On Tue, Dec 13, 2016 at 09:36:42AM +0100, 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.
>
+Paul McKenney
Per the introduction of the comment below from:
f1a11e0 futex: remove the wait queue
I believe the intent was to ensure the plist_del in ... the previous
__unqueue_futex(q) ... from getting ahead of the smp_store_release added here,
which could result in q being destroyed by the waking task before plist_del can
act on it. Is that
right?
The comment below predates the refactoring which hid plist_del under the
__unqueue_futex() making it a bit less clear as to the associated plist_del:
However, since this comment, we have moved the wake-up out of wake_futex through
the use of wake queues (wake_up_q) which now happens after the hb lock is
released (see futex_wake, futex_wake_op, and futex_requeue). Is this race still
a valid concern?
> 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);
> }
>
> static int wake_futex_pi(u32 __user *uaddr, u32 uval, struct futex_q
> *top_waiter,
>
>
>
--
Darren Hart
Intel Open Source Technology Center
