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>

1 files changed, 5 insertions, 6 deletions

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index cde848149dd6..b4935600cd85 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);