Re: [PATCH v2] tools/memory-model: Add extra ordering for locks and remove it for ordinary release/acquire

Will Deacon
2018-07-12T17:14:12Z

Hi Alan,

On Thu, Jul 12, 2018 at 01:04:27PM -0400, Alan Stern wrote:
> On Thu, 12 Jul 2018, Peter Zijlstra wrote:
> > But as you (and Will) point out, we don't so much care about rmw-acquire
> > semantics as much as that we care about unlock+lock behaviour. Another
> > way to look at this is to define:
> >
> > smp-store-release + rmw-acquire := TSO (ideally smp_mb)
> >
> > But then we also have to look at:
> >
> > rmw-release + smp-load-acquire
> > rmw-release + rmw-acquire
>
> Let's assume that rmw-release is equivalent, in terms of ordering
> strength, to smp_store_release(). Then we can focus our attention on
> just the acquire part.

I can live with that, but it does add another special case, where we could
otherwise just special case acquire/release for the load/store variants
vs everything else.

> On PowerPC, for instance, if spin_lock() used a full HWSYNC fence
> then unlock+lock would become RCsc -- even with no changes to
> spin_unlock().
>
> > for completeness sake, and I would suggest they result in (at least) the
> > same (TSO) ordering as the one we really care about.
> >
> > One alternative is to no longer use smp_store_release() for unlock(),
> > and say define atomic_set_release() to be in the rmw-release class
> > instead of being a simple smp_store_release().
> >
> > Another, and I like this proposal least, is to introduce a new barrier
> > to make this all work.
>
> This apparently boils down to two questions:
>
> Should spin_lock/spin_unlock be RCsc?

I would love that to be the case, but I'm not asking you to fight that
battle :)

> Should rmw-acquire be strong enough so that smp_store_release +
> rmw-acquire is RCtso?
>
> If both answers are No, we end up with the v3 patch. If the first
> answer is No and the second is Yes, we end up with the v2 patch. The
> problem is that different people seem to want differing answers.

Just to be extra unhelpful: I'm happy with either v2 or v3. I suspect Daniel
is the one to convince on v2, because it's RISC-V that's affected by this.

Will