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

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Message-ID: <CAFULd4YPP_5koKN5ZbdgOueXcpUgap97UkaxxyzzGcQ=GEV__Q@mail.gmail.com>
Date:   Wed, 1 Mar 2023 20:18:11 +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 7:52 PM Steven Rostedt <rostedt@...dmis.org> wrote:
>
> On Wed, 1 Mar 2023 19:43:34 +0100
> Uros Bizjak <ubizjak@...il.com> wrote:
>
> > 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.
>
> I like it!

Thanks!

> Anyway to make this more generic?

If we want to put the definition in generic headers, then we also need
to define acquire/release/relaxed and 64bit variants. ATM, we have two
sites that require this definition and I think that for now we could
live with two instances of the same definition in two separate
subsystems. But this would definitely be a good future addition. There
is some code in the form of

if (cmpxchg (ptr, 0, 1) == 0)

that can not be converted to try_cmpxchg, but can use cmpxchg_success.

Uros.