linux-kernel - Re: [PATCH 3/3] ring_buffer: Use try

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Message-ID: <CAFULd4ZQFONC-q66nCxj4B7r+PmyAf5V0KEhgrsnwR4NNxT4+g@mail.gmail.com>
Date:   Wed, 1 Mar 2023 19:08:44 +0100
From:   Uros Bizjak <ubizjak@...il.com>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     linux-trace-kernel@...r.kernel.org, linux-kernel@...r.kernel.org,
        Masami Hiramatsu <mhiramat@...nel.org>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Joel Fernandes <joel@...lfernandes.org>
Subject: Re: [PATCH 3/3] ring_buffer: Use try_cmpxchg instead of cmpxchg

On Wed, Mar 1, 2023 at 5:28 PM Steven Rostedt <rostedt@...dmis.org> wrote:
>
> On Wed, 1 Mar 2023 11:18:50 -0500
> Steven Rostedt <rostedt@...dmis.org> wrote:
>
> > But with gcc 12.2.0 I don't really see the benefit. And I'm worried that
> > the side effect of modifying the old variable could cause a bug in the
> > future, if it is used after the try_cmpxchg(). At least for the second case.
>
> Actually, I like Joel's recommendation of adding a cmpxchg_succeeded()
> function, that does the try_cmpxchg() without needing to save the old
> variable. That's my main concern, as it does have that side effect that
> could be missed when updating the code.

I understand your concern regarding updating of head_page_with_bit in
the middle of rb_insert_pages. OTOH, rb_head_page_replace is a small
utility function where update happens in a return clause, so there is
no chance of using val after the try_cmpxchg. If we can ignore the
"updating" issue in rb_head_page_replace, we can simply define
cmpxchg_success in front of rb_insert_pages (now its sole user) and be
done with it.

Uros.