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

Alan Stern
2018-07-12T20:43:49Z

On Thu, 12 Jul 2018, Andrea Parri wrote:

> > It seems reasonable to ask people to learn that locks have stronger
> > ordering guarantees than RMW atomics do. Maybe not the greatest
> > situation in the world, but one I think we could live with.
>
> Yeah, this was one of my main objections.

Does this mean you don't think you could live with it?

> > > Hence my proposal to strenghten rmw-acquire, because that is the basic
> > > primitive used to implement lock.
> >
> > That was essentially what the v2 patch did. (And my reasoning was
> > basically the same as what you have just outlined. There was one
> > additional element: smp_store_release() is already strong enough for
> > TSO; the acquire is what needs to be stronger in the memory model.)
>
> Mmh? see my comments to v2 (and your reply, in part., the part "At
> least, it's not a valid general-purpose implementation".).
>
>
> > > 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?
> >
> > 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.
>
> Again, maybe you're confonding v2 with v1?

Oops, yes, I was. v1 was the version that made RMW updates be RCtso.
v2 and v3 affected only locking, the difference being that v2 used
unlock-rf-lock-po and v3 used po-unlock-rf-lock-po.

Alan