sched: Fix a race between ttwu() and migrate_task()

Based on commit e2912009 upstream, but
done differently as this issue is not present in .33 or .34 kernels due
to rework in this area.

If a task is in the TASK_WAITING state, then try_to_wake_up() is working
on it, and it will place it on the correct cpu.

This commit ensures that neither migrate_task() nor __migrate_task()
calls set_task_cpu(p) while p is in the TASK_WAKING state.  Otherwise,
there could be two concurrent calls to set_task_cpu(p), resulting in
the task's cfs_rq being inconsistent with its cpu.
Signed-off-by: John Wright <john.wright@hp.com>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

kernel/sched.c

@@ -2116,12 +2116,10 @@ migrate_task(struct task_struct *p, int dest_cpu, struct migration_req *req)
 
 	/*
 	 * If the task is not on a runqueue (and not running), then
-	 * it is sufficient to simply update the task's cpu field.
+	 * the next wake-up will properly place the task.
 	 */
-	if (!p->se.on_rq && !task_running(rq, p)) {
-		set_task_cpu(p, dest_cpu);
+	if (!p->se.on_rq && !task_running(rq, p))
 		return 0;
-	}
 
 	init_completion(&req->done);
 	req->task = p;
@@ -7167,6 +7165,9 @@ static int __migrate_task(struct task_struct *p, int src_cpu, int dest_cpu)
 	/* Already moved. */
 	if (task_cpu(p) != src_cpu)
 		goto done;
+	/* Waking up, don't get in the way of try_to_wake_up(). */
+	if (p->state == TASK_WAKING)
+		goto fail;
 	/* Affinity changed (again). */
 	if (!cpumask_test_cpu(dest_cpu, &p->cpus_allowed))
 		goto fail;