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Code Issues Releases Activity

Fix a race which could result in some timeout buckets being skipped.

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- When a tick occurs on a cpu, iterate from cs_softticks until ticks.
   The per-cpu tick processing happens asynchronously with the actual
   adjustment of the 'ticks' variable.  Sometimes the results may
   be visible before the local call and sometimes after.  Previously this
   could cause a one tick window where we didn't evaluate the bucket.
 - In softclock fetch curticks before incrementing cc_softticks so we
   don't skip insertions which were made for the current time.

Sponsored by:	Nokia
This commit is contained in:
Jeff Roberson 2008-07-19 05:18:29 +00:00
parent e980fff622
commit 9fc51b0bf4
Notes: svn2git 2020-12-20 02:59:44 +00:00 
svn path=/head/; revision=180608
1 changed files with 11 additions and 6 deletions
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17
sys/kern/kern_timeout.c
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@ -222,19 +222,24 @@ SYSINIT(start_softclock, SI_SUB_SOFTINTR, SI_ORDER_FIRST, start_softclock, NULL)
void
callout_tick(void)
{
int need_softclock = 0;
struct callout_cpu *cc;
int need_softclock;
int bucket;
/*
* Process callouts at a very low cpu priority, so we don't keep the
* relatively high clock interrupt priority any longer than necessary.
*/
need_softclock = 0;
cc = CC_SELF();
mtx_lock_spin_flags(&cc->cc_lock, MTX_QUIET);
if (!TAILQ_EMPTY(&cc->cc_callwheel[ticks & callwheelmask])) {
need_softclock = 1;
} else if (cc->cc_softticks + 1 == ticks)
++cc->cc_softticks;
for (; cc->cc_softticks < ticks; cc->cc_softticks++) {
bucket = cc->cc_softticks & callwheelmask;
if (!TAILQ_EMPTY(&cc->cc_callwheel[bucket])) {
need_softclock = 1;
break;
}
}
mtx_unlock_spin_flags(&cc->cc_lock, MTX_QUIET);
/*
* swi_sched acquires the thread lock, so we don't want to call it
@ -308,12 +313,12 @@ softclock(void *arg)
cc = (struct callout_cpu *)arg;
CC_LOCK(cc);
while (cc->cc_softticks != ticks) {
cc->cc_softticks++;
/*
* cc_softticks may be modified by hard clock, so cache
* it while we work on a given bucket.
*/
curticks = cc->cc_softticks;
cc->cc_softticks++;
bucket = &cc->cc_callwheel[curticks & callwheelmask];
c = TAILQ_FIRST(bucket);
while (c) {