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

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Date:   Tue, 4 Oct 2016 20:57:55 -0700
From:   Davidlohr Bueso <dave@...olabs.net>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     mingo@...nel.org, tglx@...utronix.de, 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: [RFC][PATCH 2/4] futex: Use smp_store_release() in
 mark_wake_futex()

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@...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);
> }

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