linux-kernel - Re: [RFC][PATCH 2/4] futex: Use smp_store_release() in mark_wake

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Message-ID: <20161003101906.7f814829@gandalf.local.home>
Date:   Mon, 3 Oct 2016 10:19:06 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     mingo@...nel.org, tglx@...utronix.de, juri.lelli@....com,
        xlpang@...hat.com, bigeasy@...utronix.de,
        linux-kernel@...r.kernel.org, mathieu.desnoyers@...icios.com,
        jdesfossez@...icios.com, bristot@...hat.com
Subject: Re: [RFC][PATCH 2/4] futex: Use smp_store_release() in
 mark_wake_futex()

On Mon, 03 Oct 2016 11:12:36 +0200
Peter Zijlstra <peterz@...radead.org> wrote:

> Since the futex_q can dissapear the instruction after assigning NULL,
> this really should be a RELEASE barrier. That stops loads from hitting
> dead memory too.
> 
> Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
> ---
>  kernel/futex.c |    3 +--
>  1 file changed, 1 insertion(+), 2 deletions(-)
> 
> --- a/kernel/futex.c
> +++ b/kernel/futex.c
> @@ -1288,8 +1288,7 @@ static void mark_wake_futex(struct wake_
>  	 * memory barrier is required here to prevent the following
>  	 * store to lock_ptr from getting ahead of the plist_del.
>  	 */
> -	smp_wmb();
> -	q->lock_ptr = NULL;
> +	smp_store_release(&q->lock_ptr, NULL);
>  }
>  
>  static int wake_futex_pi(u32 __user *uaddr, u32 uval, struct futex_q *top_waiter,
> 

Reviewed-by: Steven Rostedt <rostedt@...dmis.org>

-- Steve