Re: [PATCH v9 3/5] locking/qspinlock: Introduce CNA into the slow path of qspinlock

Will Deacon
2020-01-23T11:23:02Z

On Thu, Jan 23, 2020 at 11:16:49AM +0100, Peter Zijlstra wrote:
> On Thu, Jan 23, 2020 at 11:06:35AM +0100, Peter Zijlstra wrote:
> > On Thu, Jan 23, 2020 at 10:26:58AM +0100, Peter Zijlstra wrote:
> > > On Tue, Jan 14, 2020 at 10:59:18PM -0500, Alex Kogan wrote:
> > > > +/* this function is called only when the primary queue is empty */
> > > > +static inline bool cna_try_change_tail(struct qspinlock *lock, u32 val,
> > > > + struct mcs_spinlock *node)
> > > > +{
> > > > + struct mcs_spinlock *head_2nd, *tail_2nd;
> > > > + u32 new;
> > > > +
> > > > + /* If the secondary queue is empty, do what MCS does. */
> > > > + if (node->locked <= 1)
> > > > + return __try_clear_tail(lock, val, node);
> > > > +
> > > > + /*
> > > > + * Try to update the tail value to the last node in the secondary queue.
> > > > + * If successful, pass the lock to the first thread in the secondary
> > > > + * queue. Doing those two actions effectively moves all nodes from the
> > > > + * secondary queue into the main one.
> > > > + */
> > > > + tail_2nd = decode_tail(node->locked);
> > > > + head_2nd = tail_2nd->next;
> > > > + new = ((struct cna_node *)tail_2nd)->encoded_tail + _Q_LOCKED_VAL;
> > > > +
> > > > + if (atomic_try_cmpxchg_relaxed(&lock->val, &val, new)) {
> > > > + /*
> > > > + * Try to reset @next in tail_2nd to NULL, but no need to check
> > > > + * the result - if failed, a new successor has updated it.
> > > > + */
> > >
> > > I think you actually have an ordering bug here; the load of head_2nd
> > > *must* happen before the atomic_try_cmpxchg(), otherwise it might
> > > observe the new next and clear a valid next pointer.
> > >
> > > What would be the best fix for that; I'm thinking:
> > >
> > > head_2nd = smp_load_acquire(&tail_2nd->next);
> > >
> > > Will?
> >
> > Hmm, given we've not passed the lock around yet; why wouldn't something
> > like this work:
> >
> > smp_store_release(&tail_2nd->next, NULL);
>
> Argh, make that:
>
> tail_2nd->next = NULL;
>
> smp_wmb();
>
> > if (!atomic_try_cmpxchg_relaxed(&lock, &val, new)) {

... or could you drop the smp_wmb() and make this
atomic_try_cmpxchg_release()?

To be honest, I've failed to understand the code prior to your changes
in this area: it appears to reply on a control-dependency from the two
cmpxchg_relaxed() calls (which isn't sufficient to order the store parts
afaict) and I also don't get how we deal with a transiently circular primary
queue.

Will