linux-kernel - Re: [PATCH v2 rcu 04/18] rcu: Weaken ->dynticks accesses and updates

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Message-ID: <CAHk-=wjwZzi=Lqsre8C8C4YJiVQNdNnco7jkLq=ohePSPA9JCA@mail.gmail.com>
Date:   Wed, 28 Jul 2021 10:58:08 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     "Paul E. McKenney" <paulmck@...nel.org>
Cc:     rcu@...r.kernel.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Kernel Team <kernel-team@...com>,
        Ingo Molnar <mingo@...nel.org>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Josh Triplett <josh@...htriplett.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        David Howells <dhowells@...hat.com>,
        Eric Dumazet <edumazet@...gle.com>,
        Frédéric Weisbecker <fweisbec@...il.com>,
        Oleg Nesterov <oleg@...hat.com>,
        Joel Fernandes <joel@...lfernandes.org>
Subject: Re: [PATCH v2 rcu 04/18] rcu: Weaken ->dynticks accesses and updates

On Wed, Jul 28, 2021 at 10:37 AM Paul E. McKenney <paulmck@...nel.org> wrote:
>
> +/*
> + * Increment the current CPU's rcu_data structure's ->dynticks field
> + * with ordering.  Return the new value.
> + */
> +static noinstr unsigned long rcu_dynticks_inc(int incby)
> +{
> +       return arch_atomic_add_return(incby, this_cpu_ptr(&rcu_data.dynticks));
> +}

Maybe inline?

But more I reacted to how we sadly don't have percpu atomics. They'd
be fairly easy to add on x86, but I guess it's not a huge deal.

And hey, if this is pretty much the only place that would use them, I
guess that's not much of an issue.

             Linus