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

From: Davidlohr Bueso
Date: Tue Oct 04 2016 - 23:58:31 EST

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:

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