[PATCH net-next 1/2] lockdep: Introduce lockdep_unregister_key_nosync()

[Vlad Poenaru]

From: Bart Van Assche <bvanassche@xxxxxxx>

Add a variant of lockdep_unregister_key() that neither sleeps nor waits
for an RCU grace period. Instead the caller must make sure that a grace
period elapses after the call before the memory holding the key is
freed or reused.

That lets callers that already free the key's memory with call_rcu() or
kfree_rcu() avoid waiting for a second grace period. The qdisc code is
such a caller: all qdiscs except ingress, clsact and noqueue embed a
dynamic lockdep key and are freed with call_rcu(), and replacing an mq
root on a multi-queue NIC destroys one qdisc per TX queue under RTNL,
each currently waiting for its own expedited grace period.

Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: Will Deacon <will@xxxxxxxxxx>
Cc: Waiman Long <longman@xxxxxxxxxx>
Cc: Boqun Feng <boqun@xxxxxxxxxx>
Link: https://lore.kernel.org/all/20250203175939.3133477-2-bvanassche@xxxxxxx/
Signed-off-by: Bart Van Assche <bvanassche@xxxxxxx>
[vlad: rebase onto synchronize_rcu_expedited(), spell out the grace
 period requirement in the comments and changelog, describe the qdisc
 use case]
Signed-off-by: Vlad Poenaru <vlad.wing@xxxxxxxxx>
---
 include/linux/lockdep.h  |  5 +++++
 kernel/locking/lockdep.c | 36 +++++++++++++++++++++++++-----------
 2 files changed, 30 insertions(+), 11 deletions(-)

diff --git a/include/linux/lockdep.h b/include/linux/lockdep.h
index a6451ecbbe9a0..80b27973b7f72 100644
--- a/include/linux/lockdep.h
+++ b/include/linux/lockdep.h
@@ -117,6 +117,7 @@ do {							\
 } while (0)
 
 extern void lockdep_register_key(struct lock_class_key *key);
+extern void lockdep_unregister_key_nosync(struct lock_class_key *key);
 extern void lockdep_unregister_key(struct lock_class_key *key);
 
 /*
@@ -375,6 +376,10 @@ static inline void lockdep_register_key(struct lock_class_key *key)
 {
 }
 
+static inline void lockdep_unregister_key_nosync(struct lock_class_key *key)
+{
+}
+
 static inline void lockdep_unregister_key(struct lock_class_key *key)
 {
 }
diff --git a/kernel/locking/lockdep.c b/kernel/locking/lockdep.c
index c3dc84a7cef26..16c8480e282e3 100644
--- a/kernel/locking/lockdep.c
+++ b/kernel/locking/lockdep.c
@@ -6669,13 +6669,19 @@ void lockdep_reset_lock(struct lockdep_map *lock)
 }
 
 /*
- * Unregister a dynamically allocated key.
+ * Unregister a dynamically allocated key without waiting for RCU readers.
  *
  * Unlike lockdep_register_key(), a search is always done to find a matching
  * key irrespective of debug_locks to avoid potential invalid access to freed
  * memory in lock_class entry.
+ *
+ * is_dynamic_key() may still be walking past @key under RCU when this
+ * returns. The memory holding @key must not be freed or reused, including by
+ * registering @key again, until an RCU grace period that starts after this
+ * call has elapsed, e.g. by freeing it with call_rcu() or kfree_rcu() queued
+ * after this call. Does not sleep.
  */
-void lockdep_unregister_key(struct lock_class_key *key)
+void lockdep_unregister_key_nosync(struct lock_class_key *key)
 {
 	struct hlist_head *hash_head = keyhashentry(key);
 	struct lock_class_key *k;
@@ -6684,8 +6690,6 @@ void lockdep_unregister_key(struct lock_class_key *key)
 	bool found = false;
 	bool need_callback = false;
 
-	might_sleep();
-
 	if (WARN_ON_ONCE(static_obj(key)))
 		return;
 
@@ -6711,15 +6715,25 @@ void lockdep_unregister_key(struct lock_class_key *key)
 
 	if (need_callback)
 		call_rcu(&delayed_free.rcu_head, free_zapped_rcu);
+}
+EXPORT_SYMBOL_GPL(lockdep_unregister_key_nosync);
+
+/*
+ * Unregister a dynamically allocated key and wait for is_dynamic_key()
+ * readers, so that the memory holding @key may be freed as soon as this
+ * returns. May sleep.
+ */
+void lockdep_unregister_key(struct lock_class_key *key)
+{
+	might_sleep();
+
+	lockdep_unregister_key_nosync(key);
 
 	/*
-	 * 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.
+	 * Expedited to keep the wait short, at the cost of IPIs. Callers that
+	 * free @key after a grace period anyway should use
+	 * lockdep_unregister_key_nosync() instead.
+	 * TODO: Replace RCU with hazptr for this.
 	 */
 	synchronize_rcu_expedited();
 }
-- 
2.53.0-Meta