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

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Message-ID: <20241023194543.GD11151@noisy.programming.kicks-ass.net>
Date: Wed, 23 Oct 2024 21:45:43 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: "Christoph Lameter (Ampere)" <cl@...two.org>,
	Will Deacon <will@...nel.org>, Thomas Gleixner <tglx@...utronix.de>,
	Catalin Marinas <catalin.marinas@....com>,
	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 Mon, Sep 23, 2024 at 09:28:31AM -0700, Linus Torvalds wrote:
> 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).

Do we want to do the complementing patch and make write_seqcount_end()
use smp_store_release() ?

I think at least ARM (the 32bit thing) has wmb but uses mb for
store_release. But I also think I don't really care about that.