[PATCH RFC v2 08/15] locking/lockdep: use hazptr to wait for dynamic key lookups

[Kunwu Chan]

lockdep_unregister_key() waits for is_dynamic_key() callers with
synchronize_rcu_expedited(), which requires system-wide coordination.
Make is_dynamic_key() protect the hash bucket with a hazard pointer
and use hazptr_synchronize() to wait for traversals of that bucket.

The hash bucket returned by keyhashentry() has a stable address, so
it can be used as the hazptr synchronization target.  Keep the key
hashlist lifetime RCU-based with hlist_del_rcu() and call_rcu().

Protect the traversal with the hazptr scoped_guard() added earlier
in this series.

This adapts the lockdep use case from Boqun Feng's shazptr series
to the current hazptr API.

Link: https://lore.kernel.org/lkml/20250625031101.12555-1-boqun.feng@xxxxxxxxx/
Signed-off-by: Kunwu Chan <kunwu.chan@xxxxxxxxx>
---
 kernel/locking/lockdep.c | 25 +++++++++++++++----------
 1 file changed, 15 insertions(+), 10 deletions(-)

diff --git a/kernel/locking/lockdep.c b/kernel/locking/lockdep.c
index c56a7f91d72e..d1ce5ace7fbf 100644
--- a/kernel/locking/lockdep.c
+++ b/kernel/locking/lockdep.c
@@ -58,6 +58,7 @@
 #include <linux/context_tracking.h>
 #include <linux/console.h>
 #include <linux/kasan.h>
+#include <linux/hazptr.h>
 
 #include <asm/sections.h>
 
@@ -1280,14 +1281,19 @@ static bool is_dynamic_key(const struct lock_class_key *key)
 
 	hash_head = keyhashentry(key);
 
-	rcu_read_lock();
-	hlist_for_each_entry_rcu(k, hash_head, hash_entry) {
-		if (k == key) {
-			found = true;
-			break;
+	{
+		struct hazptr_ctx ctx;
+		void *bucket = hash_head;
+
+		scoped_guard(hazptr, &ctx, &bucket) {
+			hlist_for_each_entry_rcu(k, hash_head, hash_entry, 1) {
+				if (k == key) {
+					found = true;
+					break;
+				}
+			}
 		}
 	}
-	rcu_read_unlock();
 
 	return found;
 }
@@ -6683,11 +6689,10 @@ void lockdep_unregister_key(struct lock_class_key *key)
 	 *
 	 * 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.
+	 * the delay of the wait matters in debugging cases. Replace the
+	 * expedited RCU wait with hazptr_synchronize().
 	 */
-	synchronize_rcu_expedited();
+	hazptr_synchronize(keyhashentry(key));
 }
 EXPORT_SYMBOL_GPL(lockdep_unregister_key);
 
-- 
2.43.0