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

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Message-ID: <87o74m1oq7.ffs@tglx>
Date: Tue, 17 Sep 2024 13:52:48 +0200
From: Thomas Gleixner <tglx@...utronix.de>
To: Christoph Lameter via B4 Relay <devnull+cl.gentwo.org@...nel.org>,
 Catalin Marinas <catalin.marinas@....com>, Will Deacon <will@...nel.org>,
 Peter Zijlstra <peterz@...radead.org>, Ingo Molnar <mingo@...hat.com>,
 Waiman Long <longman@...hat.com>, Boqun Feng <boqun.feng@...il.com>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>, linux-mm@...ck.org,
 linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
 linux-arch@...r.kernel.org, "Christoph Lameter (Ampere)" <cl@...two.org>
Subject: Re: [PATCH v3] Avoid memory barrier in read_seqcount() through load
 acquire

On Thu, Sep 12 2024 at 15:44, Christoph Lameter via wrote:

$Subject: .....

You still fail to provide a proper subsystem prefix. It's not rocket science.

https://www.kernel.org/doc/html/latest/process/maintainer-tip.html#patch-subject

> From: "Christoph Lameter (Ampere)" <cl@...two.org>
>
> Some architectures support load acquire which can save us a memory
> barrier and save some cycles.
>
> A typical sequence
>
> 	do {
> 		seq = read_seqcount_begin(&s);
> 		<something>
> 	} while (read_seqcount_retry(&s, seq);
>
> requires 13 cycles on ARM64 for an empty loop. Two read memory
> barriers are needed. One for each of the seqcount_* functions.
>
> We can replace the first read barrier with a load acquire of
> the seqcount which saves us one barrier.

We ....

Please write changelogs using passive voice. 'We' means nothing here.

Thanks,

        tglx