linux-kernel - Re: [tip:locking/core] rtmutex: Deboost before waking up the top waiter

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Date:   Wed, 05 Apr 2017 10:08:27 +0200
From:   Mike Galbraith <efault@....de>
To:     linux-tip-commits@...r.kernel.org
Cc:     hpa@...or.com, linux-kernel@...r.kernel.org, rostedt@...dmis.org,
        mingo@...nel.org, xlpang@...hat.com, tglx@...utronix.de,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: [tip:locking/core] rtmutex: Deboost before waking up the top
 waiter

locking/rtmutex: Fix preempt leak in __rt_mutex_futex_unlock()

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

Signed-off-by: Mike Galbraith <efault@....de>
---
 kernel/locking/rtmutex.c |   10 +++++-----
 1 file changed, 5 insertions(+), 5 deletions(-)

--- a/kernel/locking/rtmutex.c
+++ b/kernel/locking/rtmutex.c
@@ -1581,13 +1581,13 @@ bool __sched __rt_mutex_futex_unlock(str
 		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() */
 }