linux-kernel - Re: [PATCH v3] Avoid memory barrier in read

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Message-ID: <CAHk-=wjdOX0t45a7aHerVPv_WBM3AmMi3sEp8xb19jpLFnk0dA@mail.gmail.com>
Date: Mon, 23 Sep 2024 09:28:31 -0700
From: Linus Torvalds <torvalds@...ux-foundation.org>
To: "Christoph Lameter (Ampere)" <cl@...two.org>
Cc: Will Deacon <will@...nel.org>, Thomas Gleixner <tglx@...utronix.de>, 
	Catalin Marinas <catalin.marinas@....com>, Peter Zijlstra <peterz@...radead.org>, 
	Ingo Molnar <mingo@...hat.com>, Waiman Long <longman@...hat.com>, Boqun Feng <boqun.feng@...il.com>, 
	linux-mm@...ck.org, linux-kernel@...r.kernel.org, 
	linux-arm-kernel@...ts.infradead.org, linux-arch@...r.kernel.org
Subject: Re: [PATCH v3] Avoid memory barrier in read_seqcount() through load acquire

On Wed, 18 Sept 2024 at 08:22, Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
>
> On Wed, 18 Sept 2024 at 13:15, Christoph Lameter (Ampere) <cl@...two.org> wrote:
> >
> > Other arches do not have acquire / release and will create additional
> > barriers in the fallback implementation of smp_load_acquire. So it needs
> > to be an arch config option.
>
> Actually, I looked at a few cases, and it doesn't really seem to be true.

Bah. I ended up just committing the minimal version of this all. I
gave Christoph credit for the commit, because I stole his commit
message, and he did most of the work, I just ended up going "simplify,
simplify, simplify".

I doubt anybody will notice, and smp_load_acquire() is the future. Any
architecture that does badly on it just doesn't matter (and, as
mentioned, I don't think they even exist - "smp_rmb()" is generally at
least as expensive).

                 Linus