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

[Will Deacon]

On Mon, Aug 21, 2017 at 12:55:08PM +0200, Peter Zijlstra wrote:
> On Tue, Aug 15, 2017 at 07:40:35PM +0100, Will Deacon wrote:
> > 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.
> 
> No, I meant _from_ the LL load, not _to_ a later load.

Sorry, I'm still not following enough to give you a definitive answer on
that. Could you give an example, please? These sequences usually run in
a loop, so the conditional branch back (based on the status flag) is where
the read-after-read comes in.

Any control dependencies from the loaded data exist regardless of the status
flag.

> > > 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?
> 
> Yes. So I was looking to exclude (SC) STORE -> STORE order through
> control dependencies.

Ok, good.

Will