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

[Boqun Feng]

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@nvidia.com/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@nvidia.com/
> 
> 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 ;-)

Regards,
Boqun

>   Andrea
> 
> 
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