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Add comments describing r271604 change.

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MFC after:	3 days
This commit is contained in:
Alexander Motin 2014-09-15 11:17:36 +00:00
parent 56538e4af9
commit 7965496958
Notes: svn2git 2020-12-20 02:59:44 +00:00 
svn path=/head/; revision=271616
1 changed files with 12 additions and 0 deletions
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12
sys/kern/sched_ule.c
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@ -1037,7 +1037,14 @@ tdq_notify(struct tdq *tdq, struct thread *td)
ctd = pcpu_find(cpu)->pc_curthread;
if (!sched_shouldpreempt(pri, ctd->td_priority, 1))
return;
/*
* Make sure that tdq_load updated before calling this function
* is globally visible before we read tdq_cpu_idle. Idle thread
* accesses both of them without locks, and the order is important.
*/
mb();
if (TD_IS_IDLETHREAD(ctd)) {
/*
* If the MD code has an idle wakeup routine try that before
@ -2641,6 +2648,11 @@ sched_idletd(void *dummy)
/* Run main MD idle handler. */
tdq->tdq_cpu_idle = 1;
/*
* Make sure that tdq_cpu_idle update is globally visible
* before cpu_idle() read tdq_load. The order is important
* to avoid race with tdq_notify.
*/
mb();
cpu_idle(switchcnt * 4 > sched_idlespinthresh);
tdq->tdq_cpu_idle = 0;