sched: Fix hotplug task migration

Dan Carpenter reported:

> kernel/sched/rt.c:1347 pick_next_task_rt() warn: variable dereferenced before check 'prev' (see line 1338)
> kernel/sched/deadline.c:1011 pick_next_task_dl() warn: variable dereferenced before check 'prev' (see line 1005)

Kirill also spotted that migrate_tasks() will have an instant NULL
deref because pick_next_task() will immediately deref prev.

Instead of fixing all the corner cases because migrate_tasks() can
pass in a NULL prev task in the unlikely case of hot-un-plug, provide
a fake task such that we can remove all the NULL checks from the far
more common paths.

A further problem; not previously spotted; is that because we pushed
pre_schedule() and idle_balance() into pick_next_task() we now need to
avoid those getting called and pulling more tasks on our dying CPU.

We avoid pull_{dl,rt}_task() by setting fake_task.prio to MAX_PRIO+1.
We also note that since we call pick_next_task() exactly the amount of
times we have runnable tasks present, we should never land in
idle_balance().

Fixes: 38033c37faab ("sched: Push down pre_schedule() and idle_balance()")
Cc: Juri Lelli <juri.lelli@gmail.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Steven Rostedt <rostedt@goodmis.org>
Reported-by: Kirill Tkhai <tkhai@yandex.ru>
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/20140212094930.GB3545@laptop.programming.kicks-ass.net
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

1 files changed, 17 insertions, 1 deletions

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index fb9764fbc537..49db434a35d0 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -4681,6 +4681,22 @@ static void calc_load_migrate(struct rq *rq)
 		atomic_long_add(delta, &calc_load_tasks);
 }
 
+static void put_prev_task_fake(struct rq *rq, struct task_struct *prev)
+{
+}
+
+static const struct sched_class fake_sched_class = {
+	.put_prev_task = put_prev_task_fake,
+};
+
+static struct task_struct fake_task = {
+	/*
+	 * Avoid pull_{rt,dl}_task()
+	 */
+	.prio = MAX_PRIO + 1,
+	.sched_class = &fake_sched_class,
+};
+
 /*
  * Migrate all tasks from the rq, sleeping tasks will be migrated by
  * try_to_wake_up()->select_task_rq().
@@ -4721,7 +4737,7 @@ static void migrate_tasks(unsigned int dead_cpu)
 		if (rq->nr_running == 1)
 			break;
 
-		next = pick_next_task(rq, NULL);
+		next = pick_next_task(rq, &fake_task);
 		BUG_ON(!next);
 		next->sched_class->put_prev_task(rq, next);