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

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Date:   Wed, 10 Jun 2020 15:12:39 +0200
From:   Frederic Weisbecker <frederic@...nel.org>
To:     "Paul E. McKenney" <paulmck@...nel.org>
Cc:     Joel Fernandes <joel@...lfernandes.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 Tue, Jun 09, 2020 at 11:02:27AM -0700, Paul E. McKenney wrote:
> > > > And anyway we still want to unconditionally lock on many places,
> > > > regardless of the offloaded state. I don't know how we could have
> > > > a magic helper do the unconditional lock on some places and the
> > > > conditional on others.
> > > 
> > > I was assuming (perhaps incorrectly) that an intermediate phase between
> > > not-offloaded and offloaded would take care of all of those cases.
> > 
> > Perhaps partly but I fear that won't be enough.
> 
> One approach is to rely on RCU read-side critical sections surrounding
> the lock acquisition and to stay in the intermediate phase until a grace
> period completes, preferably call_rcu() instead of synchronize_rcu().
> 
> This of course means refusing to do a transition if the CPU is still
> in the intermediate state from a prior transition.

That sounds good. But using synchronize_rcu() would be far easier. We
need to keep the hotplug and rcu barrier locked during the transition.

> > Also I've been thinking that rcu_nocb_lock() should meet any of these
> > requirements:
> > 
> > * hotplug is locked
> > * rcu barrier is locked
> > * rnp is locked
> > 
> > Because checking the offloaded state (when nocb isn't locked yet) of
> > an rdp without any of the above locks held is racy. And that should
> > be easy to check and prevent from copy-pasta accidents.
> > 
> > What do you think?
> 
> An RCU read-side critical section might be simpler.

Ok I think I can manage that.

Thanks.