linux-kernel - Re: [PATCH] mutex: Fix mutex_can_spin_on

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Date:	Fri, 19 Jul 2013 11:36:29 -0700
From:	Davidlohr Bueso <davidlohr.bueso@...com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Waiman Long <Waiman.Long@...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: Re: [PATCH] mutex: Fix mutex_can_spin_on_owner

On Fri, 2013-07-19 at 20:31 +0200, Peter Zijlstra wrote:
> 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>

Yep, I remember this from the rwsem optimistic spinning thread.

Acked-by: Davidlohr Bueso <davidlohr.bueso@...com>

> ---
>  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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