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. > > 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.
