Re: lockdep false positive in double_lock_balance()?

[Peter Zijlstra <a.p.zijlstra@xxxxxxxxx>]

On Wed, 2012-05-16 at 13:00 -0700, Roland Dreier wrote:
> Hi scheduler hackers,
> 
> I'm very occasionally seeing the lockdep warning below on our boxes
> running 2.6.39 (PREEMPT=n, so "unfair" _double_lock_balance()).  I
> think I see the explanation, and it's probably not even worth fixing:
> 
> On the unlock side, we have:

<snip>

> while on the lock side we have:

<snip>

> So it seems we have the following (purely lockdep-related) race:
> 
>     unlock:				lock:
> 
> 					if (unlikely(!raw_spin_trylock(&busiest->lock))) { //fail to lock
> 
>     raw_spin_unlock(&busiest->lock);
> 
> 						if (busiest < this_rq) { //not true
> 						} else
> 							raw_spin_lock_nested(&busiest->lock,
> 									      SINGLE_DEPTH_NESTING);
> 
>     lock_set_subclass(&this_rq->lock.dep_map, 0, _RET_IP_); //too late
> 
> where we end up trying to take a second lock with SINGLE_DEPTH_NESTING
> before we've promoted our first lock to subclass 0.

*phew* you actually made me think there ;-)

Anyway, it all sounds very plausible, which is what threw me, but its
wrong :-)

The race you describe exists, except that's not how lockdep works. Both
cpu's would have a different task (one would hope to presume) and the
held lock stack is per task. So even if busiest_rq on cpu1 (lock case)
is the same lock as this_rq on cpu0 (unlock case), they're in different
stacks with different states.

> So does this make sense?

Almost :-)

> Here's the actual lockdep warning:

> [89945.640512]  [<ffffffff8103fa1a>] double_lock_balance+0x5a/0x90
> [89945.640568]  [<ffffffff8104c546>] push_rt_task+0xc6/0x290

this is the clue.. if you look at that code you'll find the
double_lock_balance() in question is the one in find_lock_lowest_rq()
[yay for inlining].

Now find_lock_lowest_rq() has a bug.. it fails to use
double_unlock_balance() in one exit path, if this results in a retry in
push_rt_task() we'll call double_lock_balance() again, at which point
we'll run into said issue.

Presumably this is all rather rare..

Something like this should fix it I think..


---
 kernel/sched/rt.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c
index c5565c3..b649108 100644
--- a/kernel/sched/rt.c
+++ b/kernel/sched/rt.c
@@ -1556,7 +1556,7 @@ static struct rq *find_lock_lowest_rq(struct task_struct *task, struct rq *rq)
 				     task_running(rq, task) ||
 				     !task->on_rq)) {
 
-				raw_spin_unlock(&lowest_rq->lock);
+				double_unlock_balance(rq, lowest_rq);
 				lowest_rq = NULL;
 				break;
 			}

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