Re: [PATCH V4 5/5] riscv: atomic: Optimize LRSC-pairs atomic ops with .aqrl annotation

[Guo Ren]

On Thu, Jul 7, 2022 at 8:04 AM Boqun Feng <boqun.feng@xxxxxxxxx> wrote:
>
> On Sat, Jun 25, 2022 at 01:29:50PM +0800, Guo Ren wrote:
> > On Fri, Jun 24, 2022 at 1:09 AM Dan Lustig <dlustig@xxxxxxxxxx> wrote:
> > >
> > > On 6/22/2022 11:31 PM, Boqun Feng wrote:
> > > > Hi,
> > > >
> > > > On Tue, Jun 14, 2022 at 01:03:47PM +0200, Andrea Parri wrote:
> > > > [...]
> > > >>> 5ce6c1f3535f ("riscv/atomic: Strengthen implementations with fences")
> > > >>> is about fixup wrong spinlock/unlock implementation and not relate to
> > > >>> this patch.
> > > >>
> > > >> No.  The commit in question is evidence of the fact that the changes
> > > >> you are presenting here (as an optimization) were buggy/incorrect at
> > > >> the time in which that commit was worked out.
> > > >>
> > > >>
> > > >>> Actually, sc.w.aqrl is very strong and the same with:
> > > >>> fence rw, rw
> > > >>> sc.w
> > > >>> fence rw,rw
> > > >>>
> > > >>> So "which do not give full-ordering with .aqrl" is not writen in
> > > >>> RISC-V ISA and we could use sc.w/d.aqrl with LKMM.
> > > >>>
> > > >>>>
> > > >>>>>> describes the issue more specifically, that's when we added these
> > > >>>>>> fences.  There have certainly been complains that these fences are too
> > > >>>>>> heavyweight for the HW to go fast, but IIUC it's the best option we have
> > > >>>>> Yeah, it would reduce the performance on D1 and our next-generation
> > > >>>>> processor has optimized fence performance a lot.
> > > >>>>
> > > >>>> Definately a bummer that the fences make the HW go slow, but I don't
> > > >>>> really see any other way to go about this.  If you think these mappings
> > > >>>> are valid for LKMM and RVWMO then we should figure this out, but trying
> > > >>>> to drop fences to make HW go faster in ways that violate the memory
> > > >>>> model is going to lead to insanity.
> > > >>> Actually, this patch is okay with the ISA spec, and Dan also thought
> > > >>> it was valid.
> > > >>>
> > > >>> ref: https://lore.kernel.org/lkml/41e01514-74ca-84f2-f5cc-2645c444fd8e@xxxxxxxxxx/raw
> > > >>
> > > >> "Thoughts" on this regard have _changed_.  Please compare that quote
> > > >> with, e.g.
> > > >>
> > > >>   https://lore.kernel.org/linux-riscv/ddd5ca34-805b-60c4-bf2a-d6a9d95d89e7@xxxxxxxxxx/
> > > >>
> > > >> So here's a suggestion:
> > > >>
> > > >> Reviewers of your patches have asked:  How come that code we used to
> > > >> consider as buggy is now considered "an optimization" (correct)?
> > > >>
> > > >> Denying the evidence or going around it is not making their job (and
> > > >> this upstreaming) easier, so why don't you address it?  Take time to
> > > >> review previous works and discussions in this area, understand them,
> > > >> and integrate such knowledge in future submissions.
> > > >>
> > > >
> > > > I agree with Andrea.
> > > >
> > > > And I actually took a look into this, and I think I find some
> > > > explanation. There are two versions of RISV memory model here:
> > > >
> > > > Model 2017: released at Dec 1, 2017 as a draft
> > > >
> > > >       https://groups.google.com/a/groups.riscv.org/g/isa-dev/c/hKywNHBkAXM/m/QzUtxEWLBQAJ
> > > >
> > > > Model 2018: released at May 2, 2018
> > > >
> > > >       https://groups.google.com/a/groups.riscv.org/g/isa-dev/c/xW03vmfmPuA/m/bMPk3UCWAgAJ
> > > >
> > > > Noted that previous conversation about commit 5ce6c1f3535f happened at
> > > > March 2018. So the timeline is roughly:
> > > >
> > > >       Model 2017 -> commit 5ce6c1f3535f -> Model 2018
> > > >
> > > > And in the email thread of Model 2018, the commit related to model
> > > > changes also got mentioned:
> > > >
> > > >       https://github.com/riscv/riscv-isa-manual/commit/b875fe417948635ed68b9644ffdf718cb343a81a
> > > >
> > > > in that commit, we can see the changes related to sc.aqrl are:
> > > >
> > > >        to have occurred between the LR and a successful SC.  The LR/SC
> > > >        sequence can be given acquire semantics by setting the {\em aq} bit on
> > > >       -the SC instruction.  The LR/SC sequence can be given release semantics
> > > >       -by setting the {\em rl} bit on the LR instruction.  Setting both {\em
> > > >       -  aq} and {\em rl} bits on the LR instruction, and setting the {\em
> > > >       -  aq} bit on the SC instruction makes the LR/SC sequence sequentially
> > > >       -consistent with respect to other sequentially consistent atomic
> > > >       -operations.
> > > >       +the LR instruction.  The LR/SC sequence can be given release semantics
> > > >       +by setting the {\em rl} bit on the SC instruction.  Setting the {\em
> > > >       +  aq} bit on the LR instruction, and setting both the {\em aq} and the {\em
> > > >       +  rl} bit on the SC instruction makes the LR/SC sequence sequentially
> > > >       +consistent, meaning that it cannot be reordered with earlier or
> > > >       +later memory operations from the same hart.
> > > >
> > > > note that Model 2018 explicitly says that "ld.aq+sc.aqrl" is ordered
> > > > against "earlier or later memory operations from the same hart", and
> > > > this statement was not in Model 2017.
> > > >
> > > > So my understanding of the story is that at some point between March and
> > > > May 2018, RISV memory model folks decided to add this rule, which does
> > > > look more consistent with other parts of the model and is useful.
> > > >
> > > > And this is why (and when) "ld.aq+sc.aqrl" can be used as a fully-ordered
> > > > barrier ;-)
> > > >
> > > > Now if my understanding is correct, to move forward, it's better that 1)
> > > > this patch gets resend with the above information (better rewording a
> > > > bit), and 2) gets an Acked-by from Dan to confirm this is a correct
> > > > history ;-)
> > >
> > > I'm a bit lost as to why digging into RISC-V mailing list history is
> > > relevant here...what's relevant is what was ratified in the RVWMO
> > > chapter of the RISC-V spec, and whether the code you're proposing
> > > is the most optimized code that is correct wrt RVWMO.
> > >
> > > Is your claim that the code you're proposing to fix was based on a
> > > pre-RVWMO RISC-V memory model definition, and you're updating it to
> > > be more RVWMO-compliant?
> > Could "lr + beq + sc.aqrl" provides a conditional RCsc here with
> > current spec? I only found "lr.aq + sc.aqrl" despcriton which is
> > un-conditional RCsc.
> >
>
> /me put the temporary RISCV memory model hat on and pretend to be a
> RISCV memory expert.
>
> I think the answer is yes, it's actually quite straightforwards given
> that RISCV treats PPO (Preserved Program Order) as part of GMO (Global
> Memory Order), considering the following (A and B are memory accesses):
>
>         A
>         ..
>         sc.aqrl // M
>         ..
>         B
>
> , A has a ->ppo ordering to M since "sc.aqrl" is a RELEASE, and M has
> a ->ppo ordeing to B since "sc.aqrl" is an AQUIRE, so
>
>         A ->ppo M ->ppo B
That also means M must fence.rl + sc + fence.aq. But in the release
consistency model, "rl + aq" is not legal and has no guarantee at all.

So sc.aqrl should be clarified in spec, but I only found "lr.aq +
sc.aqrl" description, see the patch commit log.

Could we treat sc.aqrl as a whole in ISA? Because in micro-arch, we
must separate it into pieces for implementation.

That is what the RVWMO should give out.

>
> And since RISCV describes that PPO is part of GMO:
>
> """
> The subset of program order that must be respected by the global memory
> order is known as preserved program order.
> """
>
> also in the herd model:
>
>         (* Main model axiom *)
>         acyclic co | rfe | fr | ppo as Model
If the herd7 model has defined that, I think it should be legal. Good catch.


>
> , therefore the ordering between A and B is GMO and GMO should be
> respected by all harts.
>
> Regards,
> Boqun
>
> > >
> > > Dan
> > >
> > > > Regards,
> > > > Boqun
> > > >
> > > >>   Andrea
> > > >>
> > > >>
> > > > [...]
> >
> >
> >
> > --
> > Best Regards
> >  Guo Ren
> >
> > ML: https://lore.kernel.org/linux-csky/



-- 
Best Regards
 Guo Ren