linux-kernel - Re: [PATCH] rcu: use try_cmpxchg in check_cpu

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Date:   Wed, 1 Mar 2023 19:43:34 +0100
From:   Uros Bizjak <ubizjak@...il.com>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     Joel Fernandes <joel@...lfernandes.org>,
        "Paul E. McKenney" <paulmck@...nel.org>, rcu@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Frederic Weisbecker <frederic@...nel.org>,
        Neeraj Upadhyay <quic_neeraju@...cinc.com>,
        Josh Triplett <josh@...htriplett.org>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Lai Jiangshan <jiangshanlai@...il.com>
Subject: Re: [PATCH] rcu: use try_cmpxchg in check_cpu_stall

On Wed, Mar 1, 2023 at 5:38 PM Steven Rostedt <rostedt@...dmis.org> wrote:
>
> On Wed, 1 Mar 2023 11:28:47 +0100
> Uros Bizjak <ubizjak@...il.com> wrote:
>
> > These benefits are the reason the change to try_cmpxchg was accepted
> > also in the linear code elsewhere in the linux kernel, e.g. [2,3] to
> > name a few commits, with a thumbs-up and a claim that the new code is
> > actually *clearer* at the merge commit [4].
>
> I'll say it here too. I really like Joel's suggestion of having a
> cmpxchg_success() that does not have the added side effect of modifying the
> old variable.
>
> I think that would allow for the arch optimizations that you are trying to
> achieve, as well as remove the side effect that might cause issues down the
> road.

Attached patch implements this suggestion.

Uros.

View attachment "rcu-2.diff.txt" of type "text/plain" (1209 bytes)