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SubjectRe: [PATCH] locking/atomic: sh: Use generic_cmpxchg_local for arch_cmpxchg_local()
On Tue, 24 Oct 2023 16:08:12 +0100
Mark Rutland <mark.rutland@arm.com> wrote:

> On Tue, Oct 24, 2023 at 11:52:54PM +0900, Masami Hiramatsu (Google) wrote:
> > From: Masami Hiramatsu (Google) <mhiramat@kernel.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), "")


Thank you,


>
> I think the right thing to do here is to define arch_cmpxchg_local() in terms
> of arch_cmpxchg(), i.e. at the bottom of arch/sh's <asm/cmpxchg.h> add:
>
> #define arch_cmpxchg_local arch_cmpxchg
>
> Mark.
>
> >
> > Reported-by: kernel test robot <lkp@intel.com>
> > Closes: https://lore.kernel.org/oe-kbuild-all/202310241310.Ir5uukOG-lkp@intel.com/
> > Signed-off-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
> > ---
> > arch/sh/include/asm/cmpxchg.h | 2 ++
> > 1 file changed, 2 insertions(+)
> >
> > diff --git a/arch/sh/include/asm/cmpxchg.h b/arch/sh/include/asm/cmpxchg.h
> > index 288f6f38d98f..e920e61fb817 100644
> > --- a/arch/sh/include/asm/cmpxchg.h
> > +++ b/arch/sh/include/asm/cmpxchg.h
> > @@ -71,4 +71,6 @@ static inline unsigned long __cmpxchg(volatile void * ptr, unsigned long old,
> > (unsigned long)_n_, sizeof(*(ptr))); \
> > })
> >
> > +#include <asm-generic/cmpxchg-local.h>
> > +
> > #endif /* __ASM_SH_CMPXCHG_H */
> >


--
Masami Hiramatsu (Google) <mhiramat@kernel.org>

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