linux-kernel - Re: [PATCH] LoongArch: Fixup cmpxchg sematic for memory barrier

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Message-ID: <CAJF2gTSLD3jwzV59dGj_RENCvE0zMOAkLHqmT55bxL-4vA1wkA@mail.gmail.com>
Date:   Tue, 1 Aug 2023 19:17:08 -0400
From:   Guo Ren <guoren@...nel.org>
To:     WANG Rui <wangrui@...ngson.cn>
Cc:     chenhuacai@...nel.or, kernel@...0n.name, arnd@...db.de,
        andi.shyti@...ux.intel.com, andrzej.hajda@...el.com,
        peterz@...radead.org, will@...nel.org, boqun.feng@...il.com,
        mark.rutland@....com, loongarch@...ts.linux.dev,
        linux-kernel@...r.kernel.org, Guo Ren <guoren@...ux.alibaba.com>
Subject: Re: [PATCH] LoongArch: Fixup cmpxchg sematic for memory barrier

On Tue, Aug 1, 2023 at 12:37 PM WANG Rui <wangrui@...ngson.cn> wrote:
>
> On Tue, Aug 1, 2023 at 6:50 PM Guo Ren <guoren@...nel.org> wrote:
> >
> > On Tue, Aug 1, 2023 at 5:32 PM WANG Rui <wangrui@...ngson.cn> wrote:
> > > No. LL and LL won't reorder because LL implies a memory barrier(though
> > > not acquire semantics).
> > That means we could remove __WEAK_LLSC_MB totally, right?
>
> More precisely, __WEAK_LLSC_MB is intended to prevent reordering
> between LL and normal LD used to fetch the expected value for cmpxchg.
Oh, that's unnecessary when cmpxchg fails.

Maybe you treat cmpxchg as a CoRR antidote in coincidence. Please
solve the CoRR problem by READ_ONCE.

See alpha architecture.

>
> Regards,
> --
> WANG Rui
>


-- 
Best Regards
 Guo Ren