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

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Date:   Thu, 28 Jan 2021 13:09:39 +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()

Hi!

On 28/01/2021 12:33, Peter Zijlstra wrote:
> On Thu, Jan 28, 2021 at 08:27:29AM +0100, Alexander Sverdlin wrote:
> 
>>>> +#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.
> 
> Random discussions on the internet do not absolve you from having to
> write coherent comments. Especially so where memory ordering is
> concerned.

I actually hoped you will remember the discussion you've participated 5 years
ago and (in my understanding) actually already agreed that the solution itself
is not broken:

https://lore.kernel.org/lkml/20151112180003.GE17308@twins.programming.kicks-ass.net/

Could you please just suggest the proper comment you expect to be added here,
because there is no doubts, you have much more experience here than me?

> This, from commit 6b07d38aaa52 ("MIPS: Octeon: Use optimized memory
> barrier primitives."):
> 
> 	#define smp_mb__before_llsc() smp_wmb()
> 	#define __smp_mb__before_llsc() __smp_wmb()
> 
> is also dodgy as hell and really wants a comment too. I'm not buying the
> Changelog of that commit either, __smp_mb__before_llsc should also
> ensure the LL cannot happen earlier, but SYNCW has no effect on loads.
> So what stops the load from being speculated?
> 
> 

-- 
Best regards,
Alexander Sverdlin.