cmpset_acq_ptr(&rw->rw_lock, x, x + RW_ONE_READER)) {
                        LOCK_LOG_TRY("RLOCK", &rw->lock_object, 0, 1, file,
                            line);
                        WITNESS_LOCK(&rw->lock_object, LOP_TRYLOCK, file, line);
                        curthread->td_locks++;
                        curthread->td_rw_rlocks++;
                        return (1);
                }
        }

        LOCK_LOG_TRY("RLOCK", &rw->lock_object, 0, 0, file, line);
        return (0);
}

void
_rw_runlock(struct rwlock *rw, const char *file, int line)
{
        struct turnstile *ts;
        uintptr_t x, v, queue;

        KASSERT(rw->rw_lock != RW_DESTROYED,
            ("rw_runlock() of destroyed rwlock @ %s:%d", file, line));
        _rw_assert(rw, RA_RLOCKED, file, line);
        curthread->td_locks--;
        curthread->td_rw_rlocks--;
        WITNESS_UNLOCK(&rw->lock_object, 0, file, line);
        LOCK_LOG_LOCK("RUNLOCK", &rw->lock_object, 0, 0, file, line);

        /* TODO: drop "owner of record" here. */

        for (;;) {
                /*
                 * See if there is more than one read lock held.  If so,
                 * just drop one and return.
                 */
                x = rw->rw_lock;
                if (RW_READERS(x) > 1) {
                        if (atomic_cmpset_ptr(&rw->rw_lock, x,
                            x - RW_ONE_READER)) {
                                if (LOCK_LOG_TEST(&rw->lock_object, 0))
                                        CTR4(KTR_LOCK,
                                            "%s: %p succeeded %p -> %p",
                                            __func__, rw, (void *)x,
                                            (void *)(x - RW_ONE_READER));
                                break;
                        }
                        continue;
                }
                /*
                 * If there aren't any waiters for a write lock, then try
                 * to drop it quickly.
                 */
                if (!(x & RW_LOCK_WAITERS)) {
                        MPASS((x & ~RW_LOCK_WRITE_SPINNER) ==
                            RW_READERS_LOCK(1));
                        if (atomic_cmpset_ptr(&rw->rw_lock, x, RW_UNLOCKED)) {
                                if (LOCK_LOG_TEST(&rw->lock_object, 0))
                                        CTR2(KTR_LOCK, "%s: %p last succeeded",
                                            __func__, rw);
                                break;
                        }
                        continue;
                }
                /*
                 * Ok, we know we have waiters and we think we are the
                 * last reader, so grab the turnstile lock.
                 */
                turnstile_chain_lock(&rw->lock_object);
                v = rw->rw_lock & (RW_LOCK_WAITERS | RW_LOCK_WRITE_SPINNER);
                MPASS(v & RW_LOCK_WAITERS);

                /*
                 * Try to drop our lock leaving the lock in a unlocked
                 * state.
                 *
                 * If you wanted to do explicit lock handoff you'd have to
                 * do it here.  You'd also want to use turnstile_signal()
                 * and you'd have to handle the race where a higher
                 * priority thread blocks on the write lock before the
                 * thread you wakeup actually runs and have the new thread
                 * "steal" the lock.  For now it's a lot simpler to just
                 * wakeup all of the waiters.
                 *
                 * As above, if we fail, then another thread might have
                 * acquired a read lock, so drop the turnstile lock and
                 * restart.
                 */
                x = RW_UNLOCKED;
                if (v & RW_LOCK_WRITE_WAITERS) {
                        queue = TS_EXCLUSIVE_QUEUE;
                        x |= (v & RW_LOCK_READ_WAITERS);
                } else
                        queue = TS_SHARED_QUEUE;
                if (!atomic_cmpset_ptr(&rw->rw_lock, RW_READERS_LOCK(1) | v,
                    x)) {
                        turnstile_chain_unlock(&rw->lock_object);
                        continue;
                }
                if (LOCK_LOG_TEST(&rw->lock_object, 0))
                        CTR2(KTR_LOCK, "%s: %p last succeeded with waiters",
                            __func__, rw);

                /*
                 * Ok.  The lock is released and all that's left is to
                 * wake up the waiters.  Note that the lock might not be
                 * free anymore, but in that case the writers will just
                 * block again if they run before the new lock holder(s)
                 * release the lock.
                 */
                ts = turnstile_lookup(&rw->lock_object);
                MPASS(ts != NULL);
                turnstile_broadcast(ts, queue);
                turnstile_unpend(ts, TS_SHARED_LOCK);
                turnstile_chain_unlock(&rw->lock_object);
                break;
        }
        lock_profile_release_lock(&rw->lock_object);
}

/*
 * This function is called when we are unable to obtain a write lock on the
 * first try.  This means that at least one other thread holds either a
 * read or write lock.
 */
void
_rw_wlock_hard(struct rwlock *rw, uintptr_t tid, const char *file, int line)
{
        struct turnstile *ts;
#ifdef ADAPTIVE_RWLOCKS
        volatile struct thread *owner;
        int spintries = 0;
        int i;
#endif
        uint64_t waittime = 0;
        uintptr_t v, x;
        int contested = 0;

        if (rw_wlocked(rw)) {
                KASSERT(rw->lock_object.lo_flags & RW_RECURSE,
                    ("%s: recursing but non-recursive rw %s @ %s:%d\n",
                    __func__, rw->lock_object.lo_name, file, line));
                rw->rw_recurse++;
                if (LOCK_LOG_TEST(&rw->lock_object, 0))
                        CTR2(KTR_LOCK, "%s: %p recursing", __func__, rw);
                return;
        }

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

        while (!_rw_write_lock(rw, tid)) {
                lock_profile_obtain_lock_failed(&rw->lock_object,
                    &contested, &waittime);
#ifdef ADAPTIVE_RWLOCKS
                /*
                 * 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.
                 */
                v = rw->rw_lock;
                owner = (struct thread *)RW_OWNER(v);
                if (!(v & RW_LOCK_READ) && TD_IS_RUNNING(owner)) {
                        if (LOCK_LOG_TEST(&rw->lock_object, 0))
                                CTR3(KTR_LOCK, "%s: spinning on %p held by %p",
                                    __func__, rw, owner);
                        while ((struct thread*)RW_OWNER(rw->rw_lock) == owner &&
                            TD_IS_RUNNING(owner))
                                cpu_spinwait();
                        continue;
                }
                if ((v & RW_LOCK_READ) && RW_READERS(v) &&
                    spintries < rowner_retries) {
                        if (!(v & RW_LOCK_WRITE_SPINNER)) {
                                if (!atomic_cmpset_ptr(&rw->rw_lock, v,
                                    v | RW_LOCK_WRITE_SPINNER)) {
                                        cpu_spinwait();
                                        continue;
                                }
                        }
                        spintries++;
                        for (i = 0; i < rowner_loops; i++) {
                                if ((rw->rw_lock & RW_LOCK_WRITE_SPINNER) == 0)
                                        break;
                                cpu_spinwait();
                        }
                        if (i != rowner_loops)
                                continue;
                }
#endif
                ts = turnstile_trywait(&rw->lock_object);
                v = rw->rw_lock;

#ifdef ADAPTIVE_RWLOCKS
                /*
                 * If the current owner of the lock is executing on another
                 * CPU quit the hard path and try to spin.
                 */
                if (!(v & RW_LOCK_READ)) {
                        owner = (struct thread *)RW_OWNER(v);
                        if (TD_IS_RUNNING(owner)) {
                                turnstile_cancel(ts);
                                cpu_spinwait();
                                continue;
                        }
                }
#endif
                /*
                 * Check for the waiters flags about this rwlock.
                 * If the lock was released, without maintain any pending
                 * waiters queue, simply try to acquire it.
                 * If a pending waiters queue is present, claim the lock
                 * ownership and maintain the pending queue.
                 */
                x = v & (RW_LOCK_WAITERS | RW_LOCK_WRITE_SPINNER);
                if ((v & ~x) == RW_UNLOCKED) {
                        x &= ~RW_LOCK_WRITE_SPINNER;
                        if (atomic_cmpset_acq_ptr(&rw->rw_lock, v, tid | x)) {
                                if (x)
                                        turnstile_claim(ts);
                                else
                                        turnstile_cancel(ts);
                                break;
                        }
                        turnstile_cancel(ts);
                        cpu_spinwait();
                        continue;
                }
                /*
                 * If the RW_LOCK_WRITE_WAITERS flag isn't set, then try to
                 * set it.  If we fail to set it, then loop back and try
                 * again.
                 */
                if (!(v & RW_LOCK_WRITE_WAITERS)) {
                        if (!atomic_cmpset_ptr(&rw->rw_lock, v,
                            v | RW_LOCK_WRITE_WAITERS)) {
                                turnstile_cancel(ts);
                                cpu_spinwait();
                                continue;
                        }
                        if (LOCK_LOG_TEST(&rw->lock_object, 0))
                                CTR2(KTR_LOCK, "%s: %p set write waiters flag",
                                    __func__, rw);
                }
                /*
                 * We were unable to acquire the lock and the write waiters
                 * flag is set, so we must block on the turnstile.
                 */
                if (LOCK_LOG_TEST(&rw->lock_object, 0))
                        CTR2(KTR_LOCK, "%s: %p blocking on turnstile", __func__,
                            rw);
                turnstile_wait(ts, rw_owner(rw), TS_EXCLUSIVE_QUEUE);
                if (LOCK_LOG_TEST(&rw->lock_object, 0))
                        CTR2(KTR_LOCK, "%s: %p resuming from turnstile",
                            __func__, rw);
#ifdef ADAPTIVE_RWLOCKS
                spintries = 0;
#endif
        }
        lock_profile_obtain_lock_success(&rw->lock_object, contested, waittime,
            file, line);
}

/*
 * This function is called if the first try at releasing a write lock failed.
 * This means that one of the 2 waiter bits must be set indicating that at
 * least one thread is waiting on this lock.
 */
void
_rw_wunlock_hard(struct rwlock *rw, uintptr_t tid, const char *file, int line)
{
        struct turnstile *ts;
        uintptr_t v;
        int queue;

        if (rw_wlocked(rw) && rw_recursed(rw)) {
                rw->rw_recurse--;
                if (LOCK_LOG_TEST(&rw->lock_object, 0))
                        CTR2(KTR_LOCK, "%s: %p unrecursing", __func__, rw);
                return;
        }
        v = rw->rw_lock;

        KASSERT(rw->rw_lock & (RW_LOCK_READ_WAITERS | RW_LOCK_WRITE_WAITERS),
            ("%s: neither of the waiter flags are set", __func__));

        if (LOCK_LOG_TEST(&rw->lock_object, 0))
                CTR2(KTR_LOCK, "%s: %p contested", __func__, rw);

        turnstile_chain_lock(&rw->lock_object);
        ts = turnstile_lookup(&rw->lock_object);

        MPASS(ts != NULL);

        /*
         * Use the same algo as sx locks for now.  Prefer waking up shared
         * waiters if we have any over writers.  This is probably not ideal.
         *
         * 'v' is the value we are going to write back to rw_lock.  If we
         * have waiters on both queues, we need to preserve the state of
         * the waiter flag for the queue we don't wake up.  For now this is
         * hardcoded for the algorithm mentioned above.
         *
         * In the case of both readers and writers waiting we wakeup the
         * readers but leave the RW_LOCK_WRITE_WAITERS flag set.  If a
         * new writer comes in before a reader it will claim the lock up
         * above.  There is probably a potential priority inversion in
         * there that could be worked around either by waking both queues
         * of waiters or doing some complicated lock handoff gymnastics.
         */
        v = RW_UNLOCKED;
        if (rw->rw_lock & RW_LOCK_WRITE_WAITERS) {
                queue = TS_EXCLUSIVE_QUEUE;
                v |= (rw->rw_lock & RW_LOCK_READ_WAITERS);
        } else
                queue = TS_SHARED_QUEUE;

        /* Wake up all waiters for the specific queue. */
        if (LOCK_LOG_TEST(&rw->lock_object, 0))
                CTR3(KTR_LOCK, "%s: %p waking up %s waiters", __func__, rw,
                    queue == TS_SHARED_QUEUE ? "read" : "write");
        turnstile_broadcast(ts, queue);
        atomic_store_rel_ptr(&rw->rw_lock, v);
        turnstile_unpend(ts, TS_EXCLUSIVE_LOCK);
        turnstile_chain_unlock(&rw->lock_object);
}

/*
 * Attempt to do a non-blocking upgrade from a read lock to a write
 * lock.  This will only succeed if this thread holds a single read
 * lock.  Returns true if the upgrade succeeded and false otherwise.
 */
int
_rw_try_upgrade(struct rwlock *rw, const char *file, int line)
{
        uintptr_t v, x, tid;
        struct turnstile *ts;
        int success;

        KASSERT(rw->rw_lock != RW_DESTROYED,
            ("rw_try_upgrade() of destroyed rwlock @ %s:%d", file, line));
        _rw_assert(rw, RA_RLOCKED, file, line);

        /*
         * Attempt to switch from one reader to a writer.  If there
         * are any write waiters, then we will have to lock the
         * turnstile first to prevent races with another writer
         * calling turnstile_wait() before we have claimed this
         * turnstile.  So, do the simple case of no waiters first.
         */
        tid = (uintptr_t)curthread;
        success = 0;
        for (;;) {
                v = rw->rw_lock;
                if (RW_READERS(v) > 1)
                        break;
                if (!(v & RW_LOCK_WAITERS)) {
                        success = atomic_cmpset_ptr(&rw->rw_lock, v, tid);
                        if (!success)
                                continue;
                        break;
                }

                /*
                 * Ok, we think we have waiters, so lock the turnstile.
                 */
                ts = turnstile_trywait(&rw->lock_object);
                v = rw->rw_lock;
                if (RW_READERS(v) > 1) {
                        turnstile_cancel(ts);
                        break;
                }
                /*
                 * Try to switch from one reader to a writer again.  This time
                 * we honor the current state of the waiters flags.
                 * If we obtain the lock with the flags set, then claim
                 * ownership of the turnstile.
                 */
                x = rw->rw_lock & RW_LOCK_WAITERS;
                success = atomic_cmpset_ptr(&rw->rw_lock, v, tid | x);
                if (success) {
                        if (x)
                                turnstile_claim(ts);
                        else
                                turnstile_cancel(ts);
                        break;
                }
                turnstile_cancel(ts);
        }
        LOCK_LOG_TRY("WUPGRADE", &rw->lock_object, 0, success, file, line);
        if (success) {
                curthread->td_rw_rlocks--;
                WITNESS_UPGRADE(&rw->lock_object, LOP_EXCLUSIVE | LOP_TRYLOCK,
                    file, line);
        }
        return (success);
}

/*
 * Downgrade a write lock into a single read lock.
 */
void
_rw_downgrade(struct rwlock *rw, const char *file, int line)
{
        struct turnstile *ts;
        uintptr_t tid, v;
        int rwait, wwait;

        KASSERT(rw->rw_lock != RW_DESTROYED,
            ("rw_downgrade() of destroyed rwlock @ %s:%d", file, line));
        _rw_assert(rw, RA_WLOCKED | RA_NOTRECURSED, file, line);
#ifndef INVARIANTS
        if (rw_recursed(rw))
                panic("downgrade of a recursed lock");
#endif

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

        /*
         * Convert from a writer to a single reader.  First we handle
         * the easy case with no waiters.  If there are any waiters, we
         * lock the turnstile and "disown" the lock.
         */
        tid = (uintptr_t)curthread;
        if (atomic_cmpset_rel_ptr(&rw->rw_lock, tid, RW_READERS_LOCK(1)))
                goto out;

        /*
         * Ok, we think we have waiters, so lock the turnstile so we can
         * read the waiter flags without any races.
         */
        turnstile_chain_lock(&rw->lock_object);
        v = rw->rw_lock & RW_LOCK_WAITERS;
        rwait = v & RW_LOCK_READ_WAITERS;
        wwait = v & RW_LOCK_WRITE_WAITERS;
        MPASS(rwait | wwait);

        /*
         * Downgrade from a write lock while preserving waiters flag
         * and give up ownership of the turnstile.
         */
        ts = turnstile_lookup(&rw->lock_object);
        MPASS(ts != NULL);
        if (!wwait)
                v &= ~RW_LOCK_READ_WAITERS;
        atomic_store_rel_ptr(&rw->rw_lock, RW_READERS_LOCK(1) | v);
        /*
         * Wake other readers if there are no writers pending.  Otherwise they
         * won't be able to acquire the lock anyway.
         */
        if (rwait && !wwait) {
                turnstile_broadcast(ts, TS_SHARED_QUEUE);
                turnstile_unpend(ts, TS_EXCLUSIVE_LOCK);
        } else
                turnstile_disown(ts);
        turnstile_chain_unlock(&rw->lock_object);
out:
        curthread->td_rw_rlocks++;
        LOCK_LOG_LOCK("WDOWNGRADE", &rw->lock_object, 0, 0, file, line);
}

#ifdef INVARIANT_SUPPORT
#ifndef INVARIANTS
#undef _rw_assert
#endif

/*
 * In the non-WITNESS case, rw_assert() can only detect that at least
 * *some* thread owns an rlock, but it cannot guarantee that *this*
 * thread owns an rlock.
 */
void
_rw_assert(struct rwlock *rw, int what, const char *file, int line)
{

        if (panicstr != NULL)
                return;
        switch (what) {
        case RA_LOCKED:
        case RA_LOCKED | RA_RECURSED:
        case RA_LOCKED | RA_NOTRECURSED:
        case RA_RLOCKED:
#ifdef WITNESS
                witness_assert(&rw->lock_object, what, file, line);
#else
                /*
                 * If some other thread has a write lock or we have one
                 * and are asserting a read lock, fail.  Also, if no one
                 * has a lock at all, fail.
                 */
                if (rw->rw_lock == RW_UNLOCKED ||
                    (!(rw->rw_lock & RW_LOCK_READ) && (what == RA_RLOCKED ||
                    rw_wowner(rw) != curthread)))
                        panic("Lock %s not %slocked @ %s:%d\n",
                            rw->lock_object.lo_name, (what == RA_RLOCKED) ?
                            "read " : "", file, line);

                if (!(rw->rw_lock & RW_LOCK_READ)) {
                        if (rw_recursed(rw)) {
                                if (what & RA_NOTRECURSED)
                                        panic("Lock %s recursed @ %s:%d\n",
                                            rw->lock_object.lo_name, file,
                                            line);
                        } else if (what & RA_RECURSED)
                                panic("Lock %s not recursed @ %s:%d\n",
                                    rw->lock_object.lo_name, file, line);
                }
#endif
                break;
        case RA_WLOCKED:
        case RA_WLOCKED | RA_RECURSED:
        case RA_WLOCKED | RA_NOTRECURSED:
                if (rw_wowner(rw) != curthread)
                        panic("Lock %s not exclusively locked @ %s:%d\n",
                            rw->lock_object.lo_name, file, line);
                if (rw_recursed(rw)) {
                        if (what & RA_NOTRECURSED)
                                panic("Lock %s recursed @ %s:%d\n",
                                    rw->lock_object.lo_name, file, line);
                } else if (what & RA_RECURSED)
                        panic("Lock %s not recursed @ %s:%d\n",
                            rw->lock_object.lo_name, file, line);
                break;
        case RA_UNLOCKED:
#ifdef WITNESS
                witness_assert(&rw->lock_object, what, file, line);
#else
                /*
                 * If we hold a write lock fail.  We can't reliably check
                 * to see if we hold a read lock or not.
                 */
                if (rw_wowner(rw) == curthread)
                        panic("Lock %s exclusively locked @ %s:%d\n",
                            rw->lock_object.lo_name, file, line);
#endif
                break;
        default:
                panic("Unknown rw lock assertion: %d @ %s:%d", what, file,
                    line);
        }
}
#endif /* INVARIANT_SUPPORT */

#ifdef DDB
void
db_show_rwlock(struct lock_object *lock)
{
        struct rwlock *rw;
        struct thread *td;

        rw = (struct rwlock *)lock;

        db_printf(" state: ");
        if (rw->rw_lock == RW_UNLOCKED)
                db_printf("UNLOCKED\n");
        else if (rw->rw_lock == RW_DESTROYED) {
                db_printf("DESTROYED\n");
                return;
        } else if (rw->rw_lock & RW_LOCK_READ)
                db_printf("RLOCK: %ju locks\n",
                    (uintmax_t)(RW_READERS(rw->rw_lock)));
        else {
                td = rw_wowner(rw);
                db_printf("WLOCK: %p (tid %d, pid %d, \"%s\")\n", td,
                    td->td_tid, td->td_proc->p_pid, td->td_name);
                if (rw_recursed(rw))
                        db_printf(" recursed: %u\n", rw->rw_recurse);
        }
        db_printf(" waiters: ");
        switch (rw->rw_lock & (RW_LOCK_READ_WAITERS | RW_LOCK_WRITE_WAITERS)) {
        case RW_LOCK_READ_WAITERS:
                db_printf("readers\n");
                break;
        case RW_LOCK_WRITE_WAITERS:
                db_printf("writers\n");
                break;
        case RW_LOCK_READ_WAITERS | RW_LOCK_WRITE_WAITERS:
                db_printf("readers and writers\n");
                break;
        default:
                db_printf("none\n");
                break;
        }
}

#endif