linux-kernel - Re: [PATCH v5 06/18] rcu: Introduce call_rcu

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Date:   Wed, 7 Sep 2022 12:03:49 +0200
From:   Frederic Weisbecker <frederic@...nel.org>
To:     Joel Fernandes <joel@...lfernandes.org>
Cc:     rcu@...r.kernel.org, linux-kernel@...r.kernel.org,
        rushikesh.s.kadam@...el.com, urezki@...il.com,
        neeraj.iitr10@...il.com, paulmck@...nel.org, rostedt@...dmis.org,
        vineeth@...byteword.org, boqun.feng@...il.com
Subject: Re: [PATCH v5 06/18] rcu: Introduce call_rcu_lazy() API
 implementation

On Tue, Sep 06, 2022 at 12:15:19PM -0400, Joel Fernandes wrote:
> >> +
> >> +	// We had CBs in the bypass list before. There is nothing else to do if:
> >> +	// There were only non-lazy CBs before, in this case, the bypass timer
> > 
> > Kind of misleading. I would replace "There were only non-lazy CBs before" with
> > "There was at least one non-lazy CBs before".
> 
> I really mean "There were only non-lazy CBs ever queued in the bypass list
> before". That's the bypass_is_lazy variable. So I did not fully understand your
> suggested comment change.

I may well be missing something but to me it seems that:

bypass_is_lazy = all bypass callbacks are lazy
!bypass_is_lazy = there is at least one non-lazy bypass callback

And indeed as long as there is at least one non-lazy callback, we don't
want to rely on the LAZY timer.

Am I overlooking something?

> 
> >> +	// or GP-thread will handle the CBs including any new lazy ones.
> >> +	// Or, the new CB is lazy and the old bypass-CBs were also lazy. In this
> >> +	// case the old lazy timer would have been setup. When that expires,
> >> +	// the new lazy one will be handled.
> >> +	if (ncbs && (!bypass_is_lazy || lazy)) {
> >>  		local_irq_restore(flags);
> >>  	} else {
> >>  		// No-CBs GP kthread might be indefinitely asleep, if so, wake.