Re: [RESEND PATCH v5] locking/pvqspinlock: Relax cmpxchg's to improve performance on some archs

[Will Deacon]

On Mon, Aug 14, 2017 at 08:47:11PM +0200, Peter Zijlstra wrote:
> On Mon, Aug 14, 2017 at 01:01:22PM +0100, Will Deacon wrote:
> > Yeah, that's right, you can't use the STXR status flag to create control
> > dependencies.
> 
> Just for my elucidation; you can't use it to create a control dependency
> on the store, but you can use it to create a control dependency on the
> corresponding load, right?

Hmm, sort of, but I'd say that the reads are really ordered due to
read-after-read ordering in that case. Control dependencies to loads
don't give you order.

> Now, IIRC, we've defined control dependencies as being LOAD->STORE
> ordering, so in that respect nothing is lost. But maybe we should
> explicitly mention that if the LOAD is part of an (otherwise) atomic RmW
> the STORE is not constrained.

I could well be misreading your suggestion, but it feels like that's too
weak. You can definitely still have control dependencies off the LL part
of the LL/SC pair, just not off the SC part.

E.g. this version of LB is forbidden on arm64:

P0:
if (atomic_inc_return_relaxed(&x) == 2)
	atomic_set(&y, 1);

P1:
if (atomic_inc_return_relaxed(&y) == 2)
	atomic_set(&x, 1);

Perhaps when you say "the STORE", you mean the store in the atomic RmW,
rather than the store in the LOAD->STORE control dependency?

Will