linux-kernel - Re: [PATCH] locking/atomic: sh: Use generic_cmpxchg_local for arch_cmpxchg

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Date:   Wed, 25 Oct 2023 15:16:16 +0200
From:   Geert Uytterhoeven <geert@...ux-m68k.org>
To:     John Paul Adrian Glaubitz <glaubitz@...sik.fu-berlin.de>,
        Masami Hiramatsu <mhiramat@...nel.org>
Cc:     Mark Rutland <mark.rutland@....com>,
        Yoshinori Sato <ysato@...rs.sourceforge.jp>,
        Rich Felker <dalias@...c.org>,
        "wuqiang . matt" <wuqiang.matt@...edance.com>,
        Peter Zijlstra <peterz@...radead.org>,
        linux-sh@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-trace-kernel@...r.kernel.org,
        Geert Uytterhoeven <geert+renesas@...der.be>
Subject: Re: [PATCH] locking/atomic: sh: Use generic_cmpxchg_local for arch_cmpxchg_local()

Hi Adrian,

On Wed, Oct 25, 2023 at 12:32 PM John Paul Adrian Glaubitz
<glaubitz@...sik.fu-berlin.de> wrote:
> On Wed, 2023-10-25 at 11:30 +0100, Mark Rutland wrote:
> > On Wed, Oct 25, 2023 at 08:42:55AM +0900, Masami Hiramatsu wrote:
> > > On Tue, 24 Oct 2023 16:08:12 +0100
> > > Mark Rutland <mark.rutland@....com> wrote:
> > > > On Tue, Oct 24, 2023 at 11:52:54PM +0900, Masami Hiramatsu (Google) wrote:
> > > > > From: Masami Hiramatsu (Google) <mhiramat@...nel.org>
> > > > >
> > > > > Use generic_cmpxchg_local() for arch_cmpxchg_local() implementation
> > > > > in SH architecture because it does not implement arch_cmpxchg_local().
> > > >
> > > > I do not think this is correct.
> > > >
> > > > The implementation in <asm-generic/cmpxchg-local.h> is UP-only (and it only
> > > > disables interrupts), whereas arch/sh can be built SMP. We should probably add
> > > > some guards into <asm-generic/cmpxchg-local.h> for that as we have in
> > > > <asm-generic/cmpxchg.h>.
> > >
> > > Isn't cmpxchg_local for the data which only needs to ensure to do cmpxchg
> > > on local CPU?
> > > So I think it doesn't care about the other CPUs (IOW, it should not touched by
> > > other CPUs), so it only considers UP case. E.g. on x86, arch_cmpxchg_local() is
> > > defined as raw "cmpxchg" without lock prefix.
> > >
> > > #define __cmpxchg_local(ptr, old, new, size)                            \
> > >         __raw_cmpxchg((ptr), (old), (new), (size), "")
> > >
> >
> > Yes, you're right; sorry for the noise.
> >
> > For your original patch:
> >
> > Acked-by: Mark Rutland <mark.rutland@....com>
>
> Geert, what's your opinion on this?

While this looks OK on first sight (ARM includes the same file, even
on SMP), it does not seem to work?

For sh-allnoconfig, as reported by kernel test robot:

$ make ARCH=sh CROSS_COMPILE=sh2-linux- allnoconfig lib/objpool.o
lib/objpool.c: In function 'objpool_try_add_slot':
./include/linux/atomic/atomic-arch-fallback.h:384:27: error: implicit
declaration of function 'arch_cmpxchg_local'; did you mean
'raw_cmpxchg_local'? [-Werror=implicit-function-declaration]
  384 | #define raw_cmpxchg_local arch_cmpxchg_local
      |                           ^~~~~~~~~~~~~~~~~~
./include/linux/atomic/atomic-arch-fallback.h:392:16: note: in
expansion of macro 'raw_cmpxchg_local'
  392 |         ___r = raw_cmpxchg_local((_ptr), ___o, (_new)); \
      |                ^~~~~~~~~~~~~~~~~
./include/linux/atomic/atomic-instrumented.h:4980:9: note: in
expansion of macro 'raw_try_cmpxchg_local'
 4980 |         raw_try_cmpxchg_local(__ai_ptr, __ai_oldp, __VA_ARGS__); \
      |         ^~~~~~~~~~~~~~~~~~~~~
lib/objpool.c:169:19: note: in expansion of macro 'try_cmpxchg_local'
  169 |         } while (!try_cmpxchg_local(&slot->tail, &tail, tail + 1));
      |                   ^~~~~~~~~~~~~~~~~

For an SMP defconfig:

$ make ARCH=sh CROSS_COMPILE=sh4-linux-gnu- sdk7786_defconfig lib/objpool.o

./include/linux/atomic/atomic-arch-fallback.h:384:27: error: implicit
declaration of function ‘arch_cmpxchg_local’; did you mean
‘try_cmpxchg_local’? [-Werror=implicit-function-declaration]
  384 | #define raw_cmpxchg_local arch_cmpxchg_local
      |                           ^~~~~~~~~~~~~~~~~~
./include/linux/atomic/atomic-arch-fallback.h:392:16: note: in
expansion of macro ‘raw_cmpxchg_local’
  392 |         ___r = raw_cmpxchg_local((_ptr), ___o, (_new)); \
      |                ^~~~~~~~~~~~~~~~~
./include/linux/atomic/atomic-instrumented.h:4980:9: note: in
expansion of macro ‘raw_try_cmpxchg_local’
 4980 |         raw_try_cmpxchg_local(__ai_ptr, __ai_oldp, __VA_ARGS__); \
      |         ^~~~~~~~~~~~~~~~~~~~~
lib/objpool.c:169:19: note: in expansion of macro ‘try_cmpxchg_local’
  169 |         } while (!try_cmpxchg_local(&slot->tail, &tail, tail + 1));
      |                   ^~~~~~~~~~~~~~~~~

Hiramatsu-san: do these build for you?

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds