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

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Message-ID: <b106edc3-2557-0669-86bf-10c08739a86a@joelfernandes.org>
Date:   Wed, 7 Sep 2022 10:01:16 -0400
From:   Joel Fernandes <joel@...lfernandes.org>
To:     Frederic Weisbecker <frederic@...nel.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 9/7/2022 6:03 AM, Frederic Weisbecker wrote:
> 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?

No you are not over looking and you are very right that I may need to change the
comment.

To clarify my intent, a wake up or timer adjustment needs to be done only if:

1. Bypass list was fully empty before (this is the first bypass list entry).

Or both the below conditions are met:

1. Bypass list had only lazy CBs before.

2. The new CB is non-lazy.

Instead of saying, "nothing needs to be done if...", I will change it to:

        // A wake up of the grace period kthread or timer adjustment needs to
        // be done only if:
        // 1. Bypass list was empty before (this is the first bypass queue).
        //      Or, both the below conditions are met:
        // 1. Bypass list had only lazy CBs before.
        // 2. The new CB is non-lazy.

That sounds less confusing...

Or, I can just make the edit you suggested... let me know either way!

Thanks!

 - Joel