linux-kernel - Re: [PATCH -v6 02/13] futex: Use smp_store_release() in mark_wake

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Date:   Fri, 24 Mar 2017 14:16:22 -0700
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
Subject: Re: [PATCH -v6 02/13] futex: Use smp_store_release() in
 mark_wake_futex()

On Wed, Mar 22, 2017 at 11:35:49AM +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.
> 
> Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>

I reviewed this carefully in the previous thread, confirming that despite the
move to wake queues, spurious wakeups can still lead to the situration Peter
describes. As such:

Reviewed-by: Darren Hart (VMware) <dvhart@...radead.org>

My only suggestion would be to clarify the language in the preceding comment to
make that obvious, as well as clarify which plist_del it is referring to since
it has been moved under the __unqueue_futex. I can do that as a follow-on though.

> ---
>  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
VMware Open Source Technology Center