FreeBSD/Linux Kernel Cross Reference
sys/kern/subr_sleepqueue.c

     /*-
      * Copyright (c) 2004 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.
     */
    
    /*
     * Implementation of sleep queues used to hold queue of threads blocked on
     * a wait channel.  Sleep queues different from turnstiles in that wait
     * channels are not owned by anyone, so there is no priority propagation.
     * Sleep queues can also provide a timeout and can also be interrupted by
     * signals.  That said, there are several similarities between the turnstile
     * and sleep queue implementations.  (Note: turnstiles were implemented
     * first.)  For example, both use a hash table of the same size where each
     * bucket is referred to as a "chain" that contains both a spin lock and
     * a linked list of queues.  An individual queue is located by using a hash
     * to pick a chain, locking the chain, and then walking the chain searching
     * for the queue.  This means that a wait channel object does not need to
     * embed it's queue head just as locks do not embed their turnstile queue
     * head.  Threads also carry around a sleep queue that they lend to the
     * wait channel when blocking.  Just as in turnstiles, the queue includes
     * a free list of the sleep queues of other threads blocked on the same
     * wait channel in the case of multiple waiters.
     *
     * Some additional functionality provided by sleep queues include the
     * ability to set a timeout.  The timeout is managed using a per-thread
     * callout that resumes a thread if it is asleep.  A thread may also
     * catch signals while it is asleep (aka an interruptible sleep).  The
     * signal code uses sleepq_abort() to interrupt a sleeping thread.  Finally,
     * sleep queues also provide some extra assertions.  One is not allowed to
     * mix the sleep/wakeup and cv APIs for a given wait channel.  Also, one
     * must consistently use the same lock to synchronize with a wait channel,
     * though this check is currently only a warning for sleep/wakeup due to
     * pre-existing abuse of that API.  The same lock must also be held when
     * awakening threads, though that is currently only enforced for condition
     * variables.
     */
    
    #include <sys/cdefs.h>
    __FBSDID("$FreeBSD: releng/6.2/sys/kern/subr_sleepqueue.c 164286 2006-11-14 20:42:41Z cvs2svn $");
    
    #include "opt_sleepqueue_profiling.h"
    #include "opt_ddb.h"
    
    #include <sys/param.h>
    #include <sys/systm.h>
    #include <sys/lock.h>
    #include <sys/kernel.h>
    #include <sys/ktr.h>
    #include <sys/malloc.h>
    #include <sys/mutex.h>
    #include <sys/proc.h>
    #include <sys/sched.h>
    #include <sys/signalvar.h>
    #include <sys/sleepqueue.h>
    #include <sys/sysctl.h>
    
    #ifdef DDB
    #include <ddb/ddb.h>
    #endif
    
    /*
     * Constants for the hash table of sleep queue chains.  These constants are
     * the same ones that 4BSD (and possibly earlier versions of BSD) used.
     * Basically, we ignore the lower 8 bits of the address since most wait
     * channel pointers are aligned and only look at the next 7 bits for the
     * hash.  SC_TABLESIZE must be a power of two for SC_MASK to work properly.
     */
    #define SC_TABLESIZE    128                     /* Must be power of 2. */
    #define SC_MASK         (SC_TABLESIZE - 1)
    #define SC_SHIFT        8
    #define SC_HASH(wc)     (((uintptr_t)(wc) >> SC_SHIFT) & SC_MASK)
    #define SC_LOOKUP(wc)   &sleepq_chains[SC_HASH(wc)]
    
    /*
     * There two different lists of sleep queues.  Both lists are connected
    * via the sq_hash entries.  The first list is the sleep queue chain list
    * that a sleep queue is on when it is attached to a wait channel.  The
    * second list is the free list hung off of a sleep queue that is attached
    * to a wait channel.
    *
    * Each sleep queue also contains the wait channel it is attached to, the
    * list of threads blocked on that wait channel, flags specific to the
    * wait channel, and the lock used to synchronize with a wait channel.
    * The flags are used to catch mismatches between the various consumers
    * of the sleep queue API (e.g. sleep/wakeup and condition variables).
    * The lock pointer is only used when invariants are enabled for various
    * debugging checks.
    *
    * Locking key:
    *  c - sleep queue chain lock
    */
   struct sleepqueue {
           TAILQ_HEAD(, thread) sq_blocked;        /* (c) Blocked threads. */
           LIST_ENTRY(sleepqueue) sq_hash;         /* (c) Chain and free list. */
           LIST_HEAD(, sleepqueue) sq_free;        /* (c) Free queues. */
           void    *sq_wchan;                      /* (c) Wait channel. */
   #ifdef INVARIANTS
           int     sq_type;                        /* (c) Queue type. */
           struct mtx *sq_lock;                    /* (c) Associated lock. */
   #endif
   };
   
   struct sleepqueue_chain {
           LIST_HEAD(, sleepqueue) sc_queues;      /* List of sleep queues. */
           struct mtx sc_lock;                     /* Spin lock for this chain. */
   #ifdef SLEEPQUEUE_PROFILING
           u_int   sc_depth;                       /* Length of sc_queues. */
           u_int   sc_max_depth;                   /* Max length of sc_queues. */
   #endif
   };
   
   #ifdef SLEEPQUEUE_PROFILING
   u_int sleepq_max_depth;
   SYSCTL_NODE(_debug, OID_AUTO, sleepq, CTLFLAG_RD, 0, "sleepq profiling");
   SYSCTL_NODE(_debug_sleepq, OID_AUTO, chains, CTLFLAG_RD, 0,
       "sleepq chain stats");
   SYSCTL_UINT(_debug_sleepq, OID_AUTO, max_depth, CTLFLAG_RD, &sleepq_max_depth,
       0, "maxmimum depth achieved of a single chain");
   #endif
   static struct sleepqueue_chain sleepq_chains[SC_TABLESIZE];
   
   static MALLOC_DEFINE(M_SLEEPQUEUE, "sleep queues", "sleep queues");
   
   /*
    * Prototypes for non-exported routines.
    */
   static int      sleepq_check_timeout(void);
   static void     sleepq_switch(void *wchan);
   static void     sleepq_timeout(void *arg);
   static void     sleepq_resume_thread(struct sleepqueue *sq, struct thread *td, int pri);
   
   /*
    * Early initialization of sleep queues that is called from the sleepinit()
    * SYSINIT.
    */
   void
   init_sleepqueues(void)
   {
   #ifdef SLEEPQUEUE_PROFILING
           struct sysctl_oid *chain_oid;
           char chain_name[10];
   #endif
           int i;
   
           for (i = 0; i < SC_TABLESIZE; i++) {
                   LIST_INIT(&sleepq_chains[i].sc_queues);
                   mtx_init(&sleepq_chains[i].sc_lock, "sleepq chain", NULL,
                       MTX_SPIN);
   #ifdef SLEEPQUEUE_PROFILING
                   snprintf(chain_name, sizeof(chain_name), "%d", i);
                   chain_oid = SYSCTL_ADD_NODE(NULL, 
                       SYSCTL_STATIC_CHILDREN(_debug_sleepq_chains), OID_AUTO,
                       chain_name, CTLFLAG_RD, NULL, "sleepq chain stats");
                   SYSCTL_ADD_UINT(NULL, SYSCTL_CHILDREN(chain_oid), OID_AUTO,
                       "depth", CTLFLAG_RD, &sleepq_chains[i].sc_depth, 0, NULL);
                   SYSCTL_ADD_UINT(NULL, SYSCTL_CHILDREN(chain_oid), OID_AUTO,
                       "max_depth", CTLFLAG_RD, &sleepq_chains[i].sc_max_depth, 0,
                       NULL);
   #endif
           }
           thread0.td_sleepqueue = sleepq_alloc();
   }
   
   /*
    * Malloc and initialize a new sleep queue for a new thread.
    */
   struct sleepqueue *
   sleepq_alloc(void)
   {
           struct sleepqueue *sq;
   
           sq = malloc(sizeof(struct sleepqueue), M_SLEEPQUEUE, M_WAITOK | M_ZERO);
           TAILQ_INIT(&sq->sq_blocked);
           LIST_INIT(&sq->sq_free);
           return (sq);
   }
   
   /*
    * Free a sleep queue when a thread is destroyed.
    */
   void
   sleepq_free(struct sleepqueue *sq)
   {
   
           MPASS(sq != NULL);
           MPASS(TAILQ_EMPTY(&sq->sq_blocked));
           free(sq, M_SLEEPQUEUE);
   }
   
   /*
    * Lock the sleep queue chain associated with the specified wait channel.
    */
   void
   sleepq_lock(void *wchan)
   {
           struct sleepqueue_chain *sc;
   
           sc = SC_LOOKUP(wchan);
           mtx_lock_spin(&sc->sc_lock);
   }
   
   /*
    * Look up the sleep queue associated with a given wait channel in the hash
    * table locking the associated sleep queue chain.  If no queue is found in
    * the table, NULL is returned.
    */
   struct sleepqueue *
   sleepq_lookup(void *wchan)
   {
           struct sleepqueue_chain *sc;
           struct sleepqueue *sq;
   
           KASSERT(wchan != NULL, ("%s: invalid NULL wait channel", __func__));
           sc = SC_LOOKUP(wchan);
           mtx_assert(&sc->sc_lock, MA_OWNED);
           LIST_FOREACH(sq, &sc->sc_queues, sq_hash)
                   if (sq->sq_wchan == wchan)
                           return (sq);
           return (NULL);
   }
   
   /*
    * Unlock the sleep queue chain associated with a given wait channel.
    */
   void
   sleepq_release(void *wchan)
   {
           struct sleepqueue_chain *sc;
   
           sc = SC_LOOKUP(wchan);
           mtx_unlock_spin(&sc->sc_lock);
   }
   
   /*
    * Places the current thread on the sleep queue for the specified wait
    * channel.  If INVARIANTS is enabled, then it associates the passed in
    * lock with the sleepq to make sure it is held when that sleep queue is
    * woken up.
    */
   void
   sleepq_add(void *wchan, struct mtx *lock, const char *wmesg, int flags)
   {
           struct sleepqueue_chain *sc;
           struct sleepqueue *sq;
           struct thread *td;
   
           td = curthread;
           sc = SC_LOOKUP(wchan);
           mtx_assert(&sc->sc_lock, MA_OWNED);
           MPASS(td->td_sleepqueue != NULL);
           MPASS(wchan != NULL);
   
           /* If this thread is not allowed to sleep, die a horrible death. */
           KASSERT(!(td->td_pflags & TDP_NOSLEEPING),
               ("trying to sleep while sleeping is prohibited"));
   
           /* Look up the sleep queue associated with the wait channel 'wchan'. */
           sq = sleepq_lookup(wchan);
   
           /*
            * If the wait channel does not already have a sleep queue, use
            * this thread's sleep queue.  Otherwise, insert the current thread
            * into the sleep queue already in use by this wait channel.
            */
           if (sq == NULL) {
   #ifdef SLEEPQUEUE_PROFILING
                   sc->sc_depth++;
                   if (sc->sc_depth > sc->sc_max_depth) {
                           sc->sc_max_depth = sc->sc_depth;
                           if (sc->sc_max_depth > sleepq_max_depth)
                                   sleepq_max_depth = sc->sc_max_depth;
                   }
   #endif
                   sq = td->td_sleepqueue;
                   LIST_INSERT_HEAD(&sc->sc_queues, sq, sq_hash);
                   KASSERT(TAILQ_EMPTY(&sq->sq_blocked),
                       ("thread's sleep queue has a non-empty queue"));
                   KASSERT(LIST_EMPTY(&sq->sq_free),
                       ("thread's sleep queue has a non-empty free list"));
                   KASSERT(sq->sq_wchan == NULL, ("stale sq_wchan pointer"));
                   sq->sq_wchan = wchan;
   #ifdef INVARIANTS
                   sq->sq_lock = lock;
                   sq->sq_type = flags & SLEEPQ_TYPE;
   #endif
           } else {
                   MPASS(wchan == sq->sq_wchan);
                   MPASS(lock == sq->sq_lock);
                   MPASS((flags & SLEEPQ_TYPE) == sq->sq_type);
                   LIST_INSERT_HEAD(&sq->sq_free, td->td_sleepqueue, sq_hash);
           }
           TAILQ_INSERT_TAIL(&sq->sq_blocked, td, td_slpq);
           td->td_sleepqueue = NULL;
           mtx_lock_spin(&sched_lock);
           td->td_wchan = wchan;
           td->td_wmesg = wmesg;
           if (flags & SLEEPQ_INTERRUPTIBLE) {
                   td->td_flags |= TDF_SINTR;
                   td->td_flags &= ~TDF_SLEEPABORT;
           }
           mtx_unlock_spin(&sched_lock);
   }
   
   /*
    * Sets a timeout that will remove the current thread from the specified
    * sleep queue after timo ticks if the thread has not already been awakened.
    */
   void
   sleepq_set_timeout(void *wchan, int timo)
   {
           struct sleepqueue_chain *sc;
           struct thread *td;
   
           td = curthread;
           sc = SC_LOOKUP(wchan);
           mtx_assert(&sc->sc_lock, MA_OWNED);
           MPASS(TD_ON_SLEEPQ(td));
           MPASS(td->td_sleepqueue == NULL);
           MPASS(wchan != NULL);
           callout_reset(&td->td_slpcallout, timo, sleepq_timeout, td);
   }
   
   /*
    * Marks the pending sleep of the current thread as interruptible and
    * makes an initial check for pending signals before putting a thread
    * to sleep. Return with sleep queue and scheduler lock held.
    */
   static int
   sleepq_catch_signals(void *wchan)
   {
           struct sleepqueue_chain *sc;
           struct sleepqueue *sq;
           struct thread *td;
           struct proc *p;
           struct sigacts *ps;
           int sig, ret;
   
           td = curthread;
           p = curproc;
           sc = SC_LOOKUP(wchan);
           mtx_assert(&sc->sc_lock, MA_OWNED);
           MPASS(wchan != NULL);
           CTR3(KTR_PROC, "sleepq catching signals: thread %p (pid %ld, %s)",
                   (void *)td, (long)p->p_pid, p->p_comm);
   
           MPASS(td->td_flags & TDF_SINTR);
           mtx_unlock_spin(&sc->sc_lock);
   
           /* See if there are any pending signals for this thread. */
           PROC_LOCK(p);
           ps = p->p_sigacts;
           mtx_lock(&ps->ps_mtx);
           sig = cursig(td);
           if (sig == 0) {
                   mtx_unlock(&ps->ps_mtx);
                   ret = thread_suspend_check(1);
                   MPASS(ret == 0 || ret == EINTR || ret == ERESTART);
           } else {
                   if (SIGISMEMBER(ps->ps_sigintr, sig))
                           ret = EINTR;
                   else
                           ret = ERESTART;
                   mtx_unlock(&ps->ps_mtx);
           }
   
           if (ret == 0) {
                   mtx_lock_spin(&sc->sc_lock);
                   /*
                    * Lock sched_lock before unlocking proc lock,
                    * without this, we could lose a race.
                    */
                   mtx_lock_spin(&sched_lock);
                   PROC_UNLOCK(p);
                   if (!(td->td_flags & TDF_INTERRUPT))
                           return (0);
                   /* KSE threads tried unblocking us. */
                   ret = td->td_intrval;
                   mtx_unlock_spin(&sched_lock);
                   MPASS(ret == EINTR || ret == ERESTART);
           } else {
                   PROC_UNLOCK(p);
                   mtx_lock_spin(&sc->sc_lock);
           }
           /*
            * There were pending signals and this thread is still
            * on the sleep queue, remove it from the sleep queue.
            */
           sq = sleepq_lookup(wchan);
           mtx_lock_spin(&sched_lock);
           if (TD_ON_SLEEPQ(td))
                   sleepq_resume_thread(sq, td, -1);
           return (ret);
   }
   
   /*
    * Switches to another thread if we are still asleep on a sleep queue and
    * drop the lock on the sleep queue chain.  Returns with sched_lock held.
    */
   static void
   sleepq_switch(void *wchan)
   {
           struct sleepqueue_chain *sc;
           struct thread *td;
   
           td = curthread;
           sc = SC_LOOKUP(wchan);
           mtx_assert(&sc->sc_lock, MA_OWNED);
           mtx_assert(&sched_lock, MA_OWNED);
   
           /* 
            * If we have a sleep queue, then we've already been woken up, so
            * just return.
            */
           if (td->td_sleepqueue != NULL) {
                   MPASS(!TD_ON_SLEEPQ(td));
                   mtx_unlock_spin(&sc->sc_lock);
                   return;
           }
   
           /*
            * Otherwise, actually go to sleep.
            */
           mtx_unlock_spin(&sc->sc_lock);
           sched_sleep(td);
           TD_SET_SLEEPING(td);
           mi_switch(SW_VOL, NULL);
           KASSERT(TD_IS_RUNNING(td), ("running but not TDS_RUNNING"));
           CTR3(KTR_PROC, "sleepq resume: thread %p (pid %ld, %s)",
               (void *)td, (long)td->td_proc->p_pid, (void *)td->td_proc->p_comm);
   }
   
   /*
    * Check to see if we timed out.
    */
   static int
   sleepq_check_timeout(void)
   {
           struct thread *td;
   
           mtx_assert(&sched_lock, MA_OWNED);
           td = curthread;
   
           /*
            * If TDF_TIMEOUT is set, we timed out.
            */
           if (td->td_flags & TDF_TIMEOUT) {
                   td->td_flags &= ~TDF_TIMEOUT;
                   return (EWOULDBLOCK);
           }
   
           /*
            * If TDF_TIMOFAIL is set, the timeout ran after we had
            * already been woken up.
            */
           if (td->td_flags & TDF_TIMOFAIL)
                   td->td_flags &= ~TDF_TIMOFAIL;
   
           /*
            * If callout_stop() fails, then the timeout is running on
            * another CPU, so synchronize with it to avoid having it
            * accidentally wake up a subsequent sleep.
            */
           else if (callout_stop(&td->td_slpcallout) == 0) {
                   td->td_flags |= TDF_TIMEOUT;
                   TD_SET_SLEEPING(td);
                   mi_switch(SW_INVOL, NULL);
           }
           return (0);
   }
   
   /*
    * Check to see if we were awoken by a signal.
    */
   static int
   sleepq_check_signals(void)
   {
           struct thread *td;
   
           mtx_assert(&sched_lock, MA_OWNED);
           td = curthread;
   
           /* We are no longer in an interruptible sleep. */
           if (td->td_flags & TDF_SINTR)
                   td->td_flags &= ~TDF_SINTR;
   
           if (td->td_flags & TDF_SLEEPABORT) {
                   td->td_flags &= ~TDF_SLEEPABORT;
                   return (td->td_intrval);
           }
   
           if (td->td_flags & TDF_INTERRUPT)
                   return (td->td_intrval);
   
           return (0);
   }
   
   /*
    * Block the current thread until it is awakened from its sleep queue.
    */
   void
   sleepq_wait(void *wchan)
   {
   
           MPASS(!(curthread->td_flags & TDF_SINTR));
           mtx_lock_spin(&sched_lock);
           sleepq_switch(wchan);
           mtx_unlock_spin(&sched_lock);
   }
   
   /*
    * Block the current thread until it is awakened from its sleep queue
    * or it is interrupted by a signal.
    */
   int
   sleepq_wait_sig(void *wchan)
   {
           int rcatch;
           int rval;
   
           rcatch = sleepq_catch_signals(wchan);
           if (rcatch == 0)
                   sleepq_switch(wchan);
           else
                   sleepq_release(wchan);
           rval = sleepq_check_signals();
           mtx_unlock_spin(&sched_lock); 
           if (rcatch)
                   return (rcatch);
           return (rval);
   }
   
   /*
    * Block the current thread until it is awakened from its sleep queue
    * or it times out while waiting.
    */
   int
   sleepq_timedwait(void *wchan)
   {
           int rval;
   
           MPASS(!(curthread->td_flags & TDF_SINTR));
           mtx_lock_spin(&sched_lock);
           sleepq_switch(wchan);
           rval = sleepq_check_timeout();
           mtx_unlock_spin(&sched_lock);
           return (rval);
   }
   
   /*
    * Block the current thread until it is awakened from its sleep queue,
    * it is interrupted by a signal, or it times out waiting to be awakened.
    */
   int
   sleepq_timedwait_sig(void *wchan)
   {
           int rcatch, rvalt, rvals;
   
           rcatch = sleepq_catch_signals(wchan);
           if (rcatch == 0)
                   sleepq_switch(wchan);
           else
                   sleepq_release(wchan);
           rvalt = sleepq_check_timeout();
           rvals = sleepq_check_signals();
           mtx_unlock_spin(&sched_lock);
           if (rcatch)
                   return (rcatch);
           if (rvals)
                   return (rvals);
           return (rvalt);
   }
   
   /*
    * Removes a thread from a sleep queue and makes it
    * runnable.
    */
   static void
   sleepq_resume_thread(struct sleepqueue *sq, struct thread *td, int pri)
   {
           struct sleepqueue_chain *sc;
   
           MPASS(td != NULL);
           MPASS(sq->sq_wchan != NULL);
           MPASS(td->td_wchan == sq->sq_wchan);
           sc = SC_LOOKUP(sq->sq_wchan);
           mtx_assert(&sc->sc_lock, MA_OWNED);
           mtx_assert(&sched_lock, MA_OWNED);
   
           /* Remove the thread from the queue. */
           TAILQ_REMOVE(&sq->sq_blocked, td, td_slpq);
   
           /*
            * Get a sleep queue for this thread.  If this is the last waiter,
            * use the queue itself and take it out of the chain, otherwise,
            * remove a queue from the free list.
            */
           if (LIST_EMPTY(&sq->sq_free)) {
                   td->td_sleepqueue = sq;
   #ifdef INVARIANTS
                   sq->sq_wchan = NULL;
   #endif
   #ifdef SLEEPQUEUE_PROFILING
                   sc->sc_depth--;
   #endif
           } else
                   td->td_sleepqueue = LIST_FIRST(&sq->sq_free);
           LIST_REMOVE(td->td_sleepqueue, sq_hash);
   
           td->td_wmesg = NULL;
           td->td_wchan = NULL;
           td->td_flags &= ~TDF_SINTR;
   
           /*
            * Note that thread td might not be sleeping if it is running
            * sleepq_catch_signals() on another CPU or is blocked on
            * its proc lock to check signals.  It doesn't hurt to clear
            * the sleeping flag if it isn't set though, so we just always
            * do it.  However, we can't assert that it is set.
            */
           CTR3(KTR_PROC, "sleepq_wakeup: thread %p (pid %ld, %s)",
               (void *)td, (long)td->td_proc->p_pid, td->td_proc->p_comm);
           TD_CLR_SLEEPING(td);
   
           /* Adjust priority if requested. */
           MPASS(pri == -1 || (pri >= PRI_MIN && pri <= PRI_MAX));
           if (pri != -1 && td->td_priority > pri)
                   sched_prio(td, pri);
           setrunnable(td);
   }
   
   /*
    * Find the highest priority thread sleeping on a wait channel and resume it.
    */
   void
   sleepq_signal(void *wchan, int flags, int pri)
   {
           struct sleepqueue *sq;
           struct thread *td, *besttd;
   
           CTR2(KTR_PROC, "sleepq_signal(%p, %d)", wchan, flags);
           KASSERT(wchan != NULL, ("%s: invalid NULL wait channel", __func__));
           sq = sleepq_lookup(wchan);
           if (sq == NULL) {
                   sleepq_release(wchan);
                   return;
           }
           KASSERT(sq->sq_type == (flags & SLEEPQ_TYPE),
               ("%s: mismatch between sleep/wakeup and cv_*", __func__));
   
           /*
            * Find the highest priority thread on the queue.  If there is a
            * tie, use the thread that first appears in the queue as it has
            * been sleeping the longest since threads are always added to
            * the tail of sleep queues.
            */
           besttd = NULL;
           TAILQ_FOREACH(td, &sq->sq_blocked, td_slpq) {
                   if (besttd == NULL || td->td_priority < besttd->td_priority)
                           besttd = td;
           }
           MPASS(besttd != NULL);
           mtx_lock_spin(&sched_lock);
           sleepq_resume_thread(sq, besttd, pri);
           mtx_unlock_spin(&sched_lock);
           sleepq_release(wchan);
   }
   
   /*
    * Resume all threads sleeping on a specified wait channel.
    */
   void
   sleepq_broadcast(void *wchan, int flags, int pri)
   {
           struct sleepqueue *sq;
   
           CTR2(KTR_PROC, "sleepq_broadcast(%p, %d)", wchan, flags);
           KASSERT(wchan != NULL, ("%s: invalid NULL wait channel", __func__));
           sq = sleepq_lookup(wchan);
           if (sq == NULL) {
                   sleepq_release(wchan);
                   return;
           }
           KASSERT(sq->sq_type == (flags & SLEEPQ_TYPE),
               ("%s: mismatch between sleep/wakeup and cv_*", __func__));
   
           /* Resume all blocked threads on the sleep queue. */
           mtx_lock_spin(&sched_lock);
           while (!TAILQ_EMPTY(&sq->sq_blocked))
                   sleepq_resume_thread(sq, TAILQ_FIRST(&sq->sq_blocked), pri);
           mtx_unlock_spin(&sched_lock);
           sleepq_release(wchan);
   }
   
   /*
    * Time sleeping threads out.  When the timeout expires, the thread is
    * removed from the sleep queue and made runnable if it is still asleep.
    */
   static void
   sleepq_timeout(void *arg)
   {
           struct sleepqueue *sq;
           struct thread *td;
           void *wchan;
   
           td = arg;
           CTR3(KTR_PROC, "sleepq_timeout: thread %p (pid %ld, %s)",
               (void *)td, (long)td->td_proc->p_pid, (void *)td->td_proc->p_comm);
   
           /*
            * First, see if the thread is asleep and get the wait channel if
            * it is.
            */
           mtx_lock_spin(&sched_lock);
           if (TD_ON_SLEEPQ(td)) {
                   wchan = td->td_wchan;
                   mtx_unlock_spin(&sched_lock);
                   sleepq_lock(wchan);
                   sq = sleepq_lookup(wchan);
                   mtx_lock_spin(&sched_lock);
           } else {
                   wchan = NULL;
                   sq = NULL;
           }
   
           /*
            * At this point, if the thread is still on the sleep queue,
            * we have that sleep queue locked as it cannot migrate sleep
            * queues while we dropped sched_lock.  If it had resumed and
            * was on another CPU while the lock was dropped, it would have
            * seen that TDF_TIMEOUT and TDF_TIMOFAIL are clear and the
            * call to callout_stop() to stop this routine would have failed
            * meaning that it would have already set TDF_TIMEOUT to
            * synchronize with this function.
            */
           if (TD_ON_SLEEPQ(td)) {
                   MPASS(td->td_wchan == wchan);
                   MPASS(sq != NULL);
                   td->td_flags |= TDF_TIMEOUT;
                   sleepq_resume_thread(sq, td, -1);
                   mtx_unlock_spin(&sched_lock);
                   sleepq_release(wchan);
                   return;
           } else if (wchan != NULL)
                   sleepq_release(wchan);
   
           /*
            * Now check for the edge cases.  First, if TDF_TIMEOUT is set,
            * then the other thread has already yielded to us, so clear
            * the flag and resume it.  If TDF_TIMEOUT is not set, then the
            * we know that the other thread is not on a sleep queue, but it
            * hasn't resumed execution yet.  In that case, set TDF_TIMOFAIL
            * to let it know that the timeout has already run and doesn't
            * need to be canceled.
            */
           if (td->td_flags & TDF_TIMEOUT) {
                   MPASS(TD_IS_SLEEPING(td));
                   td->td_flags &= ~TDF_TIMEOUT;
                   TD_CLR_SLEEPING(td);
                   setrunnable(td);
           } else
                   td->td_flags |= TDF_TIMOFAIL;
           mtx_unlock_spin(&sched_lock);
   }
   
   /*
    * Resumes a specific thread from the sleep queue associated with a specific
    * wait channel if it is on that queue.
    */
   void
   sleepq_remove(struct thread *td, void *wchan)
   {
           struct sleepqueue *sq;
   
           /*
            * Look up the sleep queue for this wait channel, then re-check
            * that the thread is asleep on that channel, if it is not, then
            * bail.
            */
           MPASS(wchan != NULL);
           sleepq_lock(wchan);
           sq = sleepq_lookup(wchan);
           mtx_lock_spin(&sched_lock);
           if (!TD_ON_SLEEPQ(td) || td->td_wchan != wchan) {
                   mtx_unlock_spin(&sched_lock);
                   sleepq_release(wchan);
                   return;
           }
           MPASS(sq != NULL);
   
           /* Thread is asleep on sleep queue sq, so wake it up. */
           sleepq_resume_thread(sq, td, -1);
           sleepq_release(wchan);
           mtx_unlock_spin(&sched_lock);
   }
   
   /*
    * Abort a thread as if an interrupt had occurred.  Only abort
    * interruptible waits (unfortunately it isn't safe to abort others).
    *
    * XXX: What in the world does the comment below mean?
    * Also, whatever the signal code does...
    */
   void
   sleepq_abort(struct thread *td, int intrval)
   {
           void *wchan;
   
           mtx_assert(&sched_lock, MA_OWNED);
           MPASS(TD_ON_SLEEPQ(td));
           MPASS(td->td_flags & TDF_SINTR);
           MPASS(intrval == EINTR || intrval == ERESTART);
   
           /*
            * If the TDF_TIMEOUT flag is set, just leave. A
            * timeout is scheduled anyhow.
            */
           if (td->td_flags & TDF_TIMEOUT)
                   return;
   
           CTR3(KTR_PROC, "sleepq_abort: thread %p (pid %ld, %s)",
               (void *)td, (long)td->td_proc->p_pid, (void *)td->td_proc->p_comm);
           wchan = td->td_wchan;
           if (wchan != NULL) {
                   td->td_intrval = intrval;
                   td->td_flags |= TDF_SLEEPABORT;
           }
           mtx_unlock_spin(&sched_lock);
           sleepq_remove(td, wchan);
           mtx_lock_spin(&sched_lock);
   }
   
   #ifdef DDB
   DB_SHOW_COMMAND(sleepq, db_show_sleepqueue)
   {
           struct sleepqueue_chain *sc;
           struct sleepqueue *sq;
   #ifdef INVARIANTS
           struct lock_object *lock;
   #endif
           struct thread *td;
           void *wchan;
           int i;
   
           if (!have_addr)
                   return;
   
           /*
            * First, see if there is an active sleep queue for the wait channel
            * indicated by the address.
            */
           wchan = (void *)addr;
           sc = SC_LOOKUP(wchan);
           LIST_FOREACH(sq, &sc->sc_queues, sq_hash)
                   if (sq->sq_wchan == wchan)
                           goto found;
   
           /*
            * Second, see if there is an active sleep queue at the address
            * indicated.
            */
           for (i = 0; i < SC_TABLESIZE; i++)
                   LIST_FOREACH(sq, &sleepq_chains[i].sc_queues, sq_hash) {
                           if (sq == (struct sleepqueue *)addr)
                                   goto found;
                   }
   
           db_printf("Unable to locate a sleep queue via %p\n", (void *)addr);
           return;
   found:
           db_printf("Wait channel: %p\n", sq->sq_wchan);
   #ifdef INVARIANTS
           db_printf("Queue type: %d\n", sq->sq_type);
           if (sq->sq_lock) {
                   lock = &sq->sq_lock->mtx_object;
                   db_printf("Associated Interlock: %p - (%s) %s\n", lock,
                       LOCK_CLASS(lock)->lc_name, lock->lo_name);
           }
   #endif
           db_printf("Blocked threads:\n");
           if (TAILQ_EMPTY(&sq->sq_blocked))
                   db_printf("\tempty\n");
           else
                   TAILQ_FOREACH(td, &sq->sq_blocked, td_slpq) {
                           db_printf("\t%p (tid %d, pid %d, \"%s\")\n", td,
                               td->td_tid, td->td_proc->p_pid,
                               td->td_proc->p_comm);
                   }       
   }
   #endif

Cache object: cce6636cdff1e772a7f94920073a5609