Author: mav
Date: Mon Sep 15 11:17:36 2014
New Revision: 271616
URL: http://svnweb.freebsd.org/changeset/base/271616
Log:
Add comments describing r271604 change.
MFC after: 3 days
Modified:
head/sys/kern/sched_ule.c
Modified: head/sys/kern/sched_ule.c
==============================================================================
--- head/sys/kern/sched_ule.c Mon Sep 15 11:14:23 2014 (r271615)
+++ head/sys/kern/sched_ule.c Mon Sep 15 11:17:36 2014 (r271616)
@@ -1037,7 +1037,14 @@ tdq_notify(struct tdq *tdq, struct threa
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;
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