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

[Peter Zijlstra]

On Tue, Oct 04, 2016 at 08:57:55PM -0700, Davidlohr Bueso wrote:
> 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;

Less obvious but would work I suppose, I didn't look too hard at the
ordering requirements on __unqueue_futex(), but an early morning glance
(without tea) doesn't show any obvious problems with that.