linux-kernel - Re: [RFC PATCH v2 0/3] Deduplicate RISCV cmpxchg.h and atomic.c macros

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Message-ID: <CAJ6HWG5mzxN=txnbdi-=c+=wLsDfSe7Me+1DQE79ZX0NN6U1_g@mail.gmail.com>
Date:   Thu, 3 Aug 2023 23:19:49 -0300
From:   Leonardo Bras Soares Passos <leobras@...hat.com>
To:     Guo Ren <guoren@...nel.org>
Cc:     Will Deacon <will@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Boqun Feng <boqun.feng@...il.com>,
        Mark Rutland <mark.rutland@....com>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Albert Ou <aou@...s.berkeley.edu>,
        Andrzej Hajda <andrzej.hajda@...el.com>,
        Arnd Bergmann <arnd@...db.de>, Ingo Molnar <mingo@...nel.org>,
        Palmer Dabbelt <palmer@...osinc.com>,
        linux-kernel@...r.kernel.org, linux-riscv@...ts.infradead.org
Subject: Re: [RFC PATCH v2 0/3] Deduplicate RISCV cmpxchg.h and atomic.c macros

On Thu, Aug 3, 2023 at 9:53 PM Guo Ren <guoren@...nel.org> wrote:
>
> On Thu, Aug 3, 2023 at 1:14 PM Leonardo Bras <leobras@...hat.com> wrote:
> >
> > I unified previous patchsets into a single one, since the work is related.
> >
> > While studying riscv's cmpxchg.h file, I got really interested in
> > understanding how RISCV asm implemented the different versions of
> > {cmp,}xchg.
> >
> > When I understood the pattern, it made sense for me to remove the
> > duplications and create macros to make it easier to understand what exactly
> > changes between the versions: Instruction sufixes & barriers.
> >
> > Also, did the same kind of work on atomic.c.
> >
> > Note to Guo Ren:
> > I did some further improvement after your previous reviews, so I ended
> > up afraid including your Reviewed-by before cheching if the changes are
> > ok for you. Please check it out again, I just removed some helper macros
> > that were not being used elsewhere in the kernel.
> I found this optimization has conflicts with the below patches:
> https://lore.kernel.org/linux-riscv/20230802164701.192791-15-guoren@kernel.org/
> https://lore.kernel.org/linux-riscv/20230802164701.192791-5-guoren@kernel.org/
>
> If yours merged, how do we support the inline cmpxchg/xchg_small
> function?

Oh, I actually introduced my series so I could introduce new xchg and
cmpxchg for size 1 and 2. Is that what your patches are about, right?

I was working on that yesterday, and decided to send the patchset
without them because I was still not sure enough.

About implementation strategy, I was introducing a new macros for xchg
& cmpxchg with asm which would work for both for size 1 & size 2, and
use the switch-case to create the mask and and_value.

You think that works enough?

> It's very struggling to use macros to implement complex
> functions.

I agree, but with this we can achieve more generic code, which makes
more clear what is the pattern for given function.

> Could you consider a more relaxed optimization in which we could
> insert inline cmpxchg/xchg_small?

What about this: I finish the patches I have been working with
(cmpxchg & xchg for sizes 1 and 2), and if they are fine we expand
this patchset with them.  If not, I try relaxing them a little so we
can merge with your set.

Does that work for you?

Best regards,
Leo


>
> >
> > Thanks!
> > Leo
> >
> >
> > Changes since squashed cmpxchg:
> > - Unified with atomic.c patchset
> > - Rebased on top of torvalds/master (thanks Andrea Parri!)
> > - Removed helper macros that were not being used elsewhere in the kernel.
> >
> > Changes since (cmpxchg) RFCv3:
> > - Squashed the 6 original patches in 2: one for cmpxchg and one for xchg
> > https://lore.kernel.org/all/20230404163741.2762165-1-leobras@redhat.com/
> >
> > Changes since (cmpxchg) RFCv2:
> > - Fixed  macros that depend on having a local variable with a magic name
> > - Previous cast to (long) is now only applied on 4-bytes cmpxchg
> > https://lore.kernel.org/all/20230321074249.2221674-1-leobras@redhat.com/
> >
> > Changes since (cmpxchg) RFCv1:
> > - Fixed patch 4/6 suffix from 'w.aqrl' to '.w.aqrl', to avoid build error
> > https://lore.kernel.org/all/20230318080059.1109286-1-leobras@redhat.com/
> >
> >
> > Leonardo Bras (3):
> >   riscv/cmpxchg: Deduplicate xchg() asm functions
> >   riscv/cmpxchg: Deduplicate cmpxchg() asm and macros
> >   riscv/atomic.h : Deduplicate arch_atomic.*
> >
> >  arch/riscv/include/asm/atomic.h  | 164 ++++++++--------
> >  arch/riscv/include/asm/cmpxchg.h | 318 +++++--------------------------
> >  2 files changed, 123 insertions(+), 359 deletions(-)
> >
> > --
> > 2.41.0
> >
>
>
> --
> Best Regards
>  Guo Ren
>