Re: [PATCH] lockdep: Speed up lockdep_unregister_key() with expedited RCU synchronization

[Peter Zijlstra <peterz@infradead.org>]

On Fri, Mar 21, 2025 at 02:30:49AM -0700, Breno Leitao wrote:
> lockdep_unregister_key() is called from critical code paths, including
> sections where rtnl_lock() is held. For example, when replacing a qdisc
> in a network device, network egress traffic is disabled while
> __qdisc_destroy() is called for every network queue.
> 
> If lockdep is enabled, __qdisc_destroy() calls lockdep_unregister_key(),
> which gets blocked waiting for synchronize_rcu() to complete.
> 
> For example, a simple tc command to replace a qdisc could take 13
> seconds:
> 
>   # time /usr/sbin/tc qdisc replace dev eth0 root handle 0x1: mq
>     real    0m13.195s
>     user    0m0.001s
>     sys     0m2.746s
> 
> During this time, network egress is completely frozen while waiting for
> RCU synchronization.
> 
> Use synchronize_rcu_expedited() instead to minimize the impact on
> critical operations like network connectivity changes.
> 
> This improves 10x the function call to tc, when replacing the qdisc for
> a network card.
> 
>    # time /usr/sbin/tc qdisc replace dev eth0 root handle 0x1: mq
>      real     0m1.789s
>      user     0m0.000s
>      sys      0m1.613s
> 
> Reported-by: Erik Lundgren <elundgren@meta.com>
> Signed-off-by: Breno Leitao <leitao@debian.org>
> Reviewed-by: "Paul E. McKenney" <paulmck@kernel.org>
> ---
>  kernel/locking/lockdep.c | 6 ++++--
>  1 file changed, 4 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/locking/lockdep.c b/kernel/locking/lockdep.c
> index 4470680f02269..a79030ac36dd4 100644
> --- a/kernel/locking/lockdep.c
> +++ b/kernel/locking/lockdep.c
> @@ -6595,8 +6595,10 @@ void lockdep_unregister_key(struct lock_class_key *key)
>  	if (need_callback)
>  		call_rcu(&delayed_free.rcu_head, free_zapped_rcu);
>  
> -	/* Wait until is_dynamic_key() has finished accessing k->hash_entry. */
> -	synchronize_rcu();
> +	/* Wait until is_dynamic_key() has finished accessing k->hash_entry.
> +	 * This needs to be quick, since it is called in critical sections
> +	 */
> +	synchronize_rcu_expedited();
>  }
>  EXPORT_SYMBOL_GPL(lockdep_unregister_key);

So I fundamentally despise synchronize_rcu_expedited(), also your
comment style is broken.

Why can't qdisc call this outside of the lock?