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

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Message-ID: <20230801083201.GB26036@willie-the-truck>
Date:   Tue, 1 Aug 2023 09:32:02 +0100
From:   Will Deacon <will@...nel.org>
To:     WANG Rui <wangrui@...ngson.cn>
Cc:     guoren@...nel.org, chenhuacai@...nel.or, kernel@...0n.name,
        arnd@...db.de, andi.shyti@...ux.intel.com, andrzej.hajda@...el.com,
        peterz@...radead.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 01, 2023 at 10:29:31AM +0800, WANG Rui wrote:
> On Tue, Aug 1, 2023 at 9:16 AM <guoren@...nel.org> wrote:
> > diff --git a/arch/loongarch/include/asm/cmpxchg.h b/arch/loongarch/include/asm/cmpxchg.h
> > index 979fde61bba8..6a05b92814b6 100644
> > --- a/arch/loongarch/include/asm/cmpxchg.h
> > +++ b/arch/loongarch/include/asm/cmpxchg.h
> > @@ -102,8 +102,8 @@ __arch_xchg(volatile void *ptr, unsigned long x, int size)
> >         "       move    $t0, %z4                        \n"             \
> >         "       " st "  $t0, %1                         \n"             \
> >         "       beqz    $t0, 1b                         \n"             \
> > -       "2:                                             \n"             \
> >         __WEAK_LLSC_MB                                                  \
> > +       "2:                                             \n"             \
> 
> Thanks for the patch.
> 
> This would look pretty good if it weren't for the special memory
> barrier semantics of the LoongArch's LL and SC instructions.
> 
> The LL/SC memory barrier behavior of LoongArch:
> 
> * LL: <memory-barrier> + <load-exclusive>
> * SC: <store-conditional> + <memory-barrier>
> 
> and the LoongArch's weak memory model allows load/load reorder for the
> same address.

Hmm, somehow this one passed me by, but I think that puts you in the naughty
corner with Itanium. It probably also means your READ_ONCE() is broken,
unless the compiler emits barriers for volatile reads (like ia64)?

Will