linux-kernel - [tip: locking/core] lockdep: Speed up lockdep_unregister

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Message-ID: <175294680733.1420.1210512165865622668.tip-bot2@tip-bot2>
Date: Sat, 19 Jul 2025 17:40:07 -0000
From: "tip-bot2 for Breno Leitao" <tip-bot2@...utronix.de>
To: linux-tip-commits@...r.kernel.org
Cc: Erik Lundgren <elundgren@...a.com>, Breno Leitao <leitao@...ian.org>,
 "Paul E. McKenney" <paulmck@...nel.org>, Eric Dumazet <edumazet@...gle.com>,
 Boqun Feng <boqun.feng@...il.com>, x86@...nel.org,
 linux-kernel@...r.kernel.org
Subject: [tip: locking/core] lockdep: Speed up lockdep_unregister_key() with
 expedited RCU synchronization

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@...ian.org>
AuthorDate:    Fri, 21 Mar 2025 02:30:49 -07:00
Committer:     Boqun Feng <boqun.feng@...il.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@...a.com>
Signed-off-by: Breno Leitao <leitao@...ian.org>
Reviewed-by: Paul E. McKenney <paulmck@...nel.org>
Reviewed-by: Eric Dumazet <edumazet@...gle.com>
Signed-off-by: Boqun Feng <boqun.feng@...il.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);