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

[Peter Zijlstra <peterz@infradead.org>]

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)) {
> 		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.