Re: [RFC][PATCH 2/4] futex: Use smp_store_release() in mark_wake_futex()

[Davidlohr Bueso]

On Mon, 03 Oct 2016, 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@xxxxxxxxxxxxx>
> ---
> 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);
> }

Hmm, what if we relied on the implicit barrier in the wake_q_add()
above and got rid of the smp_wmb altogether? We'd obviously have to
move up __unqueue_futex(), but all we care about is the publishing
store to lock_ptr being the last operation, or at least the plist_del,
such that the wakeup order is respected; ie:

  __unqueue_futex(q);
  wake_q_add(wake_q, p);
  q->lock_ptr = NULL;

Thanks,
Davidlohr