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

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Date:   Thu, 28 Jan 2021 16:15:35 +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 03:57:58PM +0100, Peter Zijlstra wrote:
> On Thu, Jan 28, 2021 at 12:52:22PM +0100, Alexander Sverdlin wrote:
> > Hello Peter,
> > 
> > On 28/01/2021 12:33, Peter Zijlstra wrote:
> > > 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?
> > 
> > hmm, the commit message you point to above, says:
> > 
> > "Since Octeon does not do speculative reads, this functions as a full barrier."
> 
> So then the only difference between SYNC and SYNCW is a pipeline drain?
> 
> I still worry about the transitivity thing.. ISTR that being a sticky
> point back then too.

Ah, there we are, it's called multi-copy-atomic these days:

  f1ab25a30ce8 ("memory-barriers: Replace uses of "transitive"")

Do those SYNCW / write-completion barriers guarantee this?