[PATCH 1/2] sched: schedule_tail() should disable preemption

[Oleg Nesterov <oleg@redhat.com>]

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.

Another problem is that finish_task_switch() itself runs with preempt
enabled after finish_lock_switch(). If nothing else this means that
->sched_in() notifier can't trust its "cpu" arg.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
---
 kernel/sched/core.c |   11 +++++------
 1 files changed, 5 insertions(+), 6 deletions(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 703c7e6..3f267e8 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -2277,15 +2277,14 @@ 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);
-- 
1.5.5.1