linux-kernel - Re: [PATCH rcu 3/3] srcu: Explain why callbacks invocations can't run concurrently

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Date:   Wed, 13 Dec 2023 09:27:09 -0500
From:   Joel Fernandes <joel@...lfernandes.org>
To:     "Neeraj Upadhyay (AMD)" <neeraj.iitr10@...il.com>
Cc:     rcu@...r.kernel.org, linux-kernel@...r.kernel.org,
        kernel-team@...a.com, rostedt@...dmis.org, paulmck@...nel.org,
        Neeraj.Upadhyay@....com, Frederic Weisbecker <frederic@...nel.org>
Subject: Re: [PATCH rcu 3/3] srcu: Explain why callbacks invocations can't run concurrently

On Tue, Dec 12, 2023 at 12:48 PM Neeraj Upadhyay (AMD)
<neeraj.iitr10@...il.com> wrote:
>
> From: Frederic Weisbecker <frederic@...nel.org>
>
> If an SRCU barrier is queued while callbacks are running and a new
> callbacks invocator for the same sdp were to run concurrently, the
> RCU barrier might execute too early. As this requirement is non-obvious,
> make sure to keep a record.
>
> Signed-off-by: Frederic Weisbecker <frederic@...nel.org>
> Reviewed-by: Joel Fernandes (Google) <joel@...lfernandes.org>
> Signed-off-by: Paul E. McKenney <paulmck@...nel.org>
> Signed-off-by: Neeraj Upadhyay (AMD) <neeraj.iitr10@...il.com>
> ---
>  kernel/rcu/srcutree.c | 6 ++++++
>  1 file changed, 6 insertions(+)
>
> diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c
> index 2bfc8ed1eed2..0351a4e83529 100644
> --- a/kernel/rcu/srcutree.c
> +++ b/kernel/rcu/srcutree.c
> @@ -1715,6 +1715,11 @@ static void srcu_invoke_callbacks(struct work_struct *work)
>         WARN_ON_ONCE(!rcu_segcblist_segempty(&sdp->srcu_cblist, RCU_NEXT_TAIL));
>         rcu_segcblist_advance(&sdp->srcu_cblist,
>                               rcu_seq_current(&ssp->srcu_sup->srcu_gp_seq));
> +       /*
> +        * Although this function is theoretically re-entrant, concurrent
> +        * callbacks invocation is disallowed to avoid executing an SRCU barrier
> +        * too early.
> +        */

Side comment:
I guess even without the barrier reasoning, it is best not to allow
concurrent CB execution anyway since it diverges from the behavior of
straight RCU :)

  - Joel