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References: <20231212174750.GA11886@neeraj.linux> <20231212174817.11919-3-neeraj.iitr10@gmail.com>
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From:   Joel Fernandes <joel@joelfernandes.org>
Date:   Wed, 13 Dec 2023 09:27:09 -0500
Message-ID: <CAEXW_YRHjdM+NA3CqNuwaRNXkRWbtypmt5Ov=YXnrpn3Eo-==Q@mail.gmail.com>
Subject: Re: [PATCH rcu 3/3] srcu: Explain why callbacks invocations can't run concurrently
To:     "Neeraj Upadhyay (AMD)" <neeraj.iitr10@gmail.com>
Cc:     rcu@vger.kernel.org, linux-kernel@vger.kernel.org,
        kernel-team@meta.com, rostedt@goodmis.org, paulmck@kernel.org,
        Neeraj.Upadhyay@amd.com, Frederic Weisbecker <frederic@kernel.org>
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On Tue, Dec 12, 2023 at 12:48=E2=80=AFPM Neeraj Upadhyay (AMD)
<neeraj.iitr10@gmail.com> wrote:
>
> From: Frederic Weisbecker <frederic@kernel.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@kernel.org>
> Reviewed-by: Joel Fernandes (Google) <joel@joelfernandes.org>
> Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
> Signed-off-by: Neeraj Upadhyay (AMD) <neeraj.iitr10@gmail.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_stru=
ct *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