Re: [PATCH -v4 04/10] futex: Use smp_store_release() in mark_wake_futex()

From: Peter Zijlstra
Date: Wed Feb 22 2017 - 09:03:34 EST

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.

futex_wait(), which calls unqueue_me() could have had a spurious wakeup
and observe our NULL store and 'free' the futex_q.