linux-kernel - Re: [RFC 1/3] mutex: In mutex_can_spin_on_owner(), return false if task need

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Date:	Wed, 15 Jan 2014 08:44:20 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Jason Low <jason.low2@...com>
Cc:	mingo@...hat.com, paulmck@...ux.vnet.ibm.com, Waiman.Long@...com,
	torvalds@...ux-foundation.org, tglx@...utronix.de,
	linux-kernel@...r.kernel.org, riel@...hat.com,
	akpm@...ux-foundation.org, davidlohr@...com, hpa@...or.com,
	aswin@...com, scott.norton@...com
Subject: Re: [RFC 1/3] mutex: In mutex_can_spin_on_owner(), return false if
 task need_resched()

On Tue, Jan 14, 2014 at 04:33:08PM -0800, Jason Low wrote:
> The mutex_can_spin_on_owner() function should also return false if the
> task needs to be rescheduled.
> 

While I was staring at mutex_can_spin_on_owner(); don't we need this?

 kernel/locking/mutex.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/kernel/locking/mutex.c b/kernel/locking/mutex.c
index 4dd6e4c219de..480d2f437964 100644
--- a/kernel/locking/mutex.c
+++ b/kernel/locking/mutex.c
@@ -214,8 +214,10 @@ static inline int mutex_can_spin_on_owner(struct mutex *lock)
 
 	rcu_read_lock();
 	owner = ACCESS_ONCE(lock->owner);
-	if (owner)
+	if (owner) {
+		smp_read_barrier_depends();
 		retval = owner->on_cpu;
+	}
 	rcu_read_unlock();
 	/*
 	 * if lock->owner is not set, the mutex owner may have just acquired
--
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