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) <pet...@infradead.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