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

[Eric Dumazet]

On Fri, Mar 21, 2025 at 10:31 AM Breno Leitao <leitao@xxxxxxxxxx> 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.

Running 'critical operations' with LOCKDEP enabled kernels seems a bit
strange :)

Reviewed-by: Eric Dumazet <edumazet@xxxxxxxxxx>