linux-kernel - Re: [PATCH RFC] srcu: Yet more detail for srcu_readers_active_idx

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Message-ID: <20221215000433.GD4001@paulmck-ThinkPad-P17-Gen-1>
Date:   Wed, 14 Dec 2022 16:04:33 -0800
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Joel Fernandes <joel@...lfernandes.org>
Cc:     boqun.feng@...il.com, frederic@...nel.org, neeraj.iitr10@...il.com,
        urezki@...il.com, rcu@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH RFC] srcu: Yet more detail for
 srcu_readers_active_idx_check() comments

On Wed, Dec 14, 2022 at 11:14:48PM +0000, Joel Fernandes wrote:
> On Wed, Dec 14, 2022 at 11:10 PM Joel Fernandes <joel@...lfernandes.org> wrote:
> >
> > On Wed, Dec 14, 2022 at 11:07 PM Joel Fernandes <joel@...lfernandes.org> wrote:
> > >
> > > On Wed, Dec 14, 2022 at 9:24 PM Paul E. McKenney <paulmck@...nel.org> wrote:
> > > > > I also did not get why you care about readers that come and ago (you
> > > > > mentioned the first reader seeing incorrect idx and the second reader
> > > > > seeing the right flipped one, etc). Those readers are irrelevant
> > > > > AFAICS since they came and went, and need not be waited on , right?.
> > > >
> > > > The comment is attempting to show (among other things) that we don't
> > > > need to care about readers that come and go more than twice during that
> > > > critical interval of time during the counter scans.
> > >
> > > Why do we need to care about readers that come and go even once? Once
> > > they are gone, they have already done an unlock() and their RSCS is
> > > over, so they need to be considered AFAICS.
> > >
> >
> > Aargh, I meant: "so they need to be considered AFAICS".
> 
> Trying again: "so they need not be considered AFAICS".

Give or take counter wrap, which can make it appear that still-present
readers have finished.

> Anyway, my 1 year old son is sick so signing off for now. Thanks.

Ouch!  I hope he recovers quickly and completely!!!

							Thanx, Paul