linux-kernel - [tip:locking/core] rtmutex: Plug preempt count leak in rt_mutex_futex

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Date:   Wed, 5 Apr 2017 08:03:50 -0700
From:   tip-bot for Mike Galbraith <tipbot@...or.com>
To:     linux-tip-commits@...r.kernel.org
Cc:     hpa@...or.com, peterz@...radead.org, tglx@...utronix.de,
        efault@....de, linux-kernel@...r.kernel.org, mingo@...nel.org
Subject: [tip:locking/core] rtmutex: Plug preempt count leak in
 rt_mutex_futex_unlock()

Commit-ID:  def34eaae5ce04b324e48e1bfac873091d945213
Gitweb:     http://git.kernel.org/tip/def34eaae5ce04b324e48e1bfac873091d945213
Author:     Mike Galbraith <efault@....de>
AuthorDate: Wed, 5 Apr 2017 10:08:27 +0200
Committer:  Thomas Gleixner <tglx@...utronix.de>
CommitDate: Wed, 5 Apr 2017 16:59:37 +0200

rtmutex: Plug preempt count leak in rt_mutex_futex_unlock()

mark_wakeup_next_waiter() already disables preemption, doing so again
leaves us with an unpaired preempt_disable().

Fixes: 2a1c60299406 ("rtmutex: Deboost before waking up the top waiter")
Signed-off-by: Mike Galbraith <efault@....de>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: xlpang@...hat.com
Cc: rostedt@...dmis.org
Link: http://lkml.kernel.org/r/1491379707.6538.2.camel@gmx.de
Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
---
 kernel/locking/rtmutex.c | 10 +++++-----
 1 file changed, 5 insertions(+), 5 deletions(-)

diff --git a/kernel/locking/rtmutex.c b/kernel/locking/rtmutex.c
index 0e641eb..b955094 100644
--- a/kernel/locking/rtmutex.c
+++ b/kernel/locking/rtmutex.c
@@ -1581,13 +1581,13 @@ bool __sched __rt_mutex_futex_unlock(struct rt_mutex *lock,
 		return false; /* done */
 	}
 
-	mark_wakeup_next_waiter(wake_q, lock);
 	/*
-	 * We've already deboosted, retain preempt_disabled when dropping
-	 * the wait_lock to avoid inversion until the wakeup. Matched
-	 * by rt_mutex_postunlock();
+	 * We've already deboosted, mark_wakeup_next_waiter() will
+	 * retain preempt_disabled when we drop the wait_lock, to
+	 * avoid inversion prior to the wakeup.  preempt_disable()
+	 * therein pairs with rt_mutex_postunlock().
 	 */
-	preempt_disable();
+	mark_wakeup_next_waiter(wake_q, lock);
 
 	return true; /* call postunlock() */
 }