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

Peter Zijlstra
2020-01-23T10:06:55Z

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);

if (!atomic_try_cmpxchg_relaxed(&lock, &val, new)) {
tail_2nd->next = head_2nd;
return false;
}

The whole second queue is only ever modified by the lock owner, and that
is us, so we can pre-terminate the secondary queue (break the circular
link), try the cmpxchg and fix it back up when it fails.

> > + cmpxchg_relaxed(&tail_2nd->next, head_2nd, NULL);
> > + arch_mcs_pass_lock(&head_2nd->locked, 1);
> > + return true;
> > + }
> > +
> > + return false;
> > +}