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: stable/10/sys/kern/kern_synch.c 337035 2018-08-01 10:35:49Z hselasky $");
   39 
   40 #include "opt_kdtrace.h"
   41 #include "opt_ktrace.h"
   42 #include "opt_sched.h"
   43 
   44 #include <sys/param.h>
   45 #include <sys/systm.h>
   46 #include <sys/condvar.h>
   47 #include <sys/kdb.h>
   48 #include <sys/kernel.h>
   49 #include <sys/ktr.h>
   50 #include <sys/lock.h>
   51 #include <sys/mutex.h>
   52 #include <sys/proc.h>
   53 #include <sys/resourcevar.h>
   54 #include <sys/sched.h>
   55 #include <sys/sdt.h>
   56 #include <sys/signalvar.h>
   57 #include <sys/sleepqueue.h>
   58 #include <sys/smp.h>
   59 #include <sys/sx.h>
   60 #include <sys/sysctl.h>
   61 #include <sys/sysproto.h>
   62 #include <sys/vmmeter.h>
   63 #ifdef KTRACE
   64 #include <sys/uio.h>
   65 #include <sys/ktrace.h>
   66 #endif
   67 
   68 #include <machine/cpu.h>
   69 
   70 #ifdef XEN
   71 #include <vm/vm.h>
   72 #include <vm/vm_param.h>
   73 #include <vm/pmap.h>
   74 #endif
   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 /*
  106  * These probes reference Solaris features that are not implemented in FreeBSD.
  107  * Create the probes anyway for compatibility with existing D scripts; they'll
  108  * just never fire.
  109  */
  110 SDT_PROBE_DEFINE(sched, , , cpucaps__sleep);
  111 SDT_PROBE_DEFINE(sched, , , cpucaps__wakeup);
  112 SDT_PROBE_DEFINE(sched, , , schedctl__nopreempt);
  113 SDT_PROBE_DEFINE(sched, , , schedctl__preempt);
  114 SDT_PROBE_DEFINE(sched, , , schedctl__yield);
  115 
  116 static void
  117 sleepinit(void *unused)
  118 {
  119 
  120         hogticks = (hz / 10) * 2;       /* Default only. */
  121         init_sleepqueues();
  122 }
  123 
  124 /*
  125  * vmem tries to lock the sleepq mutexes when free'ing kva, so make sure
  126  * it is available.
  127  */
  128 SYSINIT(sleepinit, SI_SUB_KMEM, SI_ORDER_ANY, sleepinit, 0);
  129 
  130 /*
  131  * General sleep call.  Suspends the current thread until a wakeup is
  132  * performed on the specified identifier.  The thread will then be made
  133  * runnable with the specified priority.  Sleeps at most sbt units of time
  134  * (0 means no timeout).  If pri includes the PCATCH flag, let signals
  135  * interrupt the sleep, otherwise ignore them while sleeping.  Returns 0 if
  136  * awakened, EWOULDBLOCK if the timeout expires.  If PCATCH is set and a
  137  * signal becomes pending, ERESTART is returned if the current system
  138  * call should be restarted if possible, and EINTR is returned if the system
  139  * call should be interrupted by the signal (return EINTR).
  140  *
  141  * The lock argument is unlocked before the caller is suspended, and
  142  * re-locked before _sleep() returns.  If priority includes the PDROP
  143  * flag the lock is not re-locked before returning.
  144  */
  145 int
  146 _sleep(void *ident, struct lock_object *lock, int priority,
  147     const char *wmesg, sbintime_t sbt, sbintime_t pr, int flags)
  148 {
  149         struct thread *td;
  150         struct proc *p;
  151         struct lock_class *class;
  152         uintptr_t lock_state;
  153         int catch, pri, rval, sleepq_flags;
  154         WITNESS_SAVE_DECL(lock_witness);
  155 
  156         td = curthread;
  157         p = td->td_proc;
  158 #ifdef KTRACE
  159         if (KTRPOINT(td, KTR_CSW))
  160                 ktrcsw(1, 0, wmesg);
  161 #endif
  162         WITNESS_WARN(WARN_GIANTOK | WARN_SLEEPOK, lock,
  163             "Sleeping on \"%s\"", wmesg);
  164         KASSERT(sbt != 0 || mtx_owned(&Giant) || lock != NULL,
  165             ("sleeping without a lock"));
  166         KASSERT(p != NULL, ("msleep1"));
  167         KASSERT(ident != NULL && TD_IS_RUNNING(td), ("msleep"));
  168         if (priority & PDROP)
  169                 KASSERT(lock != NULL && lock != &Giant.lock_object,
  170                     ("PDROP requires a non-Giant lock"));
  171         if (lock != NULL)
  172                 class = LOCK_CLASS(lock);
  173         else
  174                 class = NULL;
  175 
  176         if (cold || SCHEDULER_STOPPED()) {
  177                 /*
  178                  * During autoconfiguration, just return;
  179                  * don't run any other threads or panic below,
  180                  * in case this is the idle thread and already asleep.
  181                  * XXX: this used to do "s = splhigh(); splx(safepri);
  182                  * splx(s);" to give interrupts a chance, but there is
  183                  * no way to give interrupts a chance now.
  184                  */
  185                 if (lock != NULL && priority & PDROP)
  186                         class->lc_unlock(lock);
  187                 return (0);
  188         }
  189         catch = priority & PCATCH;
  190         pri = priority & PRIMASK;
  191 
  192         KASSERT(!TD_ON_SLEEPQ(td), ("recursive sleep"));
  193 
  194         if ((uint8_t *)ident >= &pause_wchan[0] &&
  195             (uint8_t *)ident <= &pause_wchan[MAXCPU - 1])
  196                 sleepq_flags = SLEEPQ_PAUSE;
  197         else
  198                 sleepq_flags = SLEEPQ_SLEEP;
  199         if (catch)
  200                 sleepq_flags |= SLEEPQ_INTERRUPTIBLE;
  201 
  202         sleepq_lock(ident);
  203         CTR5(KTR_PROC, "sleep: thread %ld (pid %ld, %s) on %s (%p)",
  204             td->td_tid, p->p_pid, td->td_name, wmesg, ident);
  205 
  206         if (lock == &Giant.lock_object)
  207                 mtx_assert(&Giant, MA_OWNED);
  208         DROP_GIANT();
  209         if (lock != NULL && lock != &Giant.lock_object &&
  210             !(class->lc_flags & LC_SLEEPABLE)) {
  211                 WITNESS_SAVE(lock, lock_witness);
  212                 lock_state = class->lc_unlock(lock);
  213         } else
  214                 /* GCC needs to follow the Yellow Brick Road */
  215                 lock_state = -1;
  216 
  217         /*
  218          * We put ourselves on the sleep queue and start our timeout
  219          * before calling thread_suspend_check, as we could stop there,
  220          * and a wakeup or a SIGCONT (or both) could occur while we were
  221          * stopped without resuming us.  Thus, we must be ready for sleep
  222          * when cursig() is called.  If the wakeup happens while we're
  223          * stopped, then td will no longer be on a sleep queue upon
  224          * return from cursig().
  225          */
  226         sleepq_add(ident, lock, wmesg, sleepq_flags, 0);
  227         if (sbt != 0)
  228                 sleepq_set_timeout_sbt(ident, sbt, pr, flags);
  229         if (lock != NULL && class->lc_flags & LC_SLEEPABLE) {
  230                 sleepq_release(ident);
  231                 WITNESS_SAVE(lock, lock_witness);
  232                 lock_state = class->lc_unlock(lock);
  233                 sleepq_lock(ident);
  234         }
  235         if (sbt != 0 && catch)
  236                 rval = sleepq_timedwait_sig(ident, pri);
  237         else if (sbt != 0)
  238                 rval = sleepq_timedwait(ident, pri);
  239         else if (catch)
  240                 rval = sleepq_wait_sig(ident, pri);
  241         else {
  242                 sleepq_wait(ident, pri);
  243                 rval = 0;
  244         }
  245 #ifdef KTRACE
  246         if (KTRPOINT(td, KTR_CSW))
  247                 ktrcsw(0, 0, wmesg);
  248 #endif
  249         PICKUP_GIANT();
  250         if (lock != NULL && lock != &Giant.lock_object && !(priority & PDROP)) {
  251                 class->lc_lock(lock, lock_state);
  252                 WITNESS_RESTORE(lock, lock_witness);
  253         }
  254         return (rval);
  255 }
  256 
  257 int
  258 msleep_spin_sbt(void *ident, struct mtx *mtx, const char *wmesg,
  259     sbintime_t sbt, sbintime_t pr, int flags)
  260 {
  261         struct thread *td;
  262         struct proc *p;
  263         int rval;
  264         WITNESS_SAVE_DECL(mtx);
  265 
  266         td = curthread;
  267         p = td->td_proc;
  268         KASSERT(mtx != NULL, ("sleeping without a mutex"));
  269         KASSERT(p != NULL, ("msleep1"));
  270         KASSERT(ident != NULL && TD_IS_RUNNING(td), ("msleep"));
  271 
  272         if (cold || SCHEDULER_STOPPED()) {
  273                 /*
  274                  * During autoconfiguration, just return;
  275                  * don't run any other threads or panic below,
  276                  * in case this is the idle thread and already asleep.
  277                  * XXX: this used to do "s = splhigh(); splx(safepri);
  278                  * splx(s);" to give interrupts a chance, but there is
  279                  * no way to give interrupts a chance now.
  280                  */
  281                 return (0);
  282         }
  283 
  284         sleepq_lock(ident);
  285         CTR5(KTR_PROC, "msleep_spin: thread %ld (pid %ld, %s) on %s (%p)",
  286             td->td_tid, p->p_pid, td->td_name, wmesg, ident);
  287 
  288         DROP_GIANT();
  289         mtx_assert(mtx, MA_OWNED | MA_NOTRECURSED);
  290         WITNESS_SAVE(&mtx->lock_object, mtx);
  291         mtx_unlock_spin(mtx);
  292 
  293         /*
  294          * We put ourselves on the sleep queue and start our timeout.
  295          */
  296         sleepq_add(ident, &mtx->lock_object, wmesg, SLEEPQ_SLEEP, 0);
  297         if (sbt != 0)
  298                 sleepq_set_timeout_sbt(ident, sbt, pr, flags);
  299 
  300         /*
  301          * Can't call ktrace with any spin locks held so it can lock the
  302          * ktrace_mtx lock, and WITNESS_WARN considers it an error to hold
  303          * any spin lock.  Thus, we have to drop the sleepq spin lock while
  304          * we handle those requests.  This is safe since we have placed our
  305          * thread on the sleep queue already.
  306          */
  307 #ifdef KTRACE
  308         if (KTRPOINT(td, KTR_CSW)) {
  309                 sleepq_release(ident);
  310                 ktrcsw(1, 0, wmesg);
  311                 sleepq_lock(ident);
  312         }
  313 #endif
  314 #ifdef WITNESS
  315         sleepq_release(ident);
  316         WITNESS_WARN(WARN_GIANTOK | WARN_SLEEPOK, NULL, "Sleeping on \"%s\"",
  317             wmesg);
  318         sleepq_lock(ident);
  319 #endif
  320         if (sbt != 0)
  321                 rval = sleepq_timedwait(ident, 0);
  322         else {
  323                 sleepq_wait(ident, 0);
  324                 rval = 0;
  325         }
  326 #ifdef KTRACE
  327         if (KTRPOINT(td, KTR_CSW))
  328                 ktrcsw(0, 0, wmesg);
  329 #endif
  330         PICKUP_GIANT();
  331         mtx_lock_spin(mtx);
  332         WITNESS_RESTORE(&mtx->lock_object, mtx);
  333         return (rval);
  334 }
  335 
  336 /*
  337  * pause_sbt() delays the calling thread by the given signed binary
  338  * time. During cold bootup, pause_sbt() uses the DELAY() function
  339  * instead of the _sleep() function to do the waiting. The "sbt"
  340  * argument must be greater than or equal to zero. A "sbt" value of
  341  * zero is equivalent to a "sbt" value of one tick.
  342  */
  343 int
  344 pause_sbt(const char *wmesg, sbintime_t sbt, sbintime_t pr, int flags)
  345 {
  346         KASSERT(sbt >= 0, ("pause_sbt: timeout must be >= 0"));
  347 
  348         /* silently convert invalid timeouts */
  349         if (sbt == 0)
  350                 sbt = tick_sbt;
  351 
  352         if (cold || kdb_active || SCHEDULER_STOPPED()) {
  353                 /*
  354                  * We delay one second at a time to avoid overflowing the
  355                  * system specific DELAY() function(s):
  356                  */
  357                 while (sbt >= SBT_1S) {
  358                         DELAY(1000000);
  359                         sbt -= SBT_1S;
  360                 }
  361                 /* Do the delay remainder, if any */
  362                 sbt = (sbt + SBT_1US - 1) / SBT_1US;
  363                 if (sbt > 0)
  364                         DELAY(sbt);
  365                 return (EWOULDBLOCK);
  366         }
  367         return (_sleep(&pause_wchan[curcpu], NULL,
  368             (flags & C_CATCH) ? PCATCH : 0, wmesg, sbt, pr, flags));
  369 }
  370 
  371 /*
  372  * Make all threads sleeping on the specified identifier runnable.
  373  */
  374 void
  375 wakeup(void *ident)
  376 {
  377         int wakeup_swapper;
  378 
  379         sleepq_lock(ident);
  380         wakeup_swapper = sleepq_broadcast(ident, SLEEPQ_SLEEP, 0, 0);
  381         sleepq_release(ident);
  382         if (wakeup_swapper) {
  383                 KASSERT(ident != &proc0,
  384                     ("wakeup and wakeup_swapper and proc0"));
  385                 kick_proc0();
  386         }
  387 }
  388 
  389 /*
  390  * Make a thread sleeping on the specified identifier runnable.
  391  * May wake more than one thread if a target thread is currently
  392  * swapped out.
  393  */
  394 void
  395 wakeup_one(void *ident)
  396 {
  397         int wakeup_swapper;
  398 
  399         sleepq_lock(ident);
  400         wakeup_swapper = sleepq_signal(ident, SLEEPQ_SLEEP, 0, 0);
  401         sleepq_release(ident);
  402         if (wakeup_swapper)
  403                 kick_proc0();
  404 }
  405 
  406 static void
  407 kdb_switch(void)
  408 {
  409         thread_unlock(curthread);
  410         kdb_backtrace();
  411         kdb_reenter();
  412         panic("%s: did not reenter debugger", __func__);
  413 }
  414 
  415 /*
  416  * The machine independent parts of context switching.
  417  */
  418 void
  419 mi_switch(int flags, struct thread *newtd)
  420 {
  421         uint64_t runtime, new_switchtime;
  422         struct thread *td;
  423 
  424         td = curthread;                 /* XXX */
  425         THREAD_LOCK_ASSERT(td, MA_OWNED | MA_NOTRECURSED);
  426         KASSERT(!TD_ON_RUNQ(td), ("mi_switch: called by old code"));
  427 #ifdef INVARIANTS
  428         if (!TD_ON_LOCK(td) && !TD_IS_RUNNING(td))
  429                 mtx_assert(&Giant, MA_NOTOWNED);
  430 #endif
  431         KASSERT(td->td_critnest == 1 || panicstr,
  432             ("mi_switch: switch in a critical section"));
  433         KASSERT((flags & (SW_INVOL | SW_VOL)) != 0,
  434             ("mi_switch: switch must be voluntary or involuntary"));
  435         KASSERT(newtd != curthread, ("mi_switch: preempting back to ourself"));
  436 
  437         /*
  438          * Don't perform context switches from the debugger.
  439          */
  440         if (kdb_active)
  441                 kdb_switch();
  442         if (SCHEDULER_STOPPED())
  443                 return;
  444         if (flags & SW_VOL) {
  445                 td->td_ru.ru_nvcsw++;
  446                 td->td_swvoltick = ticks;
  447         } else
  448                 td->td_ru.ru_nivcsw++;
  449 #ifdef SCHED_STATS
  450         SCHED_STAT_INC(sched_switch_stats[flags & SW_TYPE_MASK]);
  451 #endif
  452         /*
  453          * Compute the amount of time during which the current
  454          * thread was running, and add that to its total so far.
  455          */
  456         new_switchtime = cpu_ticks();
  457         runtime = new_switchtime - PCPU_GET(switchtime);
  458         td->td_runtime += runtime;
  459         td->td_incruntime += runtime;
  460         PCPU_SET(switchtime, new_switchtime);
  461         td->td_generation++;    /* bump preempt-detect counter */
  462         PCPU_INC(cnt.v_swtch);
  463         PCPU_SET(switchticks, ticks);
  464         CTR4(KTR_PROC, "mi_switch: old thread %ld (td_sched %p, pid %ld, %s)",
  465             td->td_tid, td->td_sched, td->td_proc->p_pid, td->td_name);
  466 #ifdef KDTRACE_HOOKS
  467         if ((flags & SW_PREEMPT) != 0 || ((flags & SW_INVOL) != 0 &&
  468             (flags & SW_TYPE_MASK) == SWT_NEEDRESCHED))
  469                 SDT_PROBE0(sched, , , preempt);
  470 #endif
  471 #ifdef XEN
  472         PT_UPDATES_FLUSH();
  473 #endif
  474         sched_switch(td, newtd, flags);
  475         CTR4(KTR_PROC, "mi_switch: new thread %ld (td_sched %p, pid %ld, %s)",
  476             td->td_tid, td->td_sched, td->td_proc->p_pid, td->td_name);
  477 
  478         /* 
  479          * If the last thread was exiting, finish cleaning it up.
  480          */
  481         if ((td = PCPU_GET(deadthread))) {
  482                 PCPU_SET(deadthread, NULL);
  483                 thread_stash(td);
  484         }
  485 }
  486 
  487 /*
  488  * Change thread state to be runnable, placing it on the run queue if
  489  * it is in memory.  If it is swapped out, return true so our caller
  490  * will know to awaken the swapper.
  491  */
  492 int
  493 setrunnable(struct thread *td)
  494 {
  495 
  496         THREAD_LOCK_ASSERT(td, MA_OWNED);
  497         KASSERT(td->td_proc->p_state != PRS_ZOMBIE,
  498             ("setrunnable: pid %d is a zombie", td->td_proc->p_pid));
  499         switch (td->td_state) {
  500         case TDS_RUNNING:
  501         case TDS_RUNQ:
  502                 return (0);
  503         case TDS_INHIBITED:
  504                 /*
  505                  * If we are only inhibited because we are swapped out
  506                  * then arange to swap in this process. Otherwise just return.
  507                  */
  508                 if (td->td_inhibitors != TDI_SWAPPED)
  509                         return (0);
  510                 /* FALLTHROUGH */
  511         case TDS_CAN_RUN:
  512                 break;
  513         default:
  514                 printf("state is 0x%x", td->td_state);
  515                 panic("setrunnable(2)");
  516         }
  517         if ((td->td_flags & TDF_INMEM) == 0) {
  518                 if ((td->td_flags & TDF_SWAPINREQ) == 0) {
  519                         td->td_flags |= TDF_SWAPINREQ;
  520                         return (1);
  521                 }
  522         } else
  523                 sched_wakeup(td);
  524         return (0);
  525 }
  526 
  527 /*
  528  * Compute a tenex style load average of a quantity on
  529  * 1, 5 and 15 minute intervals.
  530  */
  531 static void
  532 loadav(void *arg)
  533 {
  534         int i, nrun;
  535         struct loadavg *avg;
  536 
  537         nrun = sched_load();
  538         avg = &averunnable;
  539 
  540         for (i = 0; i < 3; i++)
  541                 avg->ldavg[i] = (cexp[i] * avg->ldavg[i] +
  542                     nrun * FSCALE * (FSCALE - cexp[i])) >> FSHIFT;
  543 
  544         /*
  545          * Schedule the next update to occur after 5 seconds, but add a
  546          * random variation to avoid synchronisation with processes that
  547          * run at regular intervals.
  548          */
  549         callout_reset_sbt(&loadav_callout,
  550             SBT_1US * (4000000 + (int)(random() % 2000001)), SBT_1US,
  551             loadav, NULL, C_DIRECT_EXEC | C_PREL(32));
  552 }
  553 
  554 /* ARGSUSED */
  555 static void
  556 synch_setup(void *dummy)
  557 {
  558         callout_init(&loadav_callout, 1);
  559 
  560         /* Kick off timeout driven events by calling first time. */
  561         loadav(NULL);
  562 }
  563 
  564 int
  565 should_yield(void)
  566 {
  567 
  568         return ((u_int)ticks - (u_int)curthread->td_swvoltick >= hogticks);
  569 }
  570 
  571 void
  572 maybe_yield(void)
  573 {
  574 
  575         if (should_yield())
  576                 kern_yield(PRI_USER);
  577 }
  578 
  579 void
  580 kern_yield(int prio)
  581 {
  582         struct thread *td;
  583 
  584         td = curthread;
  585         DROP_GIANT();
  586         thread_lock(td);
  587         if (prio == PRI_USER)
  588                 prio = td->td_user_pri;
  589         if (prio >= 0)
  590                 sched_prio(td, prio);
  591         mi_switch(SW_VOL | SWT_RELINQUISH, NULL);
  592         thread_unlock(td);
  593         PICKUP_GIANT();
  594 }
  595 
  596 /*
  597  * General purpose yield system call.
  598  */
  599 int
  600 sys_yield(struct thread *td, struct yield_args *uap)
  601 {
  602 
  603         thread_lock(td);
  604         if (PRI_BASE(td->td_pri_class) == PRI_TIMESHARE)
  605                 sched_prio(td, PRI_MAX_TIMESHARE);
  606         mi_switch(SW_VOL | SWT_RELINQUISH, NULL);
  607         thread_unlock(td);
  608         td->td_retval[0] = 0;
  609         return (0);
  610 }

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