linux-kernel - Re: Follow-up on Linux-kernel code accessibility

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Message-ID: <20251229104005.18b25def@gandalf.local.home>
Date: Mon, 29 Dec 2025 10:40:05 -0500
From: Steven Rostedt <rostedt@...dmis.org>
To: Julia Lawall <julia.lawall@...ia.fr>
Cc: "Paul E. McKenney" <paulmck@...nel.org>, "Dr. David Alan Gilbert"
 <dave@...blig.org>, Theodore Tso <tytso@....edu>, Sasha Levin
 <sashal@...nel.org>, Gabriele Paoloni <gpaoloni@...hat.com>, Kate Stewart
 <kstewart@...uxfoundation.org>, Chuck Wolber <chuckwolber@...il.com>,
 Dmitry Vyukov <dvyukov@...gle.com>, Mark Rutland <mark.rutland@....com>,
 Thomas Gleixner <tglx@...utronix.de>, Lorenzo Stoakes
 <lorenzo.stoakes@...cle.com>, Shuah Khan <skhan@...uxfoundation.org>, Chris
 Mason <clm@...a.com>, linux-kernel@...r.kernel.org
Subject: Re: Follow-up on Linux-kernel code accessibility

On Sun, 28 Dec 2025 10:36:39 +0100 (CET)
Julia Lawall <julia.lawall@...ia.fr> wrote:

> > > > > j = (j + 2) / 3;  
> > > >
> > > > "Divide by three rounding up."  

That's as useful as:

	/* Add one to X */
	x++;

> > >
> > > That's not *that* obvious, but ok, but then why 3?
> > >

Bingo! You win a cigar! :-)

I know that was a round up (and yes, as David pointed out, we have macros
for that too). The question is why are you dividing it by 3? I don't see
anything in that function which suggests the reason for needing to divide j
by 3.

If the comments you were adding in the past was things like "Divide by
three rounding up" then yeah, I can see why people would say you have too
many comments. The comments are not to be discussing what is being done,
but why is it being done.

			WRITE_ONCE(rcu_state.jiffies_kick_kthreads,
				   jiffies + (j ? 3 * j : 2));

Why the: (j ? 3 * j : 2) ?

Why is 3 so magical?

			/*
			 * j is the Father, Son and Holy Ghost.
			 * But only one may be active at a time.
			 * They each take a third. Father is first,
			 * Son is second, and Holy Ghost is third.
			 */
			j = (j + 2) / 3;

			/*
			 * j may not be zero, as that would lead to
			 * damnation. 
			 */
			if (j <= 0)
				j = 1;

-- Steve