linux-kernel - Re: [PATCH v2 1/4] seqlock: introduce scoped_seqlock_read() and scoped_seqlock_read

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Message-ID: <CAHk-=wh3h5cV=UiTg+gvqB-T6+pStDNH0+6w4i34qMC1BQwmpg@mail.gmail.com>
Date: Thu, 9 Oct 2025 13:24:51 -0700
From: Linus Torvalds <torvalds@...ux-foundation.org>
To: Peter Zijlstra <peterz@...radead.org>
Cc: Oleg Nesterov <oleg@...hat.com>, Alexander Viro <viro@...iv.linux.org.uk>, 
	Boqun Feng <boqun.feng@...il.com>, David Howells <dhowells@...hat.com>, 
	Ingo Molnar <mingo@...hat.com>, Li RongQing <lirongqing@...du.com>, 
	Waiman Long <longman@...hat.com>, Will Deacon <will@...nel.org>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 1/4] seqlock: introduce scoped_seqlock_read() and scoped_seqlock_read_irqsave()

On Thu, 9 Oct 2025 at 13:12, Peter Zijlstra <peterz@...radead.org> wrote:
>
> Slightly nicer version that's actually compiled :-)

I assume that "target of ss_lockless" is an intentional extension to
just make the loop never take a lock at all?

I do like it, but I think you'll find that having a separate 'seq' and
'flags' like this:

> +struct ss_tmp {
> +       enum ss_state   state;
> +       int             seq;
> +       unsigned long   flags;
> +       spinlock_t      *lock;
> +};

makes it unnecessarily waste a register.

You never need both seq and flags at the same time, since if you take
the spinlock the sequence number is pointless.

So please make that a union, and I think it will help avoid wasting a
register in the loop.

Other than that I like it. Except that "BUG()" really bugs me. It will
generate horrendous code for no reason and we *really* shouldn't add
BUG statements anyway.

Either that inline function is fine, or it isn't. Don't make it
generate stupid code for "I'm not fine" that will also be a huge pain
to debug because if that code is buggy it will presumably trigger in
context where the machine will be dead, dead, dead.

             Linus