wait_task_inactive(): don't consider task->nivcsw

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>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

diff --git a/kernel/sched.c b/kernel/sched.c
index da7c5d2..908670a 100644 (file)
--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -1921,11 +1921,8 @@ unsigned long wait_task_inactive(struct task_struct *p, long match_state)
                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);
 
                /*