Commit e9028b0f authored Mar 23, 2006 by Anton Blanchard Committed by Linus Torvalds Mar 23, 2006

[PATCH] fix scheduler deadlock



We have noticed lockups during boot when stress testing kexec on ppc64.
Two cpus would deadlock in scheduler code trying to grab already taken
spinlocks.

The double_rq_lock code uses the address of the runqueue to order the
taking of multiple locks.  This address is a per cpu variable:

	if (rq1 < rq2) {
		spin_lock(&rq1->lock);
		spin_lock(&rq2->lock);
	} else {
		spin_lock(&rq2->lock);
		spin_lock(&rq1->lock);
	}

On the other hand, the code in wake_sleeping_dependent uses the cpu id
order to grab locks:

	for_each_cpu_mask(i, sibling_map)
		spin_lock(&cpu_rq(i)->lock);

This means we rely on the address of per cpu data increasing as cpu ids
increase.  While this will be true for the generic percpu implementation it
may not be true for arch specific implementations.

One way to solve this is to always take runqueues in cpu id order. To do
this we add a cpu variable to the runqueue and check it in the
double runqueue locking functions.

Signed-off-by: Anton Blanchard <anton@samba.org>
Acked-by: Ingo Molnar <mingo@elte.hu>
Cc: <stable@kernel.org>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

parent 5be0e951

kernel/sched.c

+7 −2

Original line number	Diff line number	Diff line
		@@ -237,6 +237,7 @@ struct runqueue {

		task_t *migration_thread;
		struct list_head migration_queue;
		int cpu;
		#endif

		#ifdef CONFIG_SCHEDSTATS
		@@ -1654,6 +1655,9 @@ unsigned long nr_iowait(void)
		/*
		* double_rq_lock - safely lock two runqueues
		*
		* We must take them in cpu order to match code in
		* dependent_sleeper and wake_dependent_sleeper.
		*
		* Note this does not disable interrupts like task_rq_lock,
		* you need to do so manually before calling.
		*/
		@@ -1665,7 +1669,7 @@ static void double_rq_lock(runqueue_t rq1, runqueue_t rq2)
		spin_lock(&rq1->lock);
		__acquire(rq2->lock); /* Fake it out ;) */
		} else {
		if (rq1 < rq2) {
		if (rq1->cpu < rq2->cpu) {
		spin_lock(&rq1->lock);
		spin_lock(&rq2->lock);
		} else {
		@@ -1701,7 +1705,7 @@ static void double_lock_balance(runqueue_t this_rq, runqueue_t busiest)
		__acquires(this_rq->lock)
		{
		if (unlikely(!spin_trylock(&busiest->lock))) {
		if (busiest < this_rq) {
		if (busiest->cpu < this_rq->cpu) {
		spin_unlock(&this_rq->lock);
		spin_lock(&busiest->lock);
		spin_lock(&this_rq->lock);
		@@ -6029,6 +6033,7 @@ void __init sched_init(void)
		rq->push_cpu = 0;
		rq->migration_thread = NULL;
		INIT_LIST_HEAD(&rq->migration_queue);
		rq->cpu = i;
		#endif
		atomic_set(&rq->nr_iowait, 0);

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