linux-kernel - Re: [PATCH 3/3] rcu/exp: Remove needless CPU up quiescent state report

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

Message-ID: <be5188d7-0ae3-4f08-b6e9-759f45e45013@paulmck-laptop>
Date: Tue, 25 Feb 2025 16:00:52 -0800
From: "Paul E. McKenney" <paulmck@...nel.org>
To: Frederic Weisbecker <frederic@...nel.org>
Cc: LKML <linux-kernel@...r.kernel.org>, Boqun Feng <boqun.feng@...il.com>,
	Joel Fernandes <joel@...lfernandes.org>,
	Neeraj Upadhyay <neeraj.upadhyay@....com>,
	Uladzislau Rezki <urezki@...il.com>,
	Zqiang <qiang.zhang1211@...il.com>, rcu <rcu@...r.kernel.org>
Subject: Re: [PATCH 3/3] rcu/exp: Remove needless CPU up quiescent state
 report

On Fri, Feb 21, 2025 at 04:52:39PM +0100, Frederic Weisbecker wrote:
> Le Wed, Feb 19, 2025 at 06:58:36AM -0800, Paul E. McKenney a écrit :
> > On Sat, Feb 15, 2025 at 11:23:45PM +0100, Frederic Weisbecker wrote:
> > > > Before.  There was also some buggy debug code in play.  Also, to get the
> > > > failure, it was necessary to make TREE03 disable preemption, as stock
> > > > TREE03 has an empty sync_sched_exp_online_cleanup() function.
> > > > 
> > > > I am rerunning the test with a WARN_ON_ONCE() after the early exit from
> > > > the sync_sched_exp_online_cleanup().  Of course, lack of a failure does
> > > > not necessairly indicate
> > > 
> > > Cool, thanks!
> > 
> > No failures.  But might it be wise to put this WARN_ON_ONCE() in,
> > let things go for a year or two, and complete the removal if it never
> > triggers?  Or is the lack of forward progress warning enough?
> 
> Hmm, what prevents a WARN_ON_ONCE() after the early exit of
> sync_sched_exp_online_cleanup() to hit?
> 
> All it takes is for sync_sched_exp_online_cleanup() to execute between
> sync_exp_reset_tree() and  __sync_rcu_exp_select_node_cpus() manage
> to send an IPI.

You are right, that would do it!

> But we can warn about the lack of forward progress after a few iterations
> of the retry_ipi label in __sync_rcu_exp_select_node_cpus().

Agreed, that would make more sense.

> > > > > And if after do we know why?
> > > > 
> > > > Here are some (possibly bogus) possibilities that came to mind:
> > > > 
> > > > 1.	There is some coming-online race that deprives the incoming
> > > > 	CPU of an IPI, but nevertheless marks that CPU as blocking the
> > > > 	current grace period.
> > > 
> > > Arguably there is a tiny window between rcutree_report_cpu_starting()
> > > and set_cpu_online() that could make ->qsmaskinitnext visible before
> > > cpu_online() and therefore delay the IPI a bit. But I don't expect
> > > more than a jiffy to fill up the gap. And if that's relevant, note that
> > > only !PREEMPT_RCU is then "fixed" by sync_sched_exp_online_cleanup() here.
> > 
> > Agreed.  And I vaguely recall that there was some difference due to
> > preemptible RCU's ability to clean up at the next rcu_read_unlock(),
> > though more recently, possibly deferred.
> 
> Perhaps at the time but today at least I can't find any.

And maybe not even back then.  ;-)

							Thanx, Paul