Re: [PATCH v5 03/10] rust: sync: atomic: Add ordering annotation types

Peter Zijlstra
2025-06-19T15:10:57Z

On Thu, Jun 19, 2025 at 08:00:30AM -0700, Boqun Feng wrote:

> > So given we build locks from atomics, this has to come from somewhere.
> >
> > The simplest lock -- TAS -- is: rmw.acquire + store.release.
> >
> > So while plain store.release + load.acquire might not make TSO (although
> > IIRC ARM added variants that do just that in an effort to aid x86
> > emulation); store.release + rmw.acquire must, otherwise we cannot
> > satisfy that unlock+lock.
>
> Make sense, so something like this in the model should work:
>
> diff --git a/tools/memory-model/linux-kernel.cat b/tools/memory-model/linux-kernel.cat
> index d7e7bf13c831..90cb6db6e335 100644
> --- a/tools/memory-model/linux-kernel.cat
> +++ b/tools/memory-model/linux-kernel.cat
> @@ -27,7 +27,7 @@ include "lock.cat"
> (* Release Acquire *)
> let acq-po = [Acquire] ; po ; [M]
> let po-rel = [M] ; po ; [Release]
> -let po-unlock-lock-po = po ; [UL] ; (po|rf) ; [LKR] ; po
> +let po-unlock-lock-po = po ; (([UL] ; (po|rf) ; [LKR]) | ([Release]; (po;rf); [Acquire & RMW])) ; po
>
> (* Fences *)
> let R4rmb = R \ Noreturn (* Reads for which rmb works *)
>

I am forever struggling with cats, but that does look about right :-)

> although I'm not sure whether there will be actual users that use this
> ordering.

include/asm-generic/ticket_spinlock.h comes to mind, as I thing would
kernel/locking/qspinlock.*, no?