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

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Date:   Thu, 28 Jan 2021 16:04:39 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Alexander Sverdlin <alexander.sverdlin@...ia.com>
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()

On Thu, Jan 28, 2021 at 01:09:39PM +0100, Alexander Sverdlin wrote:
> 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)

> 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/

My memory really isn't that good. I can barely remember what I did 5
weeks ago, 5 years ago might as well have never happened.

> 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?

So for store_release I'm not too worried, and provided no read
speculation, wmb is indeed sufficient. This is because our store_release
is RCpc.

Something like:

/*
 * Because Octeon does not do read speculation, an smp_wmb()
 * is sufficient to ensure {load,store}->{store} order.
 */
#define __smp_store_release(p, v) \
do { \
	compiletime_assert_atomic_type(*p); \
	__smp_wmb(); \
	WRITE_ONCE(*p, v); \
} while (0)