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

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [day] [month] [year] [list]

Message-ID: <d2bff01d-33c6-4db3-847c-7bac514beb4e@paulmck-laptop>
Date: Thu, 25 Dec 2025 10:22:22 -0800
From: "Paul E. McKenney" <paulmck@...nel.org>
To: Theodore Tso <tytso@....edu>
Cc: Steven Rostedt <rostedt@...dmis.org>, Sasha Levin <sashal@...nel.org>,
	Julia Lawall <julia.lawall@...ia.fr>,
	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 Thu, Dec 25, 2025 at 10:03:31AM -0500, Theodore Tso wrote:
> On Wed, Dec 24, 2025 at 09:11:58AM -0500, Steven Rostedt wrote:
> > /*
> >  * Loop doing repeated quiescent-state forcing until the grace period ends.
> >  */
> > static noinline_for_stack void rcu_gp_fqs_loop(void)
> > {
> > 	bool first_gp_fqs = true;
> > 	int gf = 0;
> > 	unsigned long j;
> > 	int ret;
> > 	struct rcu_node *rnp = rcu_get_root();
> > 
> > 	j = READ_ONCE(jiffies_till_first_fqs);
> > 	if (rcu_state.cbovld)
> > 		gf = RCU_GP_FLAG_OVLD;
> > 	ret = 0;
> > 	for (;;) {
> > 		if (rcu_state.cbovld) {
> > 			j = (j + 2) / 3;
> > 			if (j <= 0)
> > 				j = 1;
> > 		}
> > 		if (!ret || time_before(jiffies + j, rcu_state.jiffies_force_qs)) {
> > 			WRITE_ONCE(rcu_state.jiffies_force_qs, jiffies + j);
> > 			/*
> > 			 * jiffies_force_qs before RCU_GP_WAIT_FQS state
> > 			 * update; required for stall checks.
> > 			 */
> > 			smp_wmb();
> > 			WRITE_ONCE(rcu_state.jiffies_kick_kthreads,
> > 				   jiffies + (j ? 3 * j : 2));
> > 		}
> > 
> > There's a bit of magical manipulation of "j" that I have no idea why it's
> > doing that. ;-)
> > 
> > I would love if Julia or Gabriele came up with some specification for that
> > function.
> 
> I think we need to separate out two different things.  The first is
> <<what>> the function is doing, and the other is <<how>> the function
> is doing it.  The first is the specification, or the interface
> contract.  The second is the implementation details that are important
> if you need to modify the function, or want to verify its corrections.

Good point, and thank you!

> I'm reminded of the ancient Unix documentation of how context
> switching was implemented on a PDP-11, "You are not expected to
> understand this".  But as long as you understood what it was doing,
> the fact the magic wasn't described in the source code might be OK.
> OTOH, it's what inspired the Lion's Commentary on the Unix Source
> Code book.   :-)

Also a good point, but one of my goals is to move RCU further out of "You
are not expected to understand this" state.  And in that spirit, I will
further celebrate this day by offering up a big "THANK YOU!" to those who
have dug into RCU's implementation, and most especially the Big Five who
have dug deeply enough into it to have sent RCU pull requests to Linus!

							Thanx, Paul