linux-kernel - Re: [PATCH RFC] Avoid memory barrier in read

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Message-ID: <5d54a189-3c2b-440a-9626-4e00e95a7f77@redhat.com>
Date: Tue, 13 Aug 2024 15:58:27 -0400
From: Waiman Long <longman@...hat.com>
To: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: cl@...two.org, Catalin Marinas <catalin.marinas@....com>,
 Will Deacon <will@...nel.org>, Peter Zijlstra <peterz@...radead.org>,
 Ingo Molnar <mingo@...hat.com>, Boqun Feng <boqun.feng@...il.com>,
 linux-mm@...ck.org, linux-kernel@...r.kernel.org,
 linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH RFC] Avoid memory barrier in read_seqcount() through load
 acquire


On 8/13/24 15:48, Linus Torvalds wrote:
> On Tue, 13 Aug 2024 at 12:01, Waiman Long <longman@...hat.com> wrote:
>> Do we need a new ARCH flag?
> I'm confused by that question.
>
> That's clearly exactly what that ARCH_HAS_ACQUIRE_RELEASE is.
>
> Obviously all architectures "have" it - in the sense that we always
> have access to a "smp_load_acquire()/smp_store_release()".

Sorry for the confusion. What you said above is actually the reason that 
I ask this question. In the same way, smp_rmb()/wmb() is available for 
all arches. I am actually asking if it should be a flag that indicates 
the arch's preference to use acquire/release over rmb/wmb.

Cheers,
Longman

>
> But if the architecture doesn't support it natively, the old rmb/wmb
> model may be preferred.
>
> Although maybe we're at the point where we don't even care about that.
>
>                Linus
>