linux-kernel - [PATCH] mutex: Fix mutex_can_spin_on

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Message-ID: <20130719183101.GA20909@twins.programming.kicks-ass.net>
Date:	Fri, 19 Jul 2013 20:31:01 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Waiman Long <Waiman.Long@...com>
Cc:	Davidlohr Bueso <davidlohr.bueso@...com>,
	Rik van Riel <riel@...hat.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	David Howells <dhowells@...hat.com>,
	Ingo Molnar <mingo@...nel.org>, linux-kernel@...r.kernel.org
Subject: [PATCH] mutex: Fix mutex_can_spin_on_owner


mutex_can_spin_on_owner() is broken in that it would allow the compiler
to load lock->owner twice, seeing a pointer first time and a MULL
pointer the second time.

Signed-off-by: Peter Zijlstra <peterz@...radead.org>
---
 kernel/mutex.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/kernel/mutex.c b/kernel/mutex.c
index ff05f4b..7ff48c5 100644
--- a/kernel/mutex.c
+++ b/kernel/mutex.c
@@ -209,11 +209,13 @@ int mutex_spin_on_owner(struct mutex *lock, struct task_struct *owner)
  */
 static inline int mutex_can_spin_on_owner(struct mutex *lock)
 {
+	struct task_struct *owner;
 	int retval = 1;
 
 	rcu_read_lock();
-	if (lock->owner)
-		retval = lock->owner->on_cpu;
+	owner = ACCESS_ONCE(lock->owner);
+	if (owner)
+		retval = owner->on_cpu;
 	rcu_read_unlock();
 	/*
 	 * if lock->owner is not set, the mutex owner may have just acquired
--
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