[tip: locking/core] lockdep: Speed up lockdep_unregister_key() with expedited RCU synchronization

tip-bot2 for Breno Leitao
2025-07-19T17:40:17Z

The following commit has been merged into the locking/core branch of tip:

Commit-ID: 7a3cedafccf8e7d038ad4cfec5b38052647ceac5
Gitweb: https://git.kernel.org/tip/7a3cedafccf8e7d038ad4cfec5b38052647ceac5
Author: Breno Leitao <leitao@debian.org>
AuthorDate: Fri, 21 Mar 2025 02:30:49 -07:00
Committer: Boqun Feng <boqun.feng@gmail.com>
CommitterDate: Mon, 14 Jul 2025 21:57:29 -07:00

lockdep: Speed up lockdep_unregister_key() with expedited RCU synchronization

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

[boqun: Fixed the comment and add more information for the temporary
workaround, and add TODO information for hazptr]

Reported-by: Erik Lundgren <elundgren@meta.com>
Signed-off-by: Breno Leitao <leitao@debian.org>
Reviewed-by: Paul E. McKenney <paulmck@kernel.org>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Boqun Feng <boqun.feng@gmail.com>
Link: https://lore.kernel.org/r/20250321-lockdep-v1-1-78b732d195fb@debian.org
---
kernel/locking/lockdep.c | 12 ++++++++++--
1 file changed, 10 insertions(+), 2 deletions(-)

diff --git a/kernel/locking/lockdep.c b/kernel/locking/lockdep.c
index 0c94141..2d4c5ba 100644
--- a/kernel/locking/lockdep.c
+++ b/kernel/locking/lockdep.c
@@ -6616,8 +6616,16 @@ 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.
+ *
+ * Some operations like __qdisc_destroy() will call this in a debug
+ * kernel, and the network traffic is disabled while waiting, hence
+ * the delay of the wait matters in debugging cases. Currently use a
+ * synchronize_rcu_expedited() to speed up the wait at the cost of
+ * system IPIs. TODO: Replace RCU with hazptr for this.
+ */
+ synchronize_rcu_expedited();
}
EXPORT_SYMBOL_GPL(lockdep_unregister_key);