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

[Breno Leitao <leitao@debian.org>]

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);
 

---
base-commit: 81e4f8d68c66da301bb881862735bd74c6241a19
change-id: 20250319-lockdep-b1eca0479665

Best regards,
-- 
Breno Leitao <leitao@debian.org>