sched: fix possible recursive rq->lock

Vaidyanathan Srinivasan reported:

 > =============================================
 > [ INFO: possible recursive locking detected ]
 > 2.6.28-autotest-tip-sv #1
 > ---------------------------------------------
 > klogd/5062 is trying to acquire lock:
 >  (&rq->lock){++..}, at: [<ffffffff8022aca2>] task_rq_lock+0x45/0x7e
 >
 > but task is already holding lock:
 >  (&rq->lock){++..}, at: [<ffffffff805f7354>] schedule+0x158/0xa31

With sched_mc at 2. (it is default-off)

Strictly speaking we'll not deadlock, because ttwu will not be able to
place the migration task on our rq, but since the code can deal with
both rqs getting unlocked, this seems the easiest way out.
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

kernel/sched.c

@@ -3728,8 +3728,13 @@ load_balance_newidle(int this_cpu, struct rq *this_rq, struct sched_domain *sd,
 		}
 
 		double_unlock_balance(this_rq, busiest);
+		/*
+		 * Should not call ttwu while holding a rq->lock
+		 */
+		spin_unlock(&this_rq->lock);
 		if (active_balance)
 			wake_up_process(busiest->migration_thread);
+		spin_lock(&this_rq->lock);
 
 	} else
 		sd->nr_balance_failed = 0;