sched: only update rq->clock while holding rq->lock

Vatsa noticed rq->clock going funny and tracked it down to an update_rq_clock()
outside a rq->lock section.

This is a problem because things like double_rq_lock() update the rq->clock
value for both rqs. Therefore disabling interrupts isn't strong enough.

Reported-by: Srivatsa Vaddagiri <vatsa@linux.vnet.ibm.com>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

kernel/sched.c

@@ -4441,12 +4441,8 @@ asmlinkage void __sched schedule(void)
 	if (sched_feat(HRTICK))
 		hrtick_clear(rq);
 
-	/*
-	 * Do the rq-clock update outside the rq lock:
-	 */
-	local_irq_disable();
+	spin_lock_irq(&rq->lock);
 	update_rq_clock(rq);
-	spin_lock(&rq->lock);
 	clear_tsk_need_resched(prev);
 
 	if (prev->state && !(preempt_count() & PREEMPT_ACTIVE)) {