From: Peter Zijlstra <a.p.zijlstra@chello.nl>
To: Roland Dreier <roland@kernel.org>
Cc: Ingo Molnar <mingo@elte.hu>,
linux-kernel@vger.kernel.org, rostedt <rostedt@goodmis.org>
Subject: Re: lockdep false positive in double_lock_balance()?
Date: Thu, 17 May 2012 21:19:46 +0200 [thread overview]
Message-ID: <1337282386.4281.77.camel@twins> (raw)
In-Reply-To: <1337198420-5062-1-git-send-email-roland@kernel.org>
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;
}
next prev parent reply other threads:[~2012-05-17 19:19 UTC|newest]
Thread overview: 5+ messages / expand[flat|nested] mbox.gz Atom feed top
[not found] <1335314100-532-1-git-send-email-roland@kernel.org>
[not found] ` <alpine.LSU.2.00.1204251411460.4188@eggly.anvils>
2012-05-16 20:00 ` lockdep false positive in double_lock_balance()? Roland Dreier
2012-05-17 19:19 ` Peter Zijlstra [this message]
2012-05-18 14:43 ` Steven Rostedt
2012-05-18 16:42 ` Roland Dreier
2012-06-06 15:54 ` [tip:sched/urgent] sched/rt: Fix lockdep annotation within find_lock_lowest_rq() tip-bot for Peter Zijlstra
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