On Wed, Feb 22, 2017 at 03:03:16PM +0100, Peter Zijlstra wrote: > On Fri, Dec 16, 2016 at 04:50:45PM -0800, Darren Hart wrote: > > 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? > > Yes I think so, since __unqueue_futex() dereferences lock_ptr and does > stores in the memory it points to, those stores must not happen _after_ > we NULL lock_ptr itself.
Are you referring to the q->lock_ptr = NULL in mark_wake_futex()? So the concern is parallel mark_wake_futex() calls on the same futex? But that can't happen because the call is wrapped by the hb locks. In what scenario can this occur? > futex_wait(), which calls unqueue_me() could have had a spurious wakeup > and observe our NULL store and 'free' the futex_q. Urg. Spurious wakeups... yes... OK, still necessary. Gah. :-( -- Darren Hart VMware Open Source Technology Center
