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

Daniel Lustig
2018-07-13T02:17:52Z

On 7/12/2018 2:45 AM, Will Deacon wrote:
> On Thu, Jul 12, 2018 at 11:34:32AM +0200, Peter Zijlstra wrote:
>> On Thu, Jul 12, 2018 at 09:40:40AM +0200, Peter Zijlstra wrote:
>>> And I think if we raise atomic*_acquire() to require TSO (but ideally
>>> raise it to RCsc) we're there.
>>
>> To clarify, just the RmW-acquire. Things like atomic_read_acquire() can
>> stay smp_load_acquire() and be RCpc.
>
> I don't have strong opinions about strengthening RmW atomics to TSO, so
> if it helps to unblock Alan's patch (which doesn't go near this!) then I'll
> go with it. The important part is that we continue to allow roach motel
> into the RmW for other accesses in the non-fully-ordered cases.
>
> Daniel -- your AMO instructions are cool with this, right? It's just the
> fence-based implementations that will need help?
>
> Will
Right, let me pull this part out of the overly-long response I just gave
on the thread with Linus :)

if we pair AMOs with AMOs, we get RCsc, and everything is fine. If we
start mixing in fences (mostly because we don't currently have native
load-acquire or store-release opcodes), then that's when all the rest of the
complexity comes in.

Dan