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

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Date:   Wed, 25 Oct 2023 08:42:55 +0900
From:   Masami Hiramatsu (Google) <mhiramat@...nel.org>
To:     Mark Rutland <mark.rutland@....com>
Cc:     Yoshinori Sato <ysato@...rs.sourceforge.jp>,
        Rich Felker <dalias@...c.org>,
        John Paul Adrian Glaubitz <glaubitz@...sik.fu-berlin.de>,
        "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
Subject: Re: [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@....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), "")


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@...el.com>
> > Closes: https://lore.kernel.org/oe-kbuild-all/202310241310.Ir5uukOG-lkp@intel.com/
> > Signed-off-by: Masami Hiramatsu (Google) <mhiramat@...nel.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@...nel.org>