linux-kernel - [tip:sched/urgent] sched/rt: Fix lockdep annotation within find_lock_lowest

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Date:	Wed, 6 Jun 2012 08:54:17 -0700
From:	tip-bot for Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	linux-tip-commits@...r.kernel.org
Cc:	linux-kernel@...r.kernel.org, hpa@...or.com, mingo@...nel.org,
	rostedt@...dmis.org, a.p.zijlstra@...llo.nl, roland@...nel.org,
	tglx@...utronix.de
Subject: [tip:sched/urgent] sched/rt:
  Fix lockdep annotation within find_lock_lowest_rq()

Commit-ID:  7f1b43936f0ecad14770634c021cf4a929aec74d
Gitweb:     http://git.kernel.org/tip/7f1b43936f0ecad14770634c021cf4a929aec74d
Author:     Peter Zijlstra <a.p.zijlstra@...llo.nl>
AuthorDate: Thu, 17 May 2012 21:19:46 +0200
Committer:  Ingo Molnar <mingo@...nel.org>
CommitDate: Wed, 6 Jun 2012 16:52:26 +0200

sched/rt: Fix lockdep annotation within find_lock_lowest_rq()

Roland Dreier reported spurious, hard to trigger lockdep warnings
within the scheduler - without any real lockup.

This bit gives us the right clue:

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

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 lockdep confusion.

Reported-by: Roland Dreier <roland@...nel.org>
Acked-by: Steven Rostedt <rostedt@...dmis.org>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@...llo.nl>
Link: http://lkml.kernel.org/r/1337282386.4281.77.camel@twins
Signed-off-by: Ingo Molnar <mingo@...nel.org>
---
 kernel/sched/rt.c |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)

diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c
index 2a4e8df..573e1ca 100644
--- a/kernel/sched/rt.c
+++ b/kernel/sched/rt.c
@@ -1562,7 +1562,7 @@ 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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