[tip:locking/core] rtmutex: Plug preempt count leak in rt_mutex_futex_unlock()

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From	tip-bot for Mike Galbraith <tipbot@zytor.com>
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Subject	[tip:locking/core] rtmutex: Plug preempt count leak in rt_mutex_futex_unlock()
Date	Wed, 05 Apr 2017 17:10:01 +0200
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Commit-ID:  def34eaae5ce04b324e48e1bfac873091d945213
Gitweb:     http://git.kernel.org/tip/def34eaae5ce04b324e48e1bfac873091d945213
Author:     Mike Galbraith <efault@gmx.de>
AuthorDate: Wed, 5 Apr 2017 10:08:27 +0200
Committer:  Thomas Gleixner <tglx@linutronix.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@gmx.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: xlpang@redhat.com
Cc: rostedt@goodmis.org
Link: http://lkml.kernel.org/r/1491379707.6538.2.camel@gmx.de
Signed-off-by: Thomas Gleixner <tglx@linutronix.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() */
 }

Thread

[tip:locking/core] rtmutex: Deboost before waking up the top waiter tip-bot for Xunlei Pang <tipbot@zytor.com> - 2017-04-04 12:00 +0200
  Re: [tip:locking/core] rtmutex: Deboost before waking up the top  waiter Mike Galbraith <efault@gmx.de> - 2017-04-05 10:20 +0200
    [tip:locking/core]  Retiplockingcore_rtmutex_Deboost_before_waking_up_the_top_waiter tip-bot for Mike Galbraith <tipbot@zytor.com> - 2017-04-05 17:00 +0200
    [tip:locking/core] rtmutex: Plug preempt count leak in  rt_mutex_futex_unlock() tip-bot for Mike Galbraith <tipbot@zytor.com> - 2017-04-05 17:10 +0200
    Re: [tip:locking/core] rtmutex: Deboost before waking up the top  waiter Xunlei Pang <xpang@redhat.com> - 2017-04-06 08:20 +0200

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