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SubjectRe: [PATCH 10/10] locking/qspinlock: Elide back-to-back RELEASE operations with smp_wmb()
Hi Boqun,

On Sat, Apr 07, 2018 at 01:47:11PM +0800, Boqun Feng wrote:
> On Thu, Apr 05, 2018 at 05:59:07PM +0100, Will Deacon wrote:
> > @@ -340,12 +341,17 @@ void queued_spin_lock_slowpath(struct qspinlock *lock, u32 val)
> > goto release;
> >
> > /*
> > + * Ensure that the initialisation of @node is complete before we
> > + * publish the updated tail and potentially link @node into the
>
> I think it might be better if we mention exactly where we "publish the
> updated tail" and "link @node", how about:
>
> * publish the update tail via xchg_tail() and potentially link
> * @node into the waitqueue via WRITE_ONCE(->next,..) below.
>
> and also add comments below like:
>
> > + * waitqueue.
> > + */
> > + smp_wmb();
> > +
> > + /*
> > * We have already touched the queueing cacheline; don't bother with
> > * pending stuff.
> > *
> > * p,*,* -> n,*,*
> > - *
> > - * RELEASE, such that the stores to @node must be complete.
>
> * publish the updated tail
>
> > */
> > old = xchg_tail(lock, tail);
> > next = NULL;
> > @@ -356,15 +362,7 @@ void queued_spin_lock_slowpath(struct qspinlock *lock, u32 val)
> > */
> > if (old & _Q_TAIL_MASK) {
> > prev = decode_tail(old);
> > -
> > - /*
> > - * We must ensure that the stores to @node are observed before
> > - * the write to prev->next. The address dependency from
> > - * xchg_tail is not sufficient to ensure this because the read
> > - * component of xchg_tail is unordered with respect to the
> > - * initialisation of @node.
> > - */
> > - smp_store_release(&prev->next, node);
>
> /* Eventually link @node to the wait queue */
>
> Thoughts?

I'll make some changes along these lines for v2. Thanks!

Will

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