import less(1)

[unleashed/tickless.git] / usr / src / lib / libc / port / threads / rwlock.c

/*
 * CDDL HEADER START
 *
 * The contents of this file are subject to the terms of the
 * Common Development and Distribution License (the "License").
 * You may not use this file except in compliance with the License.
 *
 * You can obtain a copy of the license at usr/src/OPENSOLARIS.LICENSE
 * or http://www.opensolaris.org/os/licensing.
 * See the License for the specific language governing permissions
 * and limitations under the License.
 *
 * When distributing Covered Code, include this CDDL HEADER in each
 * file and include the License file at usr/src/OPENSOLARIS.LICENSE.
 * If applicable, add the following below this CDDL HEADER, with the
 * fields enclosed by brackets "[]" replaced with your own identifying
 * information: Portions Copyright [yyyy] [name of copyright owner]
 *
 * CDDL HEADER END
 */

/*
 * Copyright (c) 1999, 2010, Oracle and/or its affiliates. All rights reserved.
 * Copyright (c) 2016 by Delphix. All rights reserved.
 */

#include "lint.h"
#include "thr_uberdata.h"
#include <sys/sdt.h>

#define TRY_FLAG                0x10
#define READ_LOCK               0
#define WRITE_LOCK              1
#define READ_LOCK_TRY           (READ_LOCK | TRY_FLAG)
#define WRITE_LOCK_TRY          (WRITE_LOCK | TRY_FLAG)

#define NLOCKS  4       /* initial number of readlock_t structs allocated */

#define ASSERT_CONSISTENT_STATE(readers)                \
        ASSERT(!((readers) & URW_WRITE_LOCKED) ||       \
                ((readers) & ~URW_HAS_WAITERS) == URW_WRITE_LOCKED)

/*
 * Find/allocate an entry for rwlp in our array of rwlocks held for reading.
 * We must be deferring signals for this to be safe.
 * Else if we are returning an entry with ul_rdlockcnt == 0,
 * it could be reassigned behind our back in a signal handler.
 */
static readlock_t *
rwl_entry(rwlock_t *rwlp)
{
        ulwp_t *self = curthread;
        readlock_t *remembered = NULL;
        readlock_t *readlockp;
        uint_t nlocks;

        /* we must be deferring signals */
        ASSERT((self->ul_critical + self->ul_sigdefer) != 0);

        if ((nlocks = self->ul_rdlockcnt) != 0)
                readlockp = self->ul_readlock.array;
        else {
                nlocks = 1;
                readlockp = &self->ul_readlock.single;
        }

        for (; nlocks; nlocks--, readlockp++) {
                if (readlockp->rd_rwlock == rwlp)
                        return (readlockp);
                if (readlockp->rd_count == 0 && remembered == NULL)
                        remembered = readlockp;
        }
        if (remembered != NULL) {
                remembered->rd_rwlock = rwlp;
                return (remembered);
        }

        /*
         * No entry available.  Allocate more space, converting the single
         * readlock_t entry into an array of readlock_t entries if necessary.
         */
        if ((nlocks = self->ul_rdlockcnt) == 0) {
                /*
                 * Initial allocation of the readlock_t array.
                 * Convert the single entry into an array.
                 */
                self->ul_rdlockcnt = nlocks = NLOCKS;
                readlockp = lmalloc(nlocks * sizeof (readlock_t));
                /*
                 * The single readlock_t becomes the first entry in the array.
                 */
                *readlockp = self->ul_readlock.single;
                self->ul_readlock.single.rd_count = 0;
                self->ul_readlock.array = readlockp;
                /*
                 * Return the next available entry in the array.
                 */
                (++readlockp)->rd_rwlock = rwlp;
                return (readlockp);
        }
        /*
         * Reallocate the array, double the size each time.
         */
        readlockp = lmalloc(nlocks * 2 * sizeof (readlock_t));
        (void) memcpy(readlockp, self->ul_readlock.array,
            nlocks * sizeof (readlock_t));
        lfree(self->ul_readlock.array, nlocks * sizeof (readlock_t));
        self->ul_readlock.array = readlockp;
        self->ul_rdlockcnt *= 2;
        /*
         * Return the next available entry in the newly allocated array.
         */
        (readlockp += nlocks)->rd_rwlock = rwlp;
        return (readlockp);
}

/*
 * Free the array of rwlocks held for reading.
 */
void
rwl_free(ulwp_t *ulwp)
{
        uint_t nlocks;

        if ((nlocks = ulwp->ul_rdlockcnt) != 0)
                lfree(ulwp->ul_readlock.array, nlocks * sizeof (readlock_t));
        ulwp->ul_rdlockcnt = 0;
        ulwp->ul_readlock.single.rd_rwlock = NULL;
        ulwp->ul_readlock.single.rd_count = 0;
}

/*
 * Check if a reader version of the lock is held by the current thread.
 */
#pragma weak _rw_read_held = rw_read_held
int
rw_read_held(rwlock_t *rwlp)
{
        volatile uint32_t *rwstate = (volatile uint32_t *)&rwlp->rwlock_readers;
        uint32_t readers;
        ulwp_t *self = curthread;
        readlock_t *readlockp;
        uint_t nlocks;
        int rval = 0;

        no_preempt(self);

        readers = *rwstate;
        ASSERT_CONSISTENT_STATE(readers);
        if (!(readers & URW_WRITE_LOCKED) &&
            (readers & URW_READERS_MASK) != 0) {
                /*
                 * The lock is held for reading by some thread.
                 * Search our array of rwlocks held for reading for a match.
                 */
                if ((nlocks = self->ul_rdlockcnt) != 0)
                        readlockp = self->ul_readlock.array;
                else {
                        nlocks = 1;
                        readlockp = &self->ul_readlock.single;
                }
                for (; nlocks; nlocks--, readlockp++) {
                        if (readlockp->rd_rwlock == rwlp) {
                                if (readlockp->rd_count)
                                        rval = 1;
                                break;
                        }
                }
        }

        preempt(self);
        return (rval);
}

/*
 * Check if a writer version of the lock is held by the current thread.
 */
#pragma weak _rw_write_held = rw_write_held
int
rw_write_held(rwlock_t *rwlp)
{
        volatile uint32_t *rwstate = (volatile uint32_t *)&rwlp->rwlock_readers;
        uint32_t readers;
        ulwp_t *self = curthread;
        int rval;

        no_preempt(self);

        readers = *rwstate;
        ASSERT_CONSISTENT_STATE(readers);
        rval = ((readers & URW_WRITE_LOCKED) &&
            rwlp->rwlock_owner == (uintptr_t)self &&
            (rwlp->rwlock_type == USYNC_THREAD ||
            rwlp->rwlock_ownerpid == self->ul_uberdata->pid));

        preempt(self);
        return (rval);
}

#pragma weak _rwlock_init = rwlock_init
/* ARGSUSED2 */
int
rwlock_init(rwlock_t *rwlp, int type, void *arg)
{
        ulwp_t *self = curthread;

        if (type != USYNC_THREAD && type != USYNC_PROCESS)
                return (EINVAL);
        /*
         * Once reinitialized, we can no longer be holding a read or write lock.
         * We can do nothing about other threads that are holding read locks.
         */
        sigoff(self);
        rwl_entry(rwlp)->rd_count = 0;
        sigon(self);
        (void) memset(rwlp, 0, sizeof (*rwlp));
        rwlp->rwlock_type = (uint16_t)type;
        rwlp->rwlock_magic = RWL_MAGIC;
        rwlp->mutex.mutex_type = (uint8_t)type;
        rwlp->mutex.mutex_flag = LOCK_INITED;
        rwlp->mutex.mutex_magic = MUTEX_MAGIC;

        /*
         * This should be at the beginning of the function,
         * but for the sake of old broken applications that
         * do not have proper alignment for their rwlocks
         * (and don't check the return code from rwlock_init),
         * we put it here, after initializing the rwlock regardless.
         */
        if (((uintptr_t)rwlp & (_LONG_LONG_ALIGNMENT - 1)) &&
            self->ul_misaligned == 0)
                return (EINVAL);

        return (0);
}

#pragma weak pthread_rwlock_destroy = rwlock_destroy
#pragma weak _rwlock_destroy = rwlock_destroy
int
rwlock_destroy(rwlock_t *rwlp)
{
        ulwp_t *self = curthread;

        /*
         * Once destroyed, we can no longer be holding a read or write lock.
         * We can do nothing about other threads that are holding read locks.
         */
        sigoff(self);
        rwl_entry(rwlp)->rd_count = 0;
        sigon(self);
        rwlp->rwlock_magic = 0;
        tdb_sync_obj_deregister(rwlp);
        return (0);
}

/*
 * The following four functions:
 *      read_lock_try()
 *      read_unlock_try()
 *      write_lock_try()
 *      write_unlock_try()
 * lie at the heart of the fast-path code for rwlocks,
 * both process-private and process-shared.
 *
 * They are called once without recourse to any other locking primitives.
 * If they succeed, we are done and the fast-path code was successful.
 * If they fail, we have to deal with lock queues, either to enqueue
 * ourself and sleep or to dequeue and wake up someone else (slow paths).
 *
 * Unless 'ignore_waiters_flag' is true (a condition that applies only
 * when read_lock_try() or write_lock_try() is called from code that
 * is already in the slow path and has already acquired the queue lock),
 * these functions will always fail if the waiters flag, URW_HAS_WAITERS,
 * is set in the 'rwstate' word.  Thus, setting the waiters flag on the
 * rwlock and acquiring the queue lock guarantees exclusive access to
 * the rwlock (and is the only way to guarantee exclusive access).
 */

/*
 * Attempt to acquire a readers lock.  Return true on success.
 */
static int
read_lock_try(rwlock_t *rwlp, int ignore_waiters_flag)
{
        volatile uint32_t *rwstate = (volatile uint32_t *)&rwlp->rwlock_readers;
        uint32_t mask = ignore_waiters_flag?
            URW_WRITE_LOCKED : (URW_HAS_WAITERS | URW_WRITE_LOCKED);
        uint32_t readers;
        ulwp_t *self = curthread;

        no_preempt(self);
        while (((readers = *rwstate) & mask) == 0) {
                if (atomic_cas_32(rwstate, readers, readers + 1) == readers) {
                        preempt(self);
                        return (1);
                }
        }
        preempt(self);
        return (0);
}

/*
 * Attempt to release a reader lock.  Return true on success.
 */
static int
read_unlock_try(rwlock_t *rwlp)
{
        volatile uint32_t *rwstate = (volatile uint32_t *)&rwlp->rwlock_readers;
        uint32_t readers;
        ulwp_t *self = curthread;

        no_preempt(self);
        while (((readers = *rwstate) & URW_HAS_WAITERS) == 0) {
                if (atomic_cas_32(rwstate, readers, readers - 1) == readers) {
                        preempt(self);
                        return (1);
                }
        }
        preempt(self);
        return (0);
}

/*
 * Attempt to acquire a writer lock.  Return true on success.
 */
static int
write_lock_try(rwlock_t *rwlp, int ignore_waiters_flag)
{
        volatile uint32_t *rwstate = (volatile uint32_t *)&rwlp->rwlock_readers;
        uint32_t mask = ignore_waiters_flag?
            (URW_WRITE_LOCKED | URW_READERS_MASK) :
            (URW_HAS_WAITERS | URW_WRITE_LOCKED | URW_READERS_MASK);
        ulwp_t *self = curthread;
        uint32_t readers;

        no_preempt(self);
        while (((readers = *rwstate) & mask) == 0) {
                if (atomic_cas_32(rwstate, readers, readers | URW_WRITE_LOCKED)
                    == readers) {
                        preempt(self);
                        return (1);
                }
        }
        preempt(self);
        return (0);
}

/*
 * Attempt to release a writer lock.  Return true on success.
 */
static int
write_unlock_try(rwlock_t *rwlp)
{
        volatile uint32_t *rwstate = (volatile uint32_t *)&rwlp->rwlock_readers;
        uint32_t readers;
        ulwp_t *self = curthread;

        no_preempt(self);
        while (((readers = *rwstate) & URW_HAS_WAITERS) == 0) {
                if (atomic_cas_32(rwstate, readers, 0) == readers) {
                        preempt(self);
                        return (1);
                }
        }
        preempt(self);
        return (0);
}

/*
 * Release a process-private rwlock and wake up any thread(s) sleeping on it.
 * This is called when a thread releases a lock that appears to have waiters.
 */
static void
rw_queue_release(rwlock_t *rwlp)
{
        volatile uint32_t *rwstate = (volatile uint32_t *)&rwlp->rwlock_readers;
        queue_head_t *qp;
        uint32_t readers;
        uint32_t writer;
        ulwp_t **ulwpp;
        ulwp_t *ulwp;
        ulwp_t *prev;
        int nlwpid = 0;
        int more;
        int maxlwps = MAXLWPS;
        lwpid_t buffer[MAXLWPS];
        lwpid_t *lwpid = buffer;

        qp = queue_lock(rwlp, MX);

        /*
         * Here is where we actually drop the lock,
         * but we retain the URW_HAS_WAITERS flag, if it is already set.
         */
        readers = *rwstate;
        ASSERT_CONSISTENT_STATE(readers);
        if (readers & URW_WRITE_LOCKED) /* drop the writer lock */
                atomic_and_32(rwstate, ~URW_WRITE_LOCKED);
        else                            /* drop the readers lock */
                atomic_dec_32(rwstate);
        if (!(readers & URW_HAS_WAITERS)) {     /* no waiters */
                queue_unlock(qp);
                return;
        }

        /*
         * The presence of the URW_HAS_WAITERS flag causes all rwlock
         * code to go through the slow path, acquiring queue_lock(qp).
         * Therefore, the rest of this code is safe because we are
         * holding the queue lock and the URW_HAS_WAITERS flag is set.
         */

        readers = *rwstate;             /* must fetch the value again */
        ASSERT_CONSISTENT_STATE(readers);
        ASSERT(readers & URW_HAS_WAITERS);
        readers &= URW_READERS_MASK;    /* count of current readers */
        writer = 0;                     /* no current writer */

        /*
         * Examine the queue of waiters in priority order and prepare
         * to wake up as many readers as we encounter before encountering
         * a writer.  If the highest priority thread on the queue is a
         * writer, stop there and wake it up.
         *
         * We keep track of lwpids that are to be unparked in lwpid[].
         * __lwp_unpark_all() is called to unpark all of them after
         * they have been removed from the sleep queue and the sleep
         * queue lock has been dropped.  If we run out of space in our
         * on-stack buffer, we need to allocate more but we can't call
         * lmalloc() because we are holding a queue lock when the overflow
         * occurs and lmalloc() acquires a lock.  We can't use alloca()
         * either because the application may have allocated a small
         * stack and we don't want to overrun the stack.  So we call
         * alloc_lwpids() to allocate a bigger buffer using the mmap()
         * system call directly since that path acquires no locks.
         */
        while ((ulwpp = queue_slot(qp, &prev, &more)) != NULL) {
                ulwp = *ulwpp;
                ASSERT(ulwp->ul_wchan == rwlp);
                if (ulwp->ul_writer) {
                        if (writer != 0 || readers != 0)
                                break;
                        /* one writer to wake */
                        writer++;
                } else {
                        if (writer != 0)
                                break;
                        /* at least one reader to wake */
                        readers++;
                        if (nlwpid == maxlwps)
                                lwpid = alloc_lwpids(lwpid, &nlwpid, &maxlwps);
                }
                queue_unlink(qp, ulwpp, prev);
                ulwp->ul_sleepq = NULL;
                ulwp->ul_wchan = NULL;
                if (writer) {
                        /*
                         * Hand off the lock to the writer we will be waking.
                         */
                        ASSERT((*rwstate & ~URW_HAS_WAITERS) == 0);
                        atomic_or_32(rwstate, URW_WRITE_LOCKED);
                        rwlp->rwlock_owner = (uintptr_t)ulwp;
                }
                lwpid[nlwpid++] = ulwp->ul_lwpid;
        }

        /*
         * This modification of rwstate must be done last.
         * The presence of the URW_HAS_WAITERS flag causes all rwlock
         * code to go through the slow path, acquiring queue_lock(qp).
         * Otherwise the read_lock_try() and write_lock_try() fast paths
         * are effective.
         */
        if (ulwpp == NULL)
                atomic_and_32(rwstate, ~URW_HAS_WAITERS);

        if (nlwpid == 0) {
                queue_unlock(qp);
        } else {
                ulwp_t *self = curthread;
                no_preempt(self);
                queue_unlock(qp);
                if (nlwpid == 1)
                        (void) __lwp_unpark(lwpid[0]);
                else
                        (void) __lwp_unpark_all(lwpid, nlwpid);
                preempt(self);
        }
        if (lwpid != buffer)
                (void) munmap((caddr_t)lwpid, maxlwps * sizeof (lwpid_t));
}

/*
 * Common code for rdlock, timedrdlock, wrlock, timedwrlock, tryrdlock,
 * and trywrlock for process-shared (USYNC_PROCESS) rwlocks.
 *
 * Note: if the lock appears to be contended we call __lwp_rwlock_rdlock()
 * or __lwp_rwlock_wrlock() holding the mutex. These return with the mutex
 * released, and if they need to sleep will release the mutex first. In the
 * event of a spurious wakeup, these will return EAGAIN (because it is much
 * easier for us to re-acquire the mutex here).
 */
int
shared_rwlock_lock(rwlock_t *rwlp, timespec_t *tsp, int rd_wr)
{
        volatile uint32_t *rwstate = (volatile uint32_t *)&rwlp->rwlock_readers;
        mutex_t *mp = &rwlp->mutex;
        uint32_t readers;
        int try_flag;
        int error;

        try_flag = (rd_wr & TRY_FLAG);
        rd_wr &= ~TRY_FLAG;
        ASSERT(rd_wr == READ_LOCK || rd_wr == WRITE_LOCK);

        if (!try_flag) {
                DTRACE_PROBE2(plockstat, rw__block, rwlp, rd_wr);
        }

        do {
                if (try_flag && (*rwstate & URW_WRITE_LOCKED)) {
                        error = EBUSY;
                        break;
                }
                if ((error = mutex_lock(mp)) != 0)
                        break;
                if (rd_wr == READ_LOCK) {
                        if (read_lock_try(rwlp, 0)) {
                                (void) mutex_unlock(mp);
                                break;
                        }
                } else {
                        if (write_lock_try(rwlp, 0)) {
                                (void) mutex_unlock(mp);
                                break;
                        }
                }
                atomic_or_32(rwstate, URW_HAS_WAITERS);
                readers = *rwstate;
                ASSERT_CONSISTENT_STATE(readers);
                /*
                 * The calls to __lwp_rwlock_*() below will release the mutex,
                 * so we need a dtrace probe here.  The owner field of the
                 * mutex is cleared in the kernel when the mutex is released,
                 * so we should not clear it here.
                 */
                DTRACE_PROBE2(plockstat, mutex__release, mp, 0);
                /*
                 * The waiters bit may be inaccurate.
                 * Only the kernel knows for sure.
                 */
                if (rd_wr == READ_LOCK) {
                        if (try_flag)
                                error = __lwp_rwlock_tryrdlock(rwlp);
                        else
                                error = __lwp_rwlock_rdlock(rwlp, tsp);
                } else {
                        if (try_flag)
                                error = __lwp_rwlock_trywrlock(rwlp);
                        else
                                error = __lwp_rwlock_wrlock(rwlp, tsp);
                }
        } while (error == EAGAIN || error == EINTR);

        if (!try_flag) {
                DTRACE_PROBE3(plockstat, rw__blocked, rwlp, rd_wr, error == 0);
        }

        return (error);
}

/*
 * Common code for rdlock, timedrdlock, wrlock, timedwrlock, tryrdlock,
 * and trywrlock for process-private (USYNC_THREAD) rwlocks.
 */
int
rwlock_lock(rwlock_t *rwlp, timespec_t *tsp, int rd_wr)
{
        volatile uint32_t *rwstate = (volatile uint32_t *)&rwlp->rwlock_readers;
        uint32_t readers;
        ulwp_t *self = curthread;
        queue_head_t *qp;
        ulwp_t *ulwp;
        int try_flag;
        int ignore_waiters_flag;
        int error = 0;

        try_flag = (rd_wr & TRY_FLAG);
        rd_wr &= ~TRY_FLAG;
        ASSERT(rd_wr == READ_LOCK || rd_wr == WRITE_LOCK);

        if (!try_flag) {
                DTRACE_PROBE2(plockstat, rw__block, rwlp, rd_wr);
        }

        qp = queue_lock(rwlp, MX);
        /* initial attempt to acquire the lock fails if there are waiters */
        ignore_waiters_flag = 0;
        while (error == 0) {
                if (rd_wr == READ_LOCK) {
                        if (read_lock_try(rwlp, ignore_waiters_flag))
                                break;
                } else {
                        if (write_lock_try(rwlp, ignore_waiters_flag))
                                break;
                }
                /* subsequent attempts do not fail due to waiters */
                ignore_waiters_flag = 1;
                atomic_or_32(rwstate, URW_HAS_WAITERS);
                readers = *rwstate;
                ASSERT_CONSISTENT_STATE(readers);
                if ((readers & URW_WRITE_LOCKED) ||
                    (rd_wr == WRITE_LOCK &&
                    (readers & URW_READERS_MASK) != 0))
                        /* EMPTY */;    /* somebody holds the lock */
                else if ((ulwp = queue_waiter(qp)) == NULL) {
                        atomic_and_32(rwstate, ~URW_HAS_WAITERS);
                        ignore_waiters_flag = 0;
                        continue;       /* no queued waiters, start over */
                } else {
                        /*
                         * Do a priority check on the queued waiter (the
                         * highest priority thread on the queue) to see
                         * if we should defer to it or just grab the lock.
                         */
                        int our_pri = real_priority(self);
                        int his_pri = real_priority(ulwp);

                        if (rd_wr == WRITE_LOCK) {
                                /*
                                 * We defer to a queued thread that has
                                 * a higher priority than ours.
                                 */
                                if (his_pri <= our_pri) {
                                        /*
                                         * Don't defer, just grab the lock.
                                         */
                                        continue;
                                }
                        } else {
                                /*
                                 * We defer to a queued thread that has
                                 * a higher priority than ours or that
                                 * is a writer whose priority equals ours.
                                 */
                                if (his_pri < our_pri ||
                                    (his_pri == our_pri && !ulwp->ul_writer)) {
                                        /*
                                         * Don't defer, just grab the lock.
                                         */
                                        continue;
                                }
                        }
                }
                /*
                 * We are about to block.
                 * If we're doing a trylock, return EBUSY instead.
                 */
                if (try_flag) {
                        error = EBUSY;
                        break;
                }
                /*
                 * Enqueue writers ahead of readers.
                 */
                self->ul_writer = rd_wr;        /* *must* be 0 or 1 */
                enqueue(qp, self, 0);
                set_parking_flag(self, 1);
                queue_unlock(qp);
                if ((error = __lwp_park(tsp, 0)) == EINTR)
                        error = 0;
                set_parking_flag(self, 0);
                qp = queue_lock(rwlp, MX);
                if (self->ul_sleepq && dequeue_self(qp) == 0) {
                        atomic_and_32(rwstate, ~URW_HAS_WAITERS);
                        ignore_waiters_flag = 0;
                }
                self->ul_writer = 0;
                if (rd_wr == WRITE_LOCK &&
                    (*rwstate & URW_WRITE_LOCKED) &&
                    rwlp->rwlock_owner == (uintptr_t)self) {
                        /*
                         * We acquired the lock by hand-off
                         * from the previous owner,
                         */
                        error = 0;      /* timedlock did not fail */
                        break;
                }
        }

        /*
         * Make one final check to see if there are any threads left
         * on the rwlock queue.  Clear the URW_HAS_WAITERS flag if not.
         */
        if (qp->qh_root == NULL || qp->qh_root->qr_head == NULL)
                atomic_and_32(rwstate, ~URW_HAS_WAITERS);

        queue_unlock(qp);

        if (!try_flag) {
                DTRACE_PROBE3(plockstat, rw__blocked, rwlp, rd_wr, error == 0);
        }

        return (error);
}

int
rw_rdlock_impl(rwlock_t *rwlp, timespec_t *tsp)
{
        ulwp_t *self = curthread;
        uberdata_t *udp = self->ul_uberdata;
        readlock_t *readlockp;
        tdb_rwlock_stats_t *rwsp = RWLOCK_STATS(rwlp, udp);
        int error;

        /*
         * If we already hold a readers lock on this rwlock,
         * just increment our reference count and return.
         */
        sigoff(self);
        readlockp = rwl_entry(rwlp);
        if (readlockp->rd_count != 0) {
                if (readlockp->rd_count == READ_LOCK_MAX) {
                        sigon(self);
                        error = EAGAIN;
                        goto out;
                }
                sigon(self);
                error = 0;
                goto out;
        }
        sigon(self);

        /*
         * If we hold the writer lock, bail out.
         */
        if (rw_write_held(rwlp)) {
                if (self->ul_error_detection)
                        rwlock_error(rwlp, "rwlock_rdlock",
                            "calling thread owns the writer lock");
                error = EDEADLK;
                goto out;
        }

        if (read_lock_try(rwlp, 0))
                error = 0;
        else if (rwlp->rwlock_type == USYNC_PROCESS)    /* kernel-level */
                error = shared_rwlock_lock(rwlp, tsp, READ_LOCK);
        else                                            /* user-level */
                error = rwlock_lock(rwlp, tsp, READ_LOCK);

out:
        if (error == 0) {
                sigoff(self);
                rwl_entry(rwlp)->rd_count++;
                sigon(self);
                if (rwsp)
                        tdb_incr(rwsp->rw_rdlock);
                DTRACE_PROBE2(plockstat, rw__acquire, rwlp, READ_LOCK);
        } else {
                DTRACE_PROBE3(plockstat, rw__error, rwlp, READ_LOCK, error);
        }

        return (error);
}

#pragma weak pthread_rwlock_rdlock = rw_rdlock
#pragma weak _rw_rdlock = rw_rdlock
int
rw_rdlock(rwlock_t *rwlp)
{
        ASSERT(!curthread->ul_critical || curthread->ul_bindflags);
        return (rw_rdlock_impl(rwlp, NULL));
}

void
lrw_rdlock(rwlock_t *rwlp)
{
        enter_critical(curthread);
        (void) rw_rdlock_impl(rwlp, NULL);
}

int
pthread_rwlock_reltimedrdlock_np(pthread_rwlock_t *_RESTRICT_KYWD rwlp,
    const struct timespec *_RESTRICT_KYWD reltime)
{
        timespec_t tslocal = *reltime;
        int error;

        ASSERT(!curthread->ul_critical || curthread->ul_bindflags);
        error = rw_rdlock_impl((rwlock_t *)rwlp, &tslocal);
        if (error == ETIME)
                error = ETIMEDOUT;
        return (error);
}

int
pthread_rwlock_timedrdlock(pthread_rwlock_t *_RESTRICT_KYWD rwlp,
    const struct timespec *_RESTRICT_KYWD abstime)
{
        timespec_t tslocal;
        int error;

        ASSERT(!curthread->ul_critical || curthread->ul_bindflags);
        abstime_to_reltime(CLOCK_REALTIME, abstime, &tslocal);
        error = rw_rdlock_impl((rwlock_t *)rwlp, &tslocal);
        if (error == ETIME)
                error = ETIMEDOUT;
        return (error);
}

int
rw_wrlock_impl(rwlock_t *rwlp, timespec_t *tsp)
{
        ulwp_t *self = curthread;
        uberdata_t *udp = self->ul_uberdata;
        tdb_rwlock_stats_t *rwsp = RWLOCK_STATS(rwlp, udp);
        int error;

        /*
         * If we hold a readers lock on this rwlock, bail out.
         */
        if (rw_read_held(rwlp)) {
                if (self->ul_error_detection)
                        rwlock_error(rwlp, "rwlock_wrlock",
                            "calling thread owns the readers lock");
                error = EDEADLK;
                goto out;
        }

        /*
         * If we hold the writer lock, bail out.
         */
        if (rw_write_held(rwlp)) {
                if (self->ul_error_detection)
                        rwlock_error(rwlp, "rwlock_wrlock",
                            "calling thread owns the writer lock");
                error = EDEADLK;
                goto out;
        }

        if (write_lock_try(rwlp, 0))
                error = 0;
        else if (rwlp->rwlock_type == USYNC_PROCESS)    /* kernel-level */
                error = shared_rwlock_lock(rwlp, tsp, WRITE_LOCK);
        else                                            /* user-level */
                error = rwlock_lock(rwlp, tsp, WRITE_LOCK);

out:
        if (error == 0) {
                rwlp->rwlock_owner = (uintptr_t)self;
                if (rwlp->rwlock_type == USYNC_PROCESS)
                        rwlp->rwlock_ownerpid = udp->pid;
                if (rwsp) {
                        tdb_incr(rwsp->rw_wrlock);
                        rwsp->rw_wrlock_begin_hold = gethrtime();
                }
                DTRACE_PROBE2(plockstat, rw__acquire, rwlp, WRITE_LOCK);
        } else {
                DTRACE_PROBE3(plockstat, rw__error, rwlp, WRITE_LOCK, error);
        }
        return (error);
}

#pragma weak pthread_rwlock_wrlock = rw_wrlock
#pragma weak _rw_wrlock = rw_wrlock
int
rw_wrlock(rwlock_t *rwlp)
{
        ASSERT(!curthread->ul_critical || curthread->ul_bindflags);
        return (rw_wrlock_impl(rwlp, NULL));
}

void
lrw_wrlock(rwlock_t *rwlp)
{
        enter_critical(curthread);
        (void) rw_wrlock_impl(rwlp, NULL);
}

int
pthread_rwlock_reltimedwrlock_np(pthread_rwlock_t *_RESTRICT_KYWD rwlp,
    const struct timespec *_RESTRICT_KYWD reltime)
{
        timespec_t tslocal = *reltime;
        int error;

        ASSERT(!curthread->ul_critical || curthread->ul_bindflags);
        error = rw_wrlock_impl((rwlock_t *)rwlp, &tslocal);
        if (error == ETIME)
                error = ETIMEDOUT;
        return (error);
}

int
pthread_rwlock_timedwrlock(pthread_rwlock_t *rwlp, const timespec_t *abstime)
{
        timespec_t tslocal;
        int error;

        ASSERT(!curthread->ul_critical || curthread->ul_bindflags);
        abstime_to_reltime(CLOCK_REALTIME, abstime, &tslocal);
        error = rw_wrlock_impl((rwlock_t *)rwlp, &tslocal);
        if (error == ETIME)
                error = ETIMEDOUT;
        return (error);
}

#pragma weak pthread_rwlock_tryrdlock = rw_tryrdlock
int
rw_tryrdlock(rwlock_t *rwlp)
{
        ulwp_t *self = curthread;
        uberdata_t *udp = self->ul_uberdata;
        tdb_rwlock_stats_t *rwsp = RWLOCK_STATS(rwlp, udp);
        readlock_t *readlockp;
        int error;

        ASSERT(!curthread->ul_critical || curthread->ul_bindflags);

        if (rwsp)
                tdb_incr(rwsp->rw_rdlock_try);

        /*
         * If we already hold a readers lock on this rwlock,
         * just increment our reference count and return.
         */
        sigoff(self);
        readlockp = rwl_entry(rwlp);
        if (readlockp->rd_count != 0) {
                if (readlockp->rd_count == READ_LOCK_MAX) {
                        sigon(self);
                        error = EAGAIN;
                        goto out;
                }
                sigon(self);
                error = 0;
                goto out;
        }
        sigon(self);

        if (read_lock_try(rwlp, 0))
                error = 0;
        else if (rwlp->rwlock_type == USYNC_PROCESS)    /* kernel-level */
                error = shared_rwlock_lock(rwlp, NULL, READ_LOCK_TRY);
        else                                            /* user-level */
                error = rwlock_lock(rwlp, NULL, READ_LOCK_TRY);

out:
        if (error == 0) {
                sigoff(self);
                rwl_entry(rwlp)->rd_count++;
                sigon(self);
                DTRACE_PROBE2(plockstat, rw__acquire, rwlp, READ_LOCK);
        } else {
                if (rwsp)
                        tdb_incr(rwsp->rw_rdlock_try_fail);
                if (error != EBUSY) {
                        DTRACE_PROBE3(plockstat, rw__error, rwlp, READ_LOCK,
                            error);
                }
        }

        return (error);
}

#pragma weak pthread_rwlock_trywrlock = rw_trywrlock
int
rw_trywrlock(rwlock_t *rwlp)
{
        ulwp_t *self = curthread;
        uberdata_t *udp = self->ul_uberdata;
        tdb_rwlock_stats_t *rwsp = RWLOCK_STATS(rwlp, udp);
        int error;

        ASSERT(!self->ul_critical || self->ul_bindflags);

        if (rwsp)
                tdb_incr(rwsp->rw_wrlock_try);

        if (write_lock_try(rwlp, 0))
                error = 0;
        else if (rwlp->rwlock_type == USYNC_PROCESS)    /* kernel-level */
                error = shared_rwlock_lock(rwlp, NULL, WRITE_LOCK_TRY);
        else                                            /* user-level */
                error = rwlock_lock(rwlp, NULL, WRITE_LOCK_TRY);

        if (error == 0) {
                rwlp->rwlock_owner = (uintptr_t)self;
                if (rwlp->rwlock_type == USYNC_PROCESS)
                        rwlp->rwlock_ownerpid = udp->pid;
                if (rwsp)
                        rwsp->rw_wrlock_begin_hold = gethrtime();
                DTRACE_PROBE2(plockstat, rw__acquire, rwlp, WRITE_LOCK);
        } else {
                if (rwsp)
                        tdb_incr(rwsp->rw_wrlock_try_fail);
                if (error != EBUSY) {
                        DTRACE_PROBE3(plockstat, rw__error, rwlp, WRITE_LOCK,
                            error);
                }
        }
        return (error);
}

#pragma weak pthread_rwlock_unlock = rw_unlock
#pragma weak _rw_unlock = rw_unlock
int
rw_unlock(rwlock_t *rwlp)
{
        volatile uint32_t *rwstate = (volatile uint32_t *)&rwlp->rwlock_readers;
        uint32_t readers;
        ulwp_t *self = curthread;
        uberdata_t *udp = self->ul_uberdata;
        tdb_rwlock_stats_t *rwsp;
        int rd_wr;

        readers = *rwstate;
        ASSERT_CONSISTENT_STATE(readers);
        if (readers & URW_WRITE_LOCKED) {
                rd_wr = WRITE_LOCK;
                readers = 0;
        } else {
                rd_wr = READ_LOCK;
                readers &= URW_READERS_MASK;
        }

        if (rd_wr == WRITE_LOCK) {
                /*
                 * Since the writer lock is held, we'd better be
                 * holding it, else we cannot legitimately be here.
                 */
                if (!rw_write_held(rwlp)) {
                        if (self->ul_error_detection)
                                rwlock_error(rwlp, "rwlock_unlock",
                                    "writer lock held, "
                                    "but not by the calling thread");
                        return (EPERM);
                }
                if ((rwsp = RWLOCK_STATS(rwlp, udp)) != NULL) {
                        if (rwsp->rw_wrlock_begin_hold)
                                rwsp->rw_wrlock_hold_time +=
                                    gethrtime() - rwsp->rw_wrlock_begin_hold;
                        rwsp->rw_wrlock_begin_hold = 0;
                }
                rwlp->rwlock_owner = 0;
                rwlp->rwlock_ownerpid = 0;
        } else if (readers > 0) {
                /*
                 * A readers lock is held; if we don't hold one, bail out.
                 */
                readlock_t *readlockp;

                sigoff(self);
                readlockp = rwl_entry(rwlp);
                if (readlockp->rd_count == 0) {
                        sigon(self);
                        if (self->ul_error_detection)
                                rwlock_error(rwlp, "rwlock_unlock",
                                    "readers lock held, "
                                    "but not by the calling thread");
                        return (EPERM);
                }
                /*
                 * If we hold more than one readers lock on this rwlock,
                 * just decrement our reference count and return.
                 */
                if (--readlockp->rd_count != 0) {
                        sigon(self);
                        goto out;
                }
                sigon(self);
        } else {
                /*
                 * This is a usage error.
                 * No thread should release an unowned lock.
                 */
                if (self->ul_error_detection)
                        rwlock_error(rwlp, "rwlock_unlock", "lock not owned");
                return (EPERM);
        }

        if (rd_wr == WRITE_LOCK && write_unlock_try(rwlp)) {
                /* EMPTY */;
        } else if (rd_wr == READ_LOCK && read_unlock_try(rwlp)) {
                /* EMPTY */;
        } else if (rwlp->rwlock_type == USYNC_PROCESS) {
                (void) mutex_lock(&rwlp->mutex);
                (void) __lwp_rwlock_unlock(rwlp);
                (void) mutex_unlock(&rwlp->mutex);
        } else {
                rw_queue_release(rwlp);
        }

out:
        DTRACE_PROBE2(plockstat, rw__release, rwlp, rd_wr);
        return (0);
}

void
lrw_unlock(rwlock_t *rwlp)
{
        (void) rw_unlock(rwlp);
        exit_critical(curthread);
}