[PATCH] wait_task_inactive: don't consider task->nivcsw

[Oleg Nesterov <oleg@tv-sign.ru>]

If wait_task_inactive() returns success the task was deactivated.
In that case schedule() always increments ->nvcsw which alone can
be used as a "generation counter".

If the next call returns the same number, we can be sure that the
task was unscheduled. Otherwise, because we know that .on_rq == 0
again, ->nvcsw should have been changed in between.

Q: perhaps it is better to do "ncsw = (p->nvcsw << 1) | 1" ? This
decreases the possibility of "was it unscheduled" false positive
when ->nvcsw == 0.

Signed-off-by: Oleg Nesterov <oleg@tv-sign.ru>

--- LINUS/kernel/sched.c~	2008-07-27 14:46:03.000000000 +0400
+++ LINUS/kernel/sched.c	2008-07-27 14:55:12.000000000 +0400
@@ -1922,11 +1922,8 @@ unsigned long wait_task_inactive(struct 
 		running = task_running(rq, p);
 		on_rq = p->se.on_rq;
 		ncsw = 0;
-		if (!match_state || p->state == match_state) {
-			ncsw = p->nivcsw + p->nvcsw;
-			if (unlikely(!ncsw))
-				ncsw = 1;
-		}
+		if (!match_state || p->state == match_state)
+			ncsw = p->nvcsw ?: 1;
 		task_rq_unlock(rq, &flags);
 
 		/*