linux-kernel - Re: [PATCH 1/6] MIPS: Octeon: Implement __smp_store

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Message-ID: <aace6ff1-9ddf-15af-3c0a-378c53c59acb@nokia.com>
Date:   Thu, 28 Jan 2021 08:27:29 +0100
From:   Alexander Sverdlin <alexander.sverdlin@...ia.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Paul Burton <paul.burton@...tec.com>, linux-mips@...r.kernel.org,
        Thomas Bogendoerfer <tsbogend@...ha.franken.de>,
        Will Deacon <will@...nel.org>,
        Boqun Feng <boqun.feng@...il.com>,
        Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/6] MIPS: Octeon: Implement __smp_store_release()

Hello Peter,

On 27/01/2021 23:32, Peter Zijlstra wrote:
>> Link: https://lore.kernel.org/lkml/5644D08D.4080206@caviumnetworks.com/

please, check the discussion pointed by the link above...

>> Signed-off-by: Alexander Sverdlin <alexander.sverdlin@...ia.com>
>> ---
>>  arch/mips/include/asm/barrier.h | 9 +++++++++
>>  1 file changed, 9 insertions(+)
>>
>> diff --git a/arch/mips/include/asm/barrier.h b/arch/mips/include/asm/barrier.h
>> index 49ff172..24c3f2c 100644
>> --- a/arch/mips/include/asm/barrier.h
>> +++ b/arch/mips/include/asm/barrier.h
>> @@ -113,6 +113,15 @@ static inline void wmb(void)
>>  					    ".set arch=octeon\n\t"	\
>>  					    "syncw\n\t"			\
>>  					    ".set pop" : : : "memory")
>> +
>> +#define __smp_store_release(p, v)					\
>> +do {									\
>> +	compiletime_assert_atomic_type(*p);				\
>> +	__smp_wmb();							\
>> +	__smp_rmb();							\
>> +	WRITE_ONCE(*p, v);						\
>> +} while (0)
> This is wrong in general since smp_rmb() will only provide order between
> two loads and smp_store_release() is a store.
> 
> If this is correct for all MIPS, this needs a giant comment on exactly
> how that smp_rmb() makes sense here.

... the macro is provided for Octeon only, and __smp_rmb() is actually a NOP
there, but I thought to "document" the flow of thoughts from the discussion
above by including it anyway.

-- 
Best regards,
Alexander Sverdlin.