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

Paul E. McKenney
2025-07-11T01:04:01Z

On Sun, Apr 13, 2025 at 11:00:55PM -0700, Boqun Feng wrote:
> 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>

From an RCU and shazptr viewpoint:

Reviewed-by: Paul E. McKenney <paulmck@kernel.org>

> ---
> 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
>