linux-kernel - Re: [PATCH 8/8] locking/lockdep: Use shazptr to protect the key hashlist

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Message-ID: <i3mukc6vgwrp3cy5eis2inyms7f5b4a6pel4cvvdx6jlxrij2g@wgrnkstlifv3>
Date: Thu, 10 Jul 2025 07:06:09 -0700
From: Breno Leitao <leitao@...ian.org>
To: Boqun Feng <boqun.feng@...il.com>
Cc: linux-kernel@...r.kernel.org, rcu@...r.kernel.org, 
	lkmm@...ts.linux.dev, Peter Zijlstra <peterz@...radead.org>, 
	Ingo Molnar <mingo@...nel.org>, Will Deacon <will@...nel.org>, Waiman Long <longman@...hat.com>, 
	Davidlohr Bueso <dave@...olabs.net>, "Paul E. McKenney" <paulmck@...nel.org>, 
	Josh Triplett <josh@...htriplett.org>, Frederic Weisbecker <frederic@...nel.org>, 
	Neeraj Upadhyay <neeraj.upadhyay@...nel.org>, Joel Fernandes <joelagnelf@...dia.com>, 
	Uladzislau Rezki <urezki@...il.com>, Steven Rostedt <rostedt@...dmis.org>, 
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>, Lai Jiangshan <jiangshanlai@...il.com>, 
	Zqiang <qiang.zhang@...ux.dev>, aeh@...a.com, netdev@...r.kernel.org, edumazet@...gle.com, 
	jhs@...atatu.com, kernel-team@...a.com, Erik Lundgren <elundgren@...a.com>
Subject: Re: [PATCH 8/8] locking/lockdep: Use shazptr to protect the key
 hashlist

Hello Boqun,

On Tue, Jun 24, 2025 at 08:11:01PM -0700, Boqun Feng wrote:
> Erik Lundgren and Breno Leitao reported [1] a case where
> lockdep_unregister_key() can be called from time critical code pathes
> where rntl_lock() may be held. And the synchronize_rcu() in it can slow
> down operations such as using tc to replace a qdisc in a network device.
> 
> In fact the synchronize_rcu() in lockdep_unregister_key() is to wait for
> all is_dynamic_key() callers to finish so that removing a key from the
> key hashlist, and we can use shazptr to protect the hashlist as well.
> 
> Compared to the proposed solution which replaces synchronize_rcu() with
> synchronize_rcu_expedited(), using shazptr here can achieve the
> same/better synchronization time without the need to send IPI. Hence use
> shazptr here.
> 
> Reported-by: Erik Lundgren <elundgren@...a.com>
> Reported-by: Breno Leitao <leitao@...ian.org>
> Link: https://lore.kernel.org/all/20250321-lockdep-v1-1-78b732d195fb@debian.org/ [1]
> Signed-off-by: Boqun Feng <boqun.feng@...il.com>

First of all, thanks for working to fix the origianl issue. I've been
able to test this in my host, and the gain is impressive.

Before:

         # time /usr/sbin/tc qdisc replace dev eth0 root handle 0x1234: mq
         real    0m13.195s
         user    0m0.001s
         sys     0m2.746s
	
With your patch:

	#  time /usr/sbin/tc qdisc replace dev eth0 root handle 0x1234: mq
	real	0m0.135s
	user	0m0.002s
	sys	0m0.116s

	#  time /usr/sbin/tc qdisc replace dev eth0 root handle 0x1: mq
	real	0m0.127s
	user	0m0.001s
	sys	0m0.112s

Please add the following to the series:

Tested-by: Breno Leitao <leitao@...ian.org>