[tip:sched/core] sched: Fix schedule_tail() to disable preemption

[tip-bot for Oleg Nesterov <tipbot@zytor.com>]

Commit-ID:  1a43a14a5bd9c32dbd7af35e35a5afa703944bcb
Gitweb:     http://git.kernel.org/tip/1a43a14a5bd9c32dbd7af35e35a5afa703944bcb
Author:     Oleg Nesterov <oleg@redhat.com>
AuthorDate: Wed, 8 Oct 2014 21:36:44 +0200
Committer:  Ingo Molnar <mingo@kernel.org>
CommitDate: Tue, 28 Oct 2014 10:47:54 +0100

sched: Fix schedule_tail() to disable preemption

finish_task_switch() enables preemption, so post_schedule(rq) can be
called on the wrong (and even dead) CPU. Afaics, nothing really bad
can happen, but in this case we can wrongly clear rq->post_schedule
on that CPU. And this simply looks wrong in any case.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Kirill Tkhai <tkhai@yandex.ru>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Link: http://lkml.kernel.org/r/20141008193644.GA32055@redhat.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>
---
 kernel/sched/core.c | 11 +++++------
 1 file changed, 5 insertions(+), 6 deletions(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index cde8481..b493560 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -2309,15 +2309,14 @@ static inline void post_schedule(struct rq *rq)
 asmlinkage __visible void schedule_tail(struct task_struct *prev)
 	__releases(rq->lock)
 {
-	struct rq *rq = this_rq();
+	struct rq *rq;
 
+	/* finish_task_switch() drops rq->lock and enables preemtion */
+	preempt_disable();
+	rq = this_rq();
 	finish_task_switch(rq, prev);
-
-	/*
-	 * FIXME: do we need to worry about rq being invalidated by the
-	 * task_switch?
-	 */
 	post_schedule(rq);
+	preempt_enable();
 
 	if (current->set_child_tid)
 		put_user(task_pid_vnr(current), current->set_child_tid);