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

[Boqun Feng <boqun.feng@gmail.com>]

On Thu, Jul 10, 2025 at 07:06:09AM -0700, Breno Leitao wrote:
> 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@meta.com>
> > Reported-by: Breno Leitao <leitao@debian.org>
> > Link: https://lore.kernel.org/all/20250321-lockdep-v1-1-78b732d195fb@debian.org/ [1]
> > Signed-off-by: Boqun Feng <boqun.feng@gmail.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@debian.org>

Thanks! Will apply ;-)

Regards,
Boqun