svn commit: r271616 - head/sys/kern

[Alexander Motin]

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;