sched: Revert 498657a478c60be092208422fefa9c7b248729c2

498657a478c60be092208422fefa9c7b248729c2 incorrectly assumed
that preempt wasn't disabled around context_switch() and thus
was fixing imaginary problem.  It also broke KVM because it
depended on ->sched_in() to be called with irq enabled so that
it can do smp calls from there.

Revert the incorrect commit and add comment describing different
contexts under with the two callbacks are invoked.

Avi: spotted transposed in/out in the added comment.

Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Avi Kivity <avi@redhat.com>
Cc: peterz@infradead.org
Cc: efault@gmx.de
Cc: rusty@rustcorp.com.au
LKML-Reference: <1259726212-30259-2-git-send-email-tj@kernel.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

include/linux/preempt.h

@@ -105,6 +105,11 @@ struct preempt_notifier;
  * @sched_out: we've just been preempted
  *    notifier: struct preempt_notifier for the task being preempted
  *    next: the task that's kicking us out
+ *
+ * Please note that sched_in and out are called under different
+ * contexts.  sched_out is called with rq lock held and irq disabled
+ * while sched_in is called without rq lock and irq enabled.  This
+ * difference is intentional and depended upon by its users.
  */
 struct preempt_ops {
 	void (*sched_in)(struct preempt_notifier *notifier, int cpu);

kernel/sched.c

@@ -2768,9 +2768,9 @@ static void finish_task_switch(struct rq *rq, struct task_struct *prev)
 	prev_state = prev->state;
 	finish_arch_switch(prev);
 	perf_event_task_sched_in(current, cpu_of(rq));
-	fire_sched_in_preempt_notifiers(current);
 	finish_lock_switch(rq, prev);
 
+	fire_sched_in_preempt_notifiers(current);
 	if (mm)
 		mmdrop(mm);
 	if (unlikely(prev_state == TASK_DEAD)) {