include: gcc 7's cpp has problems with the line continuations in .x files

[unleashed.git] / kernel / os / condvar.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 2010 Sun Microsystems, Inc.  All rights reserved.
 * Use is subject to license terms.
 */

/*
 * Copyright (c) 2012 by Delphix. All rights reserved.
 */

#include <sys/thread.h>
#include <sys/proc.h>
#include <sys/debug.h>
#include <sys/cmn_err.h>
#include <sys/systm.h>
#include <sys/sobject.h>
#include <sys/sleepq.h>
#include <sys/cpuvar.h>
#include <sys/condvar.h>
#include <sys/condvar_impl.h>
#include <sys/schedctl.h>
#include <sys/procfs.h>
#include <sys/sdt.h>
#include <sys/callo.h>

/*
 * CV_MAX_WAITERS is the maximum number of waiters we track; once
 * the number becomes higher than that, we look at the sleepq to
 * see whether there are *really* any waiters.
 */
#define CV_MAX_WAITERS          1024            /* must be power of 2 */
#define CV_WAITERS_MASK         (CV_MAX_WAITERS - 1)

/*
 * Threads don't "own" condition variables.
 */
/* ARGSUSED */
static kthread_t *
cv_owner(void *cvp)
{
        return (NULL);
}

/*
 * Unsleep a thread that's blocked on a condition variable.
 */
static void
cv_unsleep(kthread_t *t)
{
        condvar_impl_t *cvp = (condvar_impl_t *)t->t_wchan;
        sleepq_head_t *sqh = SQHASH(cvp);

        ASSERT(THREAD_LOCK_HELD(t));

        if (cvp == NULL)
                panic("cv_unsleep: thread %p not on sleepq %p",
                    (void *)t, (void *)sqh);
        DTRACE_SCHED1(wakeup, kthread_t *, t);
        sleepq_unsleep(t);
        if (cvp->cv_waiters != CV_MAX_WAITERS)
                cvp->cv_waiters--;
        disp_lock_exit_high(&sqh->sq_lock);
        CL_SETRUN(t);
}

/*
 * Change the priority of a thread that's blocked on a condition variable.
 */
static void
cv_change_pri(kthread_t *t, pri_t pri, pri_t *t_prip)
{
        condvar_impl_t *cvp = (condvar_impl_t *)t->t_wchan;
        sleepq_t *sqp = t->t_sleepq;

        ASSERT(THREAD_LOCK_HELD(t));
        ASSERT(&SQHASH(cvp)->sq_queue == sqp);

        if (cvp == NULL)
                panic("cv_change_pri: %p not on sleep queue", (void *)t);
        sleepq_dequeue(t);
        *t_prip = pri;
        sleepq_insert(sqp, t);
}

/*
 * The sobj_ops vector exports a set of functions needed when a thread
 * is asleep on a synchronization object of this type.
 */
static sobj_ops_t cv_sobj_ops = {
        SOBJ_CV, cv_owner, cv_unsleep, cv_change_pri
};

/* ARGSUSED */
void
cv_init(kcondvar_t *cvp, char *name, kcv_type_t type, void *arg)
{
        ((condvar_impl_t *)cvp)->cv_waiters = 0;
}

/*
 * cv_destroy is not currently needed, but is part of the DDI.
 * This is in case cv_init ever needs to allocate something for a cv.
 */
/* ARGSUSED */
void
cv_destroy(kcondvar_t *cvp)
{
        ASSERT((((condvar_impl_t *)cvp)->cv_waiters & CV_WAITERS_MASK) == 0);
}

/*
 * The cv_block() function blocks a thread on a condition variable
 * by putting it in a hashed sleep queue associated with the
 * synchronization object.
 *
 * Threads are taken off the hashed sleep queues via calls to
 * cv_signal(), cv_broadcast(), or cv_unsleep().
 */
static void
cv_block(condvar_impl_t *cvp)
{
        kthread_t *t = curthread;
        klwp_t *lwp = ttolwp(t);
        sleepq_head_t *sqh;

        ASSERT(THREAD_LOCK_HELD(t));
        ASSERT(t != CPU->cpu_idle_thread);
        ASSERT(CPU_ON_INTR(CPU) == 0);
        ASSERT(t->t_wchan0 == NULL && t->t_wchan == NULL);
        ASSERT(t->t_state == TS_ONPROC);

        t->t_schedflag &= ~TS_SIGNALLED;
        CL_SLEEP(t);                    /* assign kernel priority */
        t->t_wchan = (caddr_t)cvp;
        t->t_sobj_ops = &cv_sobj_ops;
        DTRACE_SCHED(sleep);

        /*
         * The check for t_intr is to avoid doing the
         * account for an interrupt thread on the still-pinned
         * lwp's statistics.
         */
        if (lwp != NULL && t->t_intr == NULL) {
                lwp->lwp_ru.nvcsw++;
                (void) new_mstate(t, LMS_SLEEP);
        }

        sqh = SQHASH(cvp);
        disp_lock_enter_high(&sqh->sq_lock);
        if (cvp->cv_waiters < CV_MAX_WAITERS)
                cvp->cv_waiters++;
        ASSERT(cvp->cv_waiters <= CV_MAX_WAITERS);
        THREAD_SLEEP(t, &sqh->sq_lock);
        sleepq_insert(&sqh->sq_queue, t);
        /*
         * THREAD_SLEEP() moves curthread->t_lockp to point to the
         * lock sqh->sq_lock. This lock is later released by the caller
         * when it calls thread_unlock() on curthread.
         */
}

#define cv_block_sig(t, cvp)    \
        { (t)->t_flag |= T_WAKEABLE; cv_block(cvp); }

/*
 * Block on the indicated condition variable and release the
 * associated kmutex while blocked.
 */
void
cv_wait(kcondvar_t *cvp, kmutex_t *mp)
{
        if (panicstr)
                return;
        ASSERT(!quiesce_active);

        thread_lock(curthread);                 /* lock the thread */
        cv_block((condvar_impl_t *)cvp);
        thread_unlock_nopreempt(curthread);     /* unlock the waiters field */
        mutex_exit(mp);
        swtch();
        mutex_enter(mp);
}

static void
cv_wakeup(void *arg)
{
        kthread_t *t = arg;

        /*
         * This mutex is acquired and released in order to make sure that
         * the wakeup does not happen before the block itself happens.
         */
        mutex_enter(&t->t_wait_mutex);
        mutex_exit(&t->t_wait_mutex);
        setrun(t);
}

/*
 * Same as cv_wait except the thread will unblock at 'tim'
 * (an absolute time) if it hasn't already unblocked.
 *
 * Returns the amount of time left from the original 'tim' value
 * when it was unblocked.
 */
clock_t
cv_timedwait(kcondvar_t *cvp, kmutex_t *mp, clock_t tim)
{
        hrtime_t hrtim;
        clock_t now = ddi_get_lbolt();

        if (tim <= now)
                return (-1);

        hrtim = TICK_TO_NSEC(tim - now);
        return (cv_timedwait_hires(cvp, mp, hrtim, nsec_per_tick, 0));
}

/*
 * Same as cv_timedwait() except that the third argument is a relative
 * timeout value, as opposed to an absolute one. There is also a fourth
 * argument that specifies how accurately the timeout must be implemented.
 */
clock_t
cv_reltimedwait(kcondvar_t *cvp, kmutex_t *mp, clock_t delta, time_res_t res)
{
        hrtime_t exp;

        ASSERT(TIME_RES_VALID(res));

        if (delta <= 0)
                return (-1);

        if ((exp = TICK_TO_NSEC(delta)) < 0)
                exp = CY_INFINITY;

        return (cv_timedwait_hires(cvp, mp, exp, time_res[res], 0));
}

clock_t
cv_timedwait_hires(kcondvar_t *cvp, kmutex_t *mp, hrtime_t tim,
    hrtime_t res, int flag)
{
        kthread_t *t = curthread;
        callout_id_t id;
        clock_t timeleft;
        hrtime_t limit;
        int signalled;

        if (panicstr)
                return (-1);
        ASSERT(!quiesce_active);

        limit = (flag & CALLOUT_FLAG_ABSOLUTE) ? gethrtime() : 0;
        if (tim <= limit)
                return (-1);
        mutex_enter(&t->t_wait_mutex);
        id = timeout_generic(CALLOUT_REALTIME, (void (*)(void *))cv_wakeup, t,
            tim, res, flag);
        thread_lock(t);         /* lock the thread */
        cv_block((condvar_impl_t *)cvp);
        thread_unlock_nopreempt(t);
        mutex_exit(&t->t_wait_mutex);
        mutex_exit(mp);
        swtch();
        signalled = (t->t_schedflag & TS_SIGNALLED);
        /*
         * Get the time left. untimeout() returns -1 if the timeout has
         * occured or the time remaining.  If the time remaining is zero,
         * the timeout has occured between when we were awoken and
         * we called untimeout.  We will treat this as if the timeout
         * has occured and set timeleft to -1.
         */
        timeleft = untimeout_default(id, 0);
        mutex_enter(mp);
        if (timeleft <= 0) {
                timeleft = -1;
                if (signalled)  /* avoid consuming the cv_signal() */
                        cv_signal(cvp);
        }
        return (timeleft);
}

int
cv_wait_sig(kcondvar_t *cvp, kmutex_t *mp)
{
        kthread_t *t = curthread;
        proc_t *p = ttoproc(t);
        klwp_t *lwp = ttolwp(t);
        int cancel_pending;
        int rval = 1;
        int signalled = 0;

        if (panicstr)
                return (rval);
        ASSERT(!quiesce_active);

        /*
         * Threads in system processes don't process signals.  This is
         * true both for standard threads of system processes and for
         * interrupt threads which have borrowed their pinned thread's LWP.
         */
        if (lwp == NULL || (p->p_flag & SSYS)) {
                cv_wait(cvp, mp);
                return (rval);
        }
        ASSERT(t->t_intr == NULL);

        cancel_pending = schedctl_cancel_pending();
        lwp->lwp_asleep = 1;
        lwp->lwp_sysabort = 0;
        thread_lock(t);
        cv_block_sig(t, (condvar_impl_t *)cvp);
        thread_unlock_nopreempt(t);
        mutex_exit(mp);
        if (ISSIG(t, JUSTLOOKING) || MUSTRETURN(p, t) || cancel_pending)
                setrun(t);
        /* ASSERT(no locks are held) */
        swtch();
        signalled = (t->t_schedflag & TS_SIGNALLED);
        t->t_flag &= ~T_WAKEABLE;
        mutex_enter(mp);
        if (ISSIG_PENDING(t, lwp, p)) {
                mutex_exit(mp);
                if (issig(FORREAL))
                        rval = 0;
                mutex_enter(mp);
        }
        if (lwp->lwp_sysabort || MUSTRETURN(p, t))
                rval = 0;
        if (rval != 0 && cancel_pending) {
                schedctl_cancel_eintr();
                rval = 0;
        }
        lwp->lwp_asleep = 0;
        lwp->lwp_sysabort = 0;
        if (rval == 0 && signalled)     /* avoid consuming the cv_signal() */
                cv_signal(cvp);
        return (rval);
}

static clock_t
cv_timedwait_sig_hires(kcondvar_t *cvp, kmutex_t *mp, hrtime_t tim,
    hrtime_t res, int flag)
{
        kthread_t *t = curthread;
        proc_t *p = ttoproc(t);
        klwp_t *lwp = ttolwp(t);
        int cancel_pending = 0;
        callout_id_t id;
        clock_t rval = 1;
        hrtime_t limit;
        int signalled = 0;

        if (panicstr)
                return (rval);
        ASSERT(!quiesce_active);

        /*
         * Threads in system processes don't process signals.  This is
         * true both for standard threads of system processes and for
         * interrupt threads which have borrowed their pinned thread's LWP.
         */
        if (lwp == NULL || (p->p_flag & SSYS))
                return (cv_timedwait_hires(cvp, mp, tim, res, flag));
        ASSERT(t->t_intr == NULL);

        /*
         * If tim is less than or equal to current hrtime, then the timeout
         * has already occured.  So just check to see if there is a signal
         * pending.  If so return 0 indicating that there is a signal pending.
         * Else return -1 indicating that the timeout occured. No need to
         * wait on anything.
         */
        limit = (flag & CALLOUT_FLAG_ABSOLUTE) ? gethrtime() : 0;
        if (tim <= limit) {
                lwp->lwp_asleep = 1;
                lwp->lwp_sysabort = 0;
                rval = -1;
                goto out;
        }

        /*
         * Set the timeout and wait.
         */
        cancel_pending = schedctl_cancel_pending();
        mutex_enter(&t->t_wait_mutex);
        id = timeout_generic(CALLOUT_REALTIME, (void (*)(void *))cv_wakeup, t,
            tim, res, flag);
        lwp->lwp_asleep = 1;
        lwp->lwp_sysabort = 0;
        thread_lock(t);
        cv_block_sig(t, (condvar_impl_t *)cvp);
        thread_unlock_nopreempt(t);
        mutex_exit(&t->t_wait_mutex);
        mutex_exit(mp);
        if (ISSIG(t, JUSTLOOKING) || MUSTRETURN(p, t) || cancel_pending)
                setrun(t);
        /* ASSERT(no locks are held) */
        swtch();
        signalled = (t->t_schedflag & TS_SIGNALLED);
        t->t_flag &= ~T_WAKEABLE;

        /*
         * Untimeout the thread.  untimeout() returns -1 if the timeout has
         * occured or the time remaining.  If the time remaining is zero,
         * the timeout has occured between when we were awoken and
         * we called untimeout.  We will treat this as if the timeout
         * has occured and set rval to -1.
         */
        rval = untimeout_default(id, 0);
        mutex_enter(mp);
        if (rval <= 0)
                rval = -1;

        /*
         * Check to see if a signal is pending.  If so, regardless of whether
         * or not we were awoken due to the signal, the signal is now pending
         * and a return of 0 has the highest priority.
         */
out:
        if (ISSIG_PENDING(t, lwp, p)) {
                mutex_exit(mp);
                if (issig(FORREAL))
                        rval = 0;
                mutex_enter(mp);
        }
        if (lwp->lwp_sysabort || MUSTRETURN(p, t))
                rval = 0;
        if (rval != 0 && cancel_pending) {
                schedctl_cancel_eintr();
                rval = 0;
        }
        lwp->lwp_asleep = 0;
        lwp->lwp_sysabort = 0;
        if (rval <= 0 && signalled)     /* avoid consuming the cv_signal() */
                cv_signal(cvp);
        return (rval);
}

/*
 * Returns:
 *      Function result in order of precedence:
 *               0 if a signal was received
 *              -1 if timeout occured
 *              >0 if awakened via cv_signal() or cv_broadcast().
 *                 (returns time remaining)
 *
 * cv_timedwait_sig() is now part of the DDI.
 *
 * This function is now just a wrapper for cv_timedwait_sig_hires().
 */
clock_t
cv_timedwait_sig(kcondvar_t *cvp, kmutex_t *mp, clock_t tim)
{
        hrtime_t hrtim;

        hrtim = TICK_TO_NSEC(tim - ddi_get_lbolt());
        return (cv_timedwait_sig_hires(cvp, mp, hrtim, nsec_per_tick, 0));
}

/*
 * Wait until the specified time.
 * If tim == -1, waits without timeout using cv_wait_sig_swap().
 */
int
cv_timedwait_sig_hrtime(kcondvar_t *cvp, kmutex_t *mp, hrtime_t tim)
{
        if (tim == -1) {
                return (cv_wait_sig_swap(cvp, mp));
        } else {
                return (cv_timedwait_sig_hires(cvp, mp, tim, 1,
                    CALLOUT_FLAG_ABSOLUTE | CALLOUT_FLAG_ROUNDUP));
        }
}

/*
 * Same as cv_timedwait_sig() except that the third argument is a relative
 * timeout value, as opposed to an absolute one. There is also a fourth
 * argument that specifies how accurately the timeout must be implemented.
 */
clock_t
cv_reltimedwait_sig(kcondvar_t *cvp, kmutex_t *mp, clock_t delta,
    time_res_t res)
{
        hrtime_t exp = 0;

        ASSERT(TIME_RES_VALID(res));

        if (delta > 0) {
                if ((exp = TICK_TO_NSEC(delta)) < 0)
                        exp = CY_INFINITY;
        }

        return (cv_timedwait_sig_hires(cvp, mp, exp, time_res[res], 0));
}

/*
 * Like cv_wait_sig_swap but allows the caller to indicate (with a
 * non-NULL sigret) that they will take care of signalling the cv
 * after wakeup, if necessary.  This is a vile hack that should only
 * be used when no other option is available; almost all callers
 * should just use cv_wait_sig_swap (which takes care of the cv_signal
 * stuff automatically) instead.
 */
int
cv_wait_sig_swap_core(kcondvar_t *cvp, kmutex_t *mp, int *sigret)
{
        kthread_t *t = curthread;
        proc_t *p = ttoproc(t);
        klwp_t *lwp = ttolwp(t);
        int cancel_pending;
        int rval = 1;
        int signalled = 0;

        if (panicstr)
                return (rval);

        /*
         * Threads in system processes don't process signals.  This is
         * true both for standard threads of system processes and for
         * interrupt threads which have borrowed their pinned thread's LWP.
         */
        if (lwp == NULL || (p->p_flag & SSYS)) {
                cv_wait(cvp, mp);
                return (rval);
        }
        ASSERT(t->t_intr == NULL);

        cancel_pending = schedctl_cancel_pending();
        lwp->lwp_asleep = 1;
        lwp->lwp_sysabort = 0;
        thread_lock(t);
        t->t_kpri_req = 0;      /* don't need kernel priority */
        cv_block_sig(t, (condvar_impl_t *)cvp);
        thread_unlock_nopreempt(t);
        mutex_exit(mp);
        if (ISSIG(t, JUSTLOOKING) || MUSTRETURN(p, t) || cancel_pending)
                setrun(t);
        /* ASSERT(no locks are held) */
        swtch();
        signalled = (t->t_schedflag & TS_SIGNALLED);
        t->t_flag &= ~T_WAKEABLE;
        mutex_enter(mp);
        if (ISSIG_PENDING(t, lwp, p)) {
                mutex_exit(mp);
                if (issig(FORREAL))
                        rval = 0;
                mutex_enter(mp);
        }
        if (lwp->lwp_sysabort || MUSTRETURN(p, t))
                rval = 0;
        if (rval != 0 && cancel_pending) {
                schedctl_cancel_eintr();
                rval = 0;
        }
        lwp->lwp_asleep = 0;
        lwp->lwp_sysabort = 0;
        if (rval == 0) {
                if (sigret != NULL)
                        *sigret = signalled;    /* just tell the caller */
                else if (signalled)
                        cv_signal(cvp); /* avoid consuming the cv_signal() */
        }
        return (rval);
}

/*
 * Same as cv_wait_sig but the thread can be swapped out while waiting.
 * This should only be used when we know we aren't holding any locks.
 */
int
cv_wait_sig_swap(kcondvar_t *cvp, kmutex_t *mp)
{
        return (cv_wait_sig_swap_core(cvp, mp, NULL));
}

void
cv_signal(kcondvar_t *cvp)
{
        condvar_impl_t *cp = (condvar_impl_t *)cvp;

        /* make sure the cv_waiters field looks sane */
        ASSERT(cp->cv_waiters <= CV_MAX_WAITERS);
        if (cp->cv_waiters > 0) {
                sleepq_head_t *sqh = SQHASH(cp);
                disp_lock_enter(&sqh->sq_lock);
                ASSERT(CPU_ON_INTR(CPU) == 0);
                if (cp->cv_waiters & CV_WAITERS_MASK) {
                        kthread_t *t;
                        cp->cv_waiters--;
                        t = sleepq_wakeone_chan(&sqh->sq_queue, cp);
                        /*
                         * If cv_waiters is non-zero (and less than
                         * CV_MAX_WAITERS) there should be a thread
                         * in the queue.
                         */
                        ASSERT(t != NULL);
                } else if (sleepq_wakeone_chan(&sqh->sq_queue, cp) == NULL) {
                        cp->cv_waiters = 0;
                }
                disp_lock_exit(&sqh->sq_lock);
        }
}

void
cv_broadcast(kcondvar_t *cvp)
{
        condvar_impl_t *cp = (condvar_impl_t *)cvp;

        /* make sure the cv_waiters field looks sane */
        ASSERT(cp->cv_waiters <= CV_MAX_WAITERS);
        if (cp->cv_waiters > 0) {
                sleepq_head_t *sqh = SQHASH(cp);
                disp_lock_enter(&sqh->sq_lock);
                ASSERT(CPU_ON_INTR(CPU) == 0);
                sleepq_wakeall_chan(&sqh->sq_queue, cp);
                cp->cv_waiters = 0;
                disp_lock_exit(&sqh->sq_lock);
        }
}

/*
 * Same as cv_wait(), but wakes up (after wakeup_time milliseconds) to check
 * for requests to stop, like cv_wait_sig() but without dealing with signals.
 * This is a horrible kludge.  It is evil.  It is vile.  It is swill.
 * If your code has to call this function then your code is the same.
 */
void
cv_wait_stop(kcondvar_t *cvp, kmutex_t *mp, int wakeup_time)
{
        kthread_t *t = curthread;
        klwp_t *lwp = ttolwp(t);
        proc_t *p = ttoproc(t);
        callout_id_t id;
        clock_t tim;

        if (panicstr)
                return;

        /*
         * Threads in system processes don't process signals.  This is
         * true both for standard threads of system processes and for
         * interrupt threads which have borrowed their pinned thread's LWP.
         */
        if (lwp == NULL || (p->p_flag & SSYS)) {
                cv_wait(cvp, mp);
                return;
        }
        ASSERT(t->t_intr == NULL);

        /*
         * Wakeup in wakeup_time milliseconds, i.e., human time.
         */
        tim = ddi_get_lbolt() + MSEC_TO_TICK(wakeup_time);
        mutex_enter(&t->t_wait_mutex);
        id = realtime_timeout_default((void (*)(void *))cv_wakeup, t,
            tim - ddi_get_lbolt());
        thread_lock(t);                 /* lock the thread */
        cv_block((condvar_impl_t *)cvp);
        thread_unlock_nopreempt(t);
        mutex_exit(&t->t_wait_mutex);
        mutex_exit(mp);
        /* ASSERT(no locks are held); */
        swtch();
        (void) untimeout_default(id, 0);

        /*
         * Check for reasons to stop, if lwp_nostop is not true.
         * See issig_forreal() for explanations of the various stops.
         */
        mutex_enter(&p->p_lock);
        while (lwp->lwp_nostop == 0 && !(p->p_flag & SEXITLWPS)) {
                /*
                 * Hold the lwp here for watchpoint manipulation.
                 */
                if (t->t_proc_flag & TP_PAUSE) {
                        stop(PR_SUSPENDED, SUSPEND_PAUSE);
                        continue;
                }
                /*
                 * System checkpoint.
                 */
                if (t->t_proc_flag & TP_CHKPT) {
                        stop(PR_CHECKPOINT, 0);
                        continue;
                }
                /*
                 * Honor fork1(), watchpoint activity (remapping a page),
                 * and lwp_suspend() requests.
                 */
                if ((p->p_flag & (SHOLDFORK1|SHOLDWATCH)) ||
                    (t->t_proc_flag & TP_HOLDLWP)) {
                        stop(PR_SUSPENDED, SUSPEND_NORMAL);
                        continue;
                }
                /*
                 * Honor /proc requested stop.
                 */
                if (t->t_proc_flag & TP_PRSTOP) {
                        stop(PR_REQUESTED, 0);
                }
                /*
                 * If some lwp in the process has already stopped
                 * showing PR_JOBCONTROL, stop in sympathy with it.
                 */
                if (p->p_stopsig && t != p->p_agenttp) {
                        stop(PR_JOBCONTROL, p->p_stopsig);
                        continue;
                }
                break;
        }
        mutex_exit(&p->p_lock);
        mutex_enter(mp);
}

/*
 * Like cv_timedwait_sig(), but takes an absolute hires future time
 * rather than a future time in clock ticks.  Will not return showing
 * that a timeout occurred until the future time is passed.
 * If 'when' is a NULL pointer, no timeout will occur.
 * Returns:
 *      Function result in order of precedence:
 *               0 if a signal was received
 *              -1 if timeout occured
 *              >0 if awakened via cv_signal() or cv_broadcast()
 *                 or by a spurious wakeup.
 *                 (might return time remaining)
 * As a special test, if someone abruptly resets the system time
 * (but not through adjtime(2); drifting of the clock is allowed and
 * expected [see timespectohz_adj()]), then we force a return of -1
 * so the caller can return a premature timeout to the calling process
 * so it can reevaluate the situation in light of the new system time.
 * (The system clock has been reset if timecheck != timechanged.)
 *
 * Generally, cv_timedwait_sig_hrtime() should be used instead of this
 * routine.  It waits based on hrtime rather than wall-clock time and therefore
 * does not need to deal with the time changing.
 */
int
cv_waituntil_sig(kcondvar_t *cvp, kmutex_t *mp,
        timestruc_t *when, int timecheck)
{
        timestruc_t now;
        timestruc_t delta;
        hrtime_t interval;
        int rval;

        if (when == NULL)
                return (cv_wait_sig_swap(cvp, mp));

        gethrestime(&now);
        delta = *when;
        timespecsub(&delta, &now);
        if (delta.tv_sec < 0 || (delta.tv_sec == 0 && delta.tv_nsec == 0)) {
                /*
                 * We have already reached the absolute future time.
                 * Call cv_timedwait_sig() just to check for signals.
                 * We will return immediately with either 0 or -1.
                 */
                rval = cv_timedwait_sig_hires(cvp, mp, 0, 1, 0);
        } else {
                if (timecheck == timechanged) {
                        /*
                         * Make sure that the interval is atleast one tick.
                         * This is to prevent a user from flooding the system
                         * with very small, high resolution timers.
                         */
                        interval = ts2hrt(&delta);
                        if (interval < nsec_per_tick)
                                interval = nsec_per_tick;
                        rval = cv_timedwait_sig_hires(cvp, mp, interval, 1,
                            CALLOUT_FLAG_HRESTIME);
                } else {
                        /*
                         * Someone reset the system time;
                         * just force an immediate timeout.
                         */
                        rval = -1;
                }
                if (rval == -1 && timecheck == timechanged) {
                        /*
                         * Even though cv_timedwait_sig() returned showing a
                         * timeout, the future time may not have passed yet.
                         * If not, change rval to indicate a normal wakeup.
                         */
                        gethrestime(&now);
                        delta = *when;
                        timespecsub(&delta, &now);
                        if (delta.tv_sec > 0 || (delta.tv_sec == 0 &&
                            delta.tv_nsec > 0))
                                rval = 1;
                }
        }
        return (rval);
}