linux-kernel - [tip:core/locking] mutex: Fix/ document access-once assumption in mutex_can_spin_on

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Date:	Tue, 23 Jul 2013 00:46:20 -0700
From:	tip-bot for Peter Zijlstra <tipbot@...or.com>
To:	linux-tip-commits@...r.kernel.org
Cc:	linux-kernel@...r.kernel.org, hpa@...or.com, mingo@...nel.org,
	davidlohr.bueso@...com, torvalds@...ux-foundation.org,
	peterz@...radead.org, paulmck@...ux.vnet.ibm.com, riel@...hat.com,
	dhowells@...hat.com, Waiman.Long@...com, tglx@...utronix.de
Subject: [tip:core/locking] mutex: Fix/
 document access-once assumption in mutex_can_spin_on_owner()

Commit-ID:  1e40c2edef2537f87f94d0baf80aeaeb7d51cc23
Gitweb:     http://git.kernel.org/tip/1e40c2edef2537f87f94d0baf80aeaeb7d51cc23
Author:     Peter Zijlstra <peterz@...radead.org>
AuthorDate: Fri, 19 Jul 2013 20:31:01 +0200
Committer:  Ingo Molnar <mingo@...nel.org>
CommitDate: Mon, 22 Jul 2013 10:33:39 +0200

mutex: Fix/document access-once assumption in mutex_can_spin_on_owner()

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

Linus pointed out that a compiler has to be seriously broken to
not compile this correctly - but nevertheless this change
is correct as it will better document the implementation.

Signed-off-by: Peter Zijlstra <peterz@...radead.org>
Acked-by: Davidlohr Bueso <davidlohr.bueso@...com>
Acked-by: Waiman Long <Waiman.Long@...com>
Acked-by: Linus Torvalds <torvalds@...ux-foundation.org>
Acked-by: Thomas Gleixner <tglx@...utronix.de>
Acked-by: Rik van Riel <riel@...hat.com>
Cc: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
Cc: David Howells <dhowells@...hat.com>
Link: http://lkml.kernel.org/r/20130719183101.GA20909@twins.programming.kicks-ass.net
Signed-off-by: Ingo Molnar <mingo@...nel.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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