The Design and Implementation of the FreeBSD Operating System, Second Edition
Now available: The Design and Implementation of the FreeBSD Operating System (Second Edition)


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

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    1 /*-
    2  * Copyright (c) 1982, 1986, 1990, 1991, 1993
    3  *      The Regents of the University of California.  All rights reserved.
    4  * (c) UNIX System Laboratories, Inc.
    5  * All or some portions of this file are derived from material licensed
    6  * to the University of California by American Telephone and Telegraph
    7  * Co. or Unix System Laboratories, Inc. and are reproduced herein with
    8  * the permission of UNIX System Laboratories, Inc.
    9  *
   10  * Redistribution and use in source and binary forms, with or without
   11  * modification, are permitted provided that the following conditions
   12  * are met:
   13  * 1. Redistributions of source code must retain the above copyright
   14  *    notice, this list of conditions and the following disclaimer.
   15  * 2. Redistributions in binary form must reproduce the above copyright
   16  *    notice, this list of conditions and the following disclaimer in the
   17  *    documentation and/or other materials provided with the distribution.
   18  * 4. Neither the name of the University nor the names of its contributors
   19  *    may be used to endorse or promote products derived from this software
   20  *    without specific prior written permission.
   21  *
   22  * THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
   23  * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
   24  * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
   25  * ARE DISCLAIMED.  IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
   26  * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
   27  * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
   28  * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
   29  * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
   30  * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
   31  * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
   32  * SUCH DAMAGE.
   33  *
   34  *      @(#)kern_synch.c        8.9 (Berkeley) 5/19/95
   35  */
   36 
   37 #include <sys/cdefs.h>
   38 __FBSDID("$FreeBSD: releng/11.0/sys/kern/kern_synch.c 301456 2016-06-05 17:04:03Z kib $");
   39 
   40 #include "opt_ktrace.h"
   41 #include "opt_sched.h"
   42 
   43 #include <sys/param.h>
   44 #include <sys/systm.h>
   45 #include <sys/condvar.h>
   46 #include <sys/kdb.h>
   47 #include <sys/kernel.h>
   48 #include <sys/ktr.h>
   49 #include <sys/lock.h>
   50 #include <sys/mutex.h>
   51 #include <sys/proc.h>
   52 #include <sys/resourcevar.h>
   53 #include <sys/sched.h>
   54 #include <sys/sdt.h>
   55 #include <sys/signalvar.h>
   56 #include <sys/sleepqueue.h>
   57 #include <sys/smp.h>
   58 #include <sys/sx.h>
   59 #include <sys/sysctl.h>
   60 #include <sys/sysproto.h>
   61 #include <sys/vmmeter.h>
   62 #ifdef KTRACE
   63 #include <sys/uio.h>
   64 #include <sys/ktrace.h>
   65 #endif
   66 
   67 #include <machine/cpu.h>
   68 
   69 #define KTDSTATE(td)                                                    \
   70         (((td)->td_inhibitors & TDI_SLEEPING) != 0 ? "sleep"  :         \
   71         ((td)->td_inhibitors & TDI_SUSPENDED) != 0 ? "suspended" :      \
   72         ((td)->td_inhibitors & TDI_SWAPPED) != 0 ? "swapped" :          \
   73         ((td)->td_inhibitors & TDI_LOCK) != 0 ? "blocked" :             \
   74         ((td)->td_inhibitors & TDI_IWAIT) != 0 ? "iwait" : "yielding")
   75 
   76 static void synch_setup(void *dummy);
   77 SYSINIT(synch_setup, SI_SUB_KICK_SCHEDULER, SI_ORDER_FIRST, synch_setup,
   78     NULL);
   79 
   80 int     hogticks;
   81 static uint8_t pause_wchan[MAXCPU];
   82 
   83 static struct callout loadav_callout;
   84 
   85 struct loadavg averunnable =
   86         { {0, 0, 0}, FSCALE };  /* load average, of runnable procs */
   87 /*
   88  * Constants for averages over 1, 5, and 15 minutes
   89  * when sampling at 5 second intervals.
   90  */
   91 static fixpt_t cexp[3] = {
   92         0.9200444146293232 * FSCALE,    /* exp(-1/12) */
   93         0.9834714538216174 * FSCALE,    /* exp(-1/60) */
   94         0.9944598480048967 * FSCALE,    /* exp(-1/180) */
   95 };
   96 
   97 /* kernel uses `FSCALE', userland (SHOULD) use kern.fscale */
   98 SYSCTL_INT(_kern, OID_AUTO, fscale, CTLFLAG_RD, SYSCTL_NULL_INT_PTR, FSCALE, "");
   99 
  100 static void     loadav(void *arg);
  101 
  102 SDT_PROVIDER_DECLARE(sched);
  103 SDT_PROBE_DEFINE(sched, , , preempt);
  104 
  105 static void
  106 sleepinit(void *unused)
  107 {
  108 
  109         hogticks = (hz / 10) * 2;       /* Default only. */
  110         init_sleepqueues();
  111 }
  112 
  113 /*
  114  * vmem tries to lock the sleepq mutexes when free'ing kva, so make sure
  115  * it is available.
  116  */
  117 SYSINIT(sleepinit, SI_SUB_KMEM, SI_ORDER_ANY, sleepinit, 0);
  118 
  119 /*
  120  * General sleep call.  Suspends the current thread until a wakeup is
  121  * performed on the specified identifier.  The thread will then be made
  122  * runnable with the specified priority.  Sleeps at most sbt units of time
  123  * (0 means no timeout).  If pri includes the PCATCH flag, let signals
  124  * interrupt the sleep, otherwise ignore them while sleeping.  Returns 0 if
  125  * awakened, EWOULDBLOCK if the timeout expires.  If PCATCH is set and a
  126  * signal becomes pending, ERESTART is returned if the current system
  127  * call should be restarted if possible, and EINTR is returned if the system
  128  * call should be interrupted by the signal (return EINTR).
  129  *
  130  * The lock argument is unlocked before the caller is suspended, and
  131  * re-locked before _sleep() returns.  If priority includes the PDROP
  132  * flag the lock is not re-locked before returning.
  133  */
  134 int
  135 _sleep(void *ident, struct lock_object *lock, int priority,
  136     const char *wmesg, sbintime_t sbt, sbintime_t pr, int flags)
  137 {
  138         struct thread *td;
  139         struct proc *p;
  140         struct lock_class *class;
  141         uintptr_t lock_state;
  142         int catch, pri, rval, sleepq_flags;
  143         WITNESS_SAVE_DECL(lock_witness);
  144 
  145         td = curthread;
  146         p = td->td_proc;
  147 #ifdef KTRACE
  148         if (KTRPOINT(td, KTR_CSW))
  149                 ktrcsw(1, 0, wmesg);
  150 #endif
  151         WITNESS_WARN(WARN_GIANTOK | WARN_SLEEPOK, lock,
  152             "Sleeping on \"%s\"", wmesg);
  153         KASSERT(sbt != 0 || mtx_owned(&Giant) || lock != NULL,
  154             ("sleeping without a lock"));
  155         KASSERT(p != NULL, ("msleep1"));
  156         KASSERT(ident != NULL && TD_IS_RUNNING(td), ("msleep"));
  157         if (priority & PDROP)
  158                 KASSERT(lock != NULL && lock != &Giant.lock_object,
  159                     ("PDROP requires a non-Giant lock"));
  160         if (lock != NULL)
  161                 class = LOCK_CLASS(lock);
  162         else
  163                 class = NULL;
  164 
  165         if (SCHEDULER_STOPPED()) {
  166                 if (lock != NULL && priority & PDROP)
  167                         class->lc_unlock(lock);
  168                 return (0);
  169         }
  170         catch = priority & PCATCH;
  171         pri = priority & PRIMASK;
  172 
  173         /*
  174          * If we are already on a sleep queue, then remove us from that
  175          * sleep queue first.  We have to do this to handle recursive
  176          * sleeps.
  177          */
  178         if (TD_ON_SLEEPQ(td))
  179                 sleepq_remove(td, td->td_wchan);
  180 
  181         if ((uint8_t *)ident >= &pause_wchan[0] &&
  182             (uint8_t *)ident <= &pause_wchan[MAXCPU - 1])
  183                 sleepq_flags = SLEEPQ_PAUSE;
  184         else
  185                 sleepq_flags = SLEEPQ_SLEEP;
  186         if (catch)
  187                 sleepq_flags |= SLEEPQ_INTERRUPTIBLE;
  188 
  189         sleepq_lock(ident);
  190         CTR5(KTR_PROC, "sleep: thread %ld (pid %ld, %s) on %s (%p)",
  191             td->td_tid, p->p_pid, td->td_name, wmesg, ident);
  192 
  193         if (lock == &Giant.lock_object)
  194                 mtx_assert(&Giant, MA_OWNED);
  195         DROP_GIANT();
  196         if (lock != NULL && lock != &Giant.lock_object &&
  197             !(class->lc_flags & LC_SLEEPABLE)) {
  198                 WITNESS_SAVE(lock, lock_witness);
  199                 lock_state = class->lc_unlock(lock);
  200         } else
  201                 /* GCC needs to follow the Yellow Brick Road */
  202                 lock_state = -1;
  203 
  204         /*
  205          * We put ourselves on the sleep queue and start our timeout
  206          * before calling thread_suspend_check, as we could stop there,
  207          * and a wakeup or a SIGCONT (or both) could occur while we were
  208          * stopped without resuming us.  Thus, we must be ready for sleep
  209          * when cursig() is called.  If the wakeup happens while we're
  210          * stopped, then td will no longer be on a sleep queue upon
  211          * return from cursig().
  212          */
  213         sleepq_add(ident, lock, wmesg, sleepq_flags, 0);
  214         if (sbt != 0)
  215                 sleepq_set_timeout_sbt(ident, sbt, pr, flags);
  216         if (lock != NULL && class->lc_flags & LC_SLEEPABLE) {
  217                 sleepq_release(ident);
  218                 WITNESS_SAVE(lock, lock_witness);
  219                 lock_state = class->lc_unlock(lock);
  220                 sleepq_lock(ident);
  221         }
  222         if (sbt != 0 && catch)
  223                 rval = sleepq_timedwait_sig(ident, pri);
  224         else if (sbt != 0)
  225                 rval = sleepq_timedwait(ident, pri);
  226         else if (catch)
  227                 rval = sleepq_wait_sig(ident, pri);
  228         else {
  229                 sleepq_wait(ident, pri);
  230                 rval = 0;
  231         }
  232 #ifdef KTRACE
  233         if (KTRPOINT(td, KTR_CSW))
  234                 ktrcsw(0, 0, wmesg);
  235 #endif
  236         PICKUP_GIANT();
  237         if (lock != NULL && lock != &Giant.lock_object && !(priority & PDROP)) {
  238                 class->lc_lock(lock, lock_state);
  239                 WITNESS_RESTORE(lock, lock_witness);
  240         }
  241         return (rval);
  242 }
  243 
  244 int
  245 msleep_spin_sbt(void *ident, struct mtx *mtx, const char *wmesg,
  246     sbintime_t sbt, sbintime_t pr, int flags)
  247 {
  248         struct thread *td;
  249         struct proc *p;
  250         int rval;
  251         WITNESS_SAVE_DECL(mtx);
  252 
  253         td = curthread;
  254         p = td->td_proc;
  255         KASSERT(mtx != NULL, ("sleeping without a mutex"));
  256         KASSERT(p != NULL, ("msleep1"));
  257         KASSERT(ident != NULL && TD_IS_RUNNING(td), ("msleep"));
  258 
  259         if (SCHEDULER_STOPPED())
  260                 return (0);
  261 
  262         sleepq_lock(ident);
  263         CTR5(KTR_PROC, "msleep_spin: thread %ld (pid %ld, %s) on %s (%p)",
  264             td->td_tid, p->p_pid, td->td_name, wmesg, ident);
  265 
  266         DROP_GIANT();
  267         mtx_assert(mtx, MA_OWNED | MA_NOTRECURSED);
  268         WITNESS_SAVE(&mtx->lock_object, mtx);
  269         mtx_unlock_spin(mtx);
  270 
  271         /*
  272          * We put ourselves on the sleep queue and start our timeout.
  273          */
  274         sleepq_add(ident, &mtx->lock_object, wmesg, SLEEPQ_SLEEP, 0);
  275         if (sbt != 0)
  276                 sleepq_set_timeout_sbt(ident, sbt, pr, flags);
  277 
  278         /*
  279          * Can't call ktrace with any spin locks held so it can lock the
  280          * ktrace_mtx lock, and WITNESS_WARN considers it an error to hold
  281          * any spin lock.  Thus, we have to drop the sleepq spin lock while
  282          * we handle those requests.  This is safe since we have placed our
  283          * thread on the sleep queue already.
  284          */
  285 #ifdef KTRACE
  286         if (KTRPOINT(td, KTR_CSW)) {
  287                 sleepq_release(ident);
  288                 ktrcsw(1, 0, wmesg);
  289                 sleepq_lock(ident);
  290         }
  291 #endif
  292 #ifdef WITNESS
  293         sleepq_release(ident);
  294         WITNESS_WARN(WARN_GIANTOK | WARN_SLEEPOK, NULL, "Sleeping on \"%s\"",
  295             wmesg);
  296         sleepq_lock(ident);
  297 #endif
  298         if (sbt != 0)
  299                 rval = sleepq_timedwait(ident, 0);
  300         else {
  301                 sleepq_wait(ident, 0);
  302                 rval = 0;
  303         }
  304 #ifdef KTRACE
  305         if (KTRPOINT(td, KTR_CSW))
  306                 ktrcsw(0, 0, wmesg);
  307 #endif
  308         PICKUP_GIANT();
  309         mtx_lock_spin(mtx);
  310         WITNESS_RESTORE(&mtx->lock_object, mtx);
  311         return (rval);
  312 }
  313 
  314 /*
  315  * pause() delays the calling thread by the given number of system ticks.
  316  * During cold bootup, pause() uses the DELAY() function instead of
  317  * the tsleep() function to do the waiting. The "timo" argument must be
  318  * greater than or equal to zero. A "timo" value of zero is equivalent
  319  * to a "timo" value of one.
  320  */
  321 int
  322 pause_sbt(const char *wmesg, sbintime_t sbt, sbintime_t pr, int flags)
  323 {
  324         KASSERT(sbt >= 0, ("pause: timeout must be >= 0"));
  325 
  326         /* silently convert invalid timeouts */
  327         if (sbt == 0)
  328                 sbt = tick_sbt;
  329 
  330         if (cold || kdb_active || SCHEDULER_STOPPED()) {
  331                 /*
  332                  * We delay one second at a time to avoid overflowing the
  333                  * system specific DELAY() function(s):
  334                  */
  335                 while (sbt >= SBT_1S) {
  336                         DELAY(1000000);
  337                         sbt -= SBT_1S;
  338                 }
  339                 /* Do the delay remainder, if any */
  340                 sbt = howmany(sbt, SBT_1US);
  341                 if (sbt > 0)
  342                         DELAY(sbt);
  343                 return (0);
  344         }
  345         return (_sleep(&pause_wchan[curcpu], NULL, 0, wmesg, sbt, pr, flags));
  346 }
  347 
  348 /*
  349  * Make all threads sleeping on the specified identifier runnable.
  350  */
  351 void
  352 wakeup(void *ident)
  353 {
  354         int wakeup_swapper;
  355 
  356         sleepq_lock(ident);
  357         wakeup_swapper = sleepq_broadcast(ident, SLEEPQ_SLEEP, 0, 0);
  358         sleepq_release(ident);
  359         if (wakeup_swapper) {
  360                 KASSERT(ident != &proc0,
  361                     ("wakeup and wakeup_swapper and proc0"));
  362                 kick_proc0();
  363         }
  364 }
  365 
  366 /*
  367  * Make a thread sleeping on the specified identifier runnable.
  368  * May wake more than one thread if a target thread is currently
  369  * swapped out.
  370  */
  371 void
  372 wakeup_one(void *ident)
  373 {
  374         int wakeup_swapper;
  375 
  376         sleepq_lock(ident);
  377         wakeup_swapper = sleepq_signal(ident, SLEEPQ_SLEEP, 0, 0);
  378         sleepq_release(ident);
  379         if (wakeup_swapper)
  380                 kick_proc0();
  381 }
  382 
  383 static void
  384 kdb_switch(void)
  385 {
  386         thread_unlock(curthread);
  387         kdb_backtrace();
  388         kdb_reenter();
  389         panic("%s: did not reenter debugger", __func__);
  390 }
  391 
  392 /*
  393  * The machine independent parts of context switching.
  394  */
  395 void
  396 mi_switch(int flags, struct thread *newtd)
  397 {
  398         uint64_t runtime, new_switchtime;
  399         struct thread *td;
  400 
  401         td = curthread;                 /* XXX */
  402         THREAD_LOCK_ASSERT(td, MA_OWNED | MA_NOTRECURSED);
  403         KASSERT(!TD_ON_RUNQ(td), ("mi_switch: called by old code"));
  404 #ifdef INVARIANTS
  405         if (!TD_ON_LOCK(td) && !TD_IS_RUNNING(td))
  406                 mtx_assert(&Giant, MA_NOTOWNED);
  407 #endif
  408         KASSERT(td->td_critnest == 1 || panicstr,
  409             ("mi_switch: switch in a critical section"));
  410         KASSERT((flags & (SW_INVOL | SW_VOL)) != 0,
  411             ("mi_switch: switch must be voluntary or involuntary"));
  412         KASSERT(newtd != curthread, ("mi_switch: preempting back to ourself"));
  413 
  414         /*
  415          * Don't perform context switches from the debugger.
  416          */
  417         if (kdb_active)
  418                 kdb_switch();
  419         if (SCHEDULER_STOPPED())
  420                 return;
  421         if (flags & SW_VOL) {
  422                 td->td_ru.ru_nvcsw++;
  423                 td->td_swvoltick = ticks;
  424         } else {
  425                 td->td_ru.ru_nivcsw++;
  426                 td->td_swinvoltick = ticks;
  427         }
  428 #ifdef SCHED_STATS
  429         SCHED_STAT_INC(sched_switch_stats[flags & SW_TYPE_MASK]);
  430 #endif
  431         /*
  432          * Compute the amount of time during which the current
  433          * thread was running, and add that to its total so far.
  434          */
  435         new_switchtime = cpu_ticks();
  436         runtime = new_switchtime - PCPU_GET(switchtime);
  437         td->td_runtime += runtime;
  438         td->td_incruntime += runtime;
  439         PCPU_SET(switchtime, new_switchtime);
  440         td->td_generation++;    /* bump preempt-detect counter */
  441         PCPU_INC(cnt.v_swtch);
  442         PCPU_SET(switchticks, ticks);
  443         CTR4(KTR_PROC, "mi_switch: old thread %ld (td_sched %p, pid %ld, %s)",
  444             td->td_tid, td_get_sched(td), td->td_proc->p_pid, td->td_name);
  445 #if (KTR_COMPILE & KTR_SCHED) != 0
  446         if (TD_IS_IDLETHREAD(td))
  447                 KTR_STATE1(KTR_SCHED, "thread", sched_tdname(td), "idle",
  448                     "prio:%d", td->td_priority);
  449         else
  450                 KTR_STATE3(KTR_SCHED, "thread", sched_tdname(td), KTDSTATE(td),
  451                     "prio:%d", td->td_priority, "wmesg:\"%s\"", td->td_wmesg,
  452                     "lockname:\"%s\"", td->td_lockname);
  453 #endif
  454         SDT_PROBE0(sched, , , preempt);
  455         sched_switch(td, newtd, flags);
  456         KTR_STATE1(KTR_SCHED, "thread", sched_tdname(td), "running",
  457             "prio:%d", td->td_priority);
  458 
  459         CTR4(KTR_PROC, "mi_switch: new thread %ld (td_sched %p, pid %ld, %s)",
  460             td->td_tid, td_get_sched(td), td->td_proc->p_pid, td->td_name);
  461 
  462         /* 
  463          * If the last thread was exiting, finish cleaning it up.
  464          */
  465         if ((td = PCPU_GET(deadthread))) {
  466                 PCPU_SET(deadthread, NULL);
  467                 thread_stash(td);
  468         }
  469 }
  470 
  471 /*
  472  * Change thread state to be runnable, placing it on the run queue if
  473  * it is in memory.  If it is swapped out, return true so our caller
  474  * will know to awaken the swapper.
  475  */
  476 int
  477 setrunnable(struct thread *td)
  478 {
  479 
  480         THREAD_LOCK_ASSERT(td, MA_OWNED);
  481         KASSERT(td->td_proc->p_state != PRS_ZOMBIE,
  482             ("setrunnable: pid %d is a zombie", td->td_proc->p_pid));
  483         switch (td->td_state) {
  484         case TDS_RUNNING:
  485         case TDS_RUNQ:
  486                 return (0);
  487         case TDS_INHIBITED:
  488                 /*
  489                  * If we are only inhibited because we are swapped out
  490                  * then arange to swap in this process. Otherwise just return.
  491                  */
  492                 if (td->td_inhibitors != TDI_SWAPPED)
  493                         return (0);
  494                 /* FALLTHROUGH */
  495         case TDS_CAN_RUN:
  496                 break;
  497         default:
  498                 printf("state is 0x%x", td->td_state);
  499                 panic("setrunnable(2)");
  500         }
  501         if ((td->td_flags & TDF_INMEM) == 0) {
  502                 if ((td->td_flags & TDF_SWAPINREQ) == 0) {
  503                         td->td_flags |= TDF_SWAPINREQ;
  504                         return (1);
  505                 }
  506         } else
  507                 sched_wakeup(td);
  508         return (0);
  509 }
  510 
  511 /*
  512  * Compute a tenex style load average of a quantity on
  513  * 1, 5 and 15 minute intervals.
  514  */
  515 static void
  516 loadav(void *arg)
  517 {
  518         int i, nrun;
  519         struct loadavg *avg;
  520 
  521         nrun = sched_load();
  522         avg = &averunnable;
  523 
  524         for (i = 0; i < 3; i++)
  525                 avg->ldavg[i] = (cexp[i] * avg->ldavg[i] +
  526                     nrun * FSCALE * (FSCALE - cexp[i])) >> FSHIFT;
  527 
  528         /*
  529          * Schedule the next update to occur after 5 seconds, but add a
  530          * random variation to avoid synchronisation with processes that
  531          * run at regular intervals.
  532          */
  533         callout_reset_sbt(&loadav_callout,
  534             SBT_1US * (4000000 + (int)(random() % 2000001)), SBT_1US,
  535             loadav, NULL, C_DIRECT_EXEC | C_PREL(32));
  536 }
  537 
  538 /* ARGSUSED */
  539 static void
  540 synch_setup(void *dummy)
  541 {
  542         callout_init(&loadav_callout, 1);
  543 
  544         /* Kick off timeout driven events by calling first time. */
  545         loadav(NULL);
  546 }
  547 
  548 int
  549 should_yield(void)
  550 {
  551 
  552         return ((u_int)ticks - (u_int)curthread->td_swvoltick >= hogticks);
  553 }
  554 
  555 void
  556 maybe_yield(void)
  557 {
  558 
  559         if (should_yield())
  560                 kern_yield(PRI_USER);
  561 }
  562 
  563 void
  564 kern_yield(int prio)
  565 {
  566         struct thread *td;
  567 
  568         td = curthread;
  569         DROP_GIANT();
  570         thread_lock(td);
  571         if (prio == PRI_USER)
  572                 prio = td->td_user_pri;
  573         if (prio >= 0)
  574                 sched_prio(td, prio);
  575         mi_switch(SW_VOL | SWT_RELINQUISH, NULL);
  576         thread_unlock(td);
  577         PICKUP_GIANT();
  578 }
  579 
  580 /*
  581  * General purpose yield system call.
  582  */
  583 int
  584 sys_yield(struct thread *td, struct yield_args *uap)
  585 {
  586 
  587         thread_lock(td);
  588         if (PRI_BASE(td->td_pri_class) == PRI_TIMESHARE)
  589                 sched_prio(td, PRI_MAX_TIMESHARE);
  590         mi_switch(SW_VOL | SWT_RELINQUISH, NULL);
  591         thread_unlock(td);
  592         td->td_retval[0] = 0;
  593         return (0);
  594 }

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