[RFC PATCH 8/8] locking/lockdep: Use shazptr to protect the key hashlist

Boqun Feng
2025-04-14T06:01:31Z

Erik Lundgren and Breno Leitao reported [1] a case where
lockdep_unregister_key() can be called from time critical code pathes
where rntl_lock() may be held. And the synchronize_rcu() in it can slow
down operations such as using tc to replace a qdisc in a network device.

In fact the synchronize_rcu() in lockdep_unregister_key() is to wait for
all is_dynamic_key() callers to finish so that removing a key from the
key hashlist, and we can use shazptr to protect the hashlist as well.

Compared to the proposed solution which replaces synchronize_rcu() with
synchronize_rcu_expedited(), using shazptr here can achieve the
same/better synchronization time without the need to send IPI. Hence use
shazptr here.

Reported-by: Erik Lundgren <elundgren@meta.com>
Reported-by: Breno Leitao <leitao@debian.org>
Link: https://lore.kernel.org/lkml/20250321-lockdep-v1-1-78b732d195fb@debian.org/
Signed-off-by: Boqun Feng <boqun.feng@gmail.com>
---
kernel/locking/lockdep.c | 11 ++++++++---
1 file changed, 8 insertions(+), 3 deletions(-)

diff --git a/kernel/locking/lockdep.c b/kernel/locking/lockdep.c
index 58d78a33ac65..c5781d2dc8c6 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/shazptr.h>

#include <asm/sections.h>

@@ -1265,14 +1266,18 @@ static bool is_dynamic_key(const struct lock_class_key *key)

hash_head = keyhashentry(key);

- rcu_read_lock();
+ /* Need preemption disable for using shazptr. */
+ guard(preempt)();
+
+ /* Protect the list search with shazptr. */
+ guard(shazptr)(hash_head);
+
hlist_for_each_entry_rcu(k, hash_head, hash_entry) {
if (k == key) {
found = true;
break;
}
}
- rcu_read_unlock();

return found;
}
@@ -6614,7 +6619,7 @@ void lockdep_unregister_key(struct lock_class_key *key)
call_rcu(&delayed_free.rcu_head, free_zapped_rcu);

/* Wait until is_dynamic_key() has finished accessing k->hash_entry. */
- synchronize_rcu();
+ synchronize_shazptr(keyhashentry(key));
}
EXPORT_SYMBOL_GPL(lockdep_unregister_key);

--
2.47.1