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 <[email protected]>
> Reported-by: Breno Leitao <[email protected]>
> Link: https://lore.kernel.org/all/[email protected]/ [1]
> Signed-off-by: Boqun Feng <[email protected]>

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 <[email protected]>