Re: [PATCH] locking/atomic: sh: Use generic_cmpxchg_local for arch_cmpxchg_local()

On 2023/10/25 07:42, Masami Hiramatsu (Google) wrote:
> On Tue, 24 Oct 2023 16:08:12 +0100
> Mark Rutland <[email protected]> wrote:
> 
>> On Tue, Oct 24, 2023 at 11:52:54PM +0900, Masami Hiramatsu (Google) wrote:
>>> From: Masami Hiramatsu (Google) <[email protected]>
>>>
>>> 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?

asm-generic/cmpxchg.h is only for UP, will throw an error for SMP building:

#ifdef CONFIG_SMP
#error "Cannot use generic cmpxchg on SMP"
#endif

SH arch seems it does have SMP systems. The arch/sh/include/asm/cmpxchg.h has
the following codes:

#if defined(CONFIG_GUSA_RB)
#include <asm/cmpxchg-grb.h>
#elif defined(CONFIG_CPU_SH4A)
#include <asm/cmpxchg-llsc.h>
#elif defined(CONFIG_CPU_J2) && defined(CONFIG_SMP)
#include <asm/cmpxchg-cas.h>
#else
#include <asm/cmpxchg-irq.h>
#endif

> 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

I agree too. Might not be performance optimized but guarantees correctness.

>> Mark.
>>
>>>
>>> Reported-by: kernel test robot <[email protected]>
>>> Closes: https://lore.kernel.org/oe-kbuild-all/[email protected]/
>>> Signed-off-by: Masami Hiramatsu (Google) <[email protected]>
>>> ---
>>>   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 */
>>>
> 
>