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 09:52:35 -0800
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Joel Fernandes <joel@...lfernandes.org>
Cc:     "Neeraj Upadhyay (AMD)" <neeraj.iitr10@...il.com>,
        rcu@...r.kernel.org, linux-kernel@...r.kernel.org,
        kernel-team@...a.com, rostedt@...dmis.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 Wed, Dec 13, 2023 at 09:27:09AM -0500, Joel Fernandes wrote:
> 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 :)

Good point!

But please do not forget item 12 on the list in checklist.rst.  ;-)
(Which I just updated to include the other call_rcu*() functions.)

							Thanx, Paul