linux-kernel - Re: [PATCH 01/10] rcu: Directly lock rdp->nocb

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Message-ID: <20200611013224.GA9005@google.com>
Date:   Wed, 10 Jun 2020 21:32:24 -0400
From:   Joel Fernandes <joel@...lfernandes.org>
To:     "Paul E. McKenney" <paulmck@...nel.org>
Cc:     Frederic Weisbecker <frederic@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        Josh Triplett <josh@...htriplett.org>
Subject: Re: [PATCH 01/10] rcu: Directly lock rdp->nocb_lock on nocb code
 entrypoints

On Wed, Jun 10, 2020 at 04:21:42PM -0700, Paul E. McKenney wrote:
[...]
> > > 8.	To softirq 3.  Either GP or CB kthread for the transitioning
> > > 	CPU advances to next.
> > > 	At this point, the no-CBs setup is fully shut down.
> > > 9.	To softirq 4.  Transitioning code advances to next,
> > > 	which is the first, "In softirq".
> > > 	(This one -might- be unnecessary, but...)
> > > 
> > > All transitions are of course with the ->nocb_lock held.
> > > 
> > > When there is only one CPU during early boot near rcu_init() time,
> > > the transition from "In softirq" to "No-CB" can remain be instantaneous.
> > > 
> > > This has the advantage of not slowing things down just because there
> > > is an RCU callback flood in progress.  It also uses an explicit
> > > protocol that should (give or take bugs) maintain full safety both
> > > in protection of ->cblist and in dealing with RCU callback floods.
> > > 
> > > Thoughts?
> > 
> > Agreed. And I really like that it details the whole process in a very
> > explicit way.
> > 
> > Thanks!
> 
> Glad you like it!  And of course please adjust it as needed, up to and
> including doing something completely different that works better.  ;-)

Makes sense to me too, thanks!

 - Joel