[PATCH] move_task_off_dead_cpu() should be called with disabled ints

move_task_off_dead_cpu() requires interrupts to be disabled, while
migrate_dead() calls it with enabled interrupts.  Added appropriate
comments to functions and added BUG_ON(!irqs_disabled()) into
double_rq_lock() and double_lock_balance() which are the origin sources of
such bugs.
Signed-off-by: Kirill Korotaev <dev@openvz.org>
Acked-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>
Signed-off-by: Chris Wright <chrisw@sous-sol.org>

kernel/sched.c

@@ -1941,6 +1941,7 @@ static void double_rq_lock(struct rq *rq1, struct rq *rq2)
 	__acquires(rq1->lock)
 	__acquires(rq2->lock)
 {
+	BUG_ON(!irqs_disabled());
 	if (rq1 == rq2) {
 		spin_lock(&rq1->lock);
 		__acquire(rq2->lock);	/* Fake it out ;) */
@@ -1980,6 +1981,11 @@ static void double_lock_balance(struct rq *this_rq, struct rq *busiest)
 	__acquires(busiest->lock)
 	__acquires(this_rq->lock)
 {
+	if (unlikely(!irqs_disabled())) {
+		/* printk() doesn't work good under rq->lock */
+		spin_unlock(&this_rq->lock);
+		BUG_ON(1);
+	}
 	if (unlikely(!spin_trylock(&busiest->lock))) {
 		if (busiest < this_rq) {
 			spin_unlock(&this_rq->lock);
@@ -5050,7 +5056,10 @@ static int migration_thread(void *data)
 }
 
 #ifdef CONFIG_HOTPLUG_CPU
-/* Figure out where task on dead CPU should go, use force if neccessary. */
+/*
+ * Figure out where task on dead CPU should go, use force if neccessary.
+ * NOTE: interrupts should be disabled by the caller
+ */
 static void move_task_off_dead_cpu(int dead_cpu, struct task_struct *p)
 {
 	unsigned long flags;
@@ -5170,6 +5179,7 @@ void idle_task_exit(void)
 	mmdrop(mm);
 }
 
+/* called under rq->lock with disabled interrupts */
 static void migrate_dead(unsigned int dead_cpu, struct task_struct *p)
 {
 	struct rq *rq = cpu_rq(dead_cpu);
@@ -5186,10 +5196,11 @@ static void migrate_dead(unsigned int dead_cpu, struct task_struct *p)
 	 * Drop lock around migration; if someone else moves it,
 	 * that's OK.  No task can be added to this CPU, so iteration is
 	 * fine.
+	 * NOTE: interrupts should be left disabled  --dev@
 	 */
-	spin_unlock_irq(&rq->lock);
+	spin_unlock(&rq->lock);
 	move_task_off_dead_cpu(dead_cpu, p);
-	spin_lock_irq(&rq->lock);
+	spin_lock(&rq->lock);
 
 	put_task_struct(p);
 }