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

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Message-ID: <20230301113813.4f16a689@gandalf.local.home>
Date:   Wed, 1 Mar 2023 11:38:13 -0500
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Uros Bizjak <ubizjak@...il.com>
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, 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.

-- Steve