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 <elundg...@meta.com> Reported-by: Breno Leitao <lei...@debian.org> Link: https://lore.kernel.org/lkml/20250321-lockdep-v1-1-78b732d19...@debian.org/ Signed-off-by: Boqun Feng <boqun.f...@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
