linux-kernel - Re: [PATCH -v4 04/10] futex: Use smp_store_release() in mark_wake

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Date:   Fri, 16 Dec 2016 16:50:45 -0800
From:   Darren Hart <dvhart@...radead.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     tglx@...utronix.de, mingo@...nel.org, juri.lelli@....com,
        rostedt@...dmis.org, xlpang@...hat.com, bigeasy@...utronix.de,
        linux-kernel@...r.kernel.org, mathieu.desnoyers@...icios.com,
        jdesfossez@...icios.com, bristot@...hat.com,
        paulmck@...ux.vnet.ibm.com
Subject: Re: [PATCH -v4 04/10] futex: Use smp_store_release() in
 mark_wake_futex()

On Tue, Dec 13, 2016 at 09:36:42AM +0100, Peter Zijlstra 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.
> 

+Paul McKenney

Per the introduction of the comment below from:

	f1a11e0 futex: remove the wait queue

I believe the intent was to ensure the plist_del in ... the previous
__unqueue_futex(q) ... from getting ahead of the smp_store_release added here,
which could result in q being destroyed by the waking task before plist_del can
act on it. Is that
right?

The comment below predates the refactoring which hid plist_del under the
__unqueue_futex() making it a bit less clear as to the associated plist_del:

However, since this comment, we have moved the wake-up out of wake_futex through
the use of wake queues (wake_up_q) which now happens after the hb lock is
released (see futex_wake, futex_wake_op, and futex_requeue). Is this race still
a valid concern?

> 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,
> 
> 
> 

-- 
Darren Hart
Intel Open Source Technology Center