On 3/26/25 2:08 PM, Boqun Feng wrote:

Currently, when a lock class is allocated, nr_unused_locks will be increased by 1, until it gets used: nr_unused_locks will be decreased by 1 in mark_lock(). However, one scenario is missed: a lock class may be zapped without even being used once. This could result into a situation that nr_unused_locks != 0 but no unused lock class is active in the system, and when `cat /proc/lockdep_stats`, a WARN_ON() will be triggered in a CONFIG_DEBUG_LOCKDEP=y kernel:

[...] DEBUG_LOCKS_WARN_ON(debug_atomic_read(nr_unused_locks) != nr_unused) [...] WARNING: CPU: 41 PID: 1121 at kernel/locking/lockdep_proc.c:283 lockdep_stats_show+0xba9/0xbd0

And as a result, lockdep will be disabled after this.

Therefore, nr_unused_locks needs to be accounted correctly at zap_class() time.

Cc: stable@vger.kernel.org Signee-off-by: Boqun Feng boqun.feng@gmail.com

Typo: "Signee-off-by"?

Other than that, LGTM

Reviewed-by: Waiman Long longman@redhat.com

---

kernel/locking/lockdep.c | 3 +++ 1 file changed, 3 insertions(+)

diff --git a/kernel/locking/lockdep.c b/kernel/locking/lockdep.c index b15757e63626..686546d52337 100644 --- a/kernel/locking/lockdep.c +++ b/kernel/locking/lockdep.c @@ -6264,6 +6264,9 @@ static void zap_class(struct pending_free *pf, struct lock_class *class) hlist_del_rcu(&class->hash_entry); WRITE_ONCE(class->key, NULL); WRITE_ONCE(class->name, NULL);

• /* class allocated but not used, -1 in nr_unused_locks */
  

• if (class->usage_mask == 0)
  

• 	debug_atomic_dec(nr_unused_locks);
  

  nr_lock_classes--; __clear_bit(class - lock_classes, lock_classes_in_use); if (class - lock_classes == max_lock_class_idx)

* Boqun Feng boqun.feng@gmail.com wrote:

diff --git a/kernel/locking/lockdep.c b/kernel/locking/lockdep.c index b15757e63626..686546d52337 100644 --- a/kernel/locking/lockdep.c +++ b/kernel/locking/lockdep.c @@ -6264,6 +6264,9 @@ static void zap_class(struct pending_free *pf, struct lock_class *class) hlist_del_rcu(&class->hash_entry); WRITE_ONCE(class->key, NULL); WRITE_ONCE(class->name, NULL);

• /* class allocated but not used, -1 in nr_unused_locks */
  

• if (class->usage_mask == 0)
  

• 	debug_atomic_dec(nr_unused_locks);
  

Nit: capitalization in comments should follow the style of the surrounding code - ie. I did the change below.

Thanks,

Ingo

======================> kernel/locking/lockdep.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/locking/lockdep.c b/kernel/locking/lockdep.c index 686546d52337..58d78a33ac65 100644 --- a/kernel/locking/lockdep.c +++ b/kernel/locking/lockdep.c @@ -6264,7 +6264,7 @@ static void zap_class(struct pending_free *pf, struct lock_class *class) hlist_del_rcu(&class->hash_entry); WRITE_ONCE(class->key, NULL); WRITE_ONCE(class->name, NULL); - /* class allocated but not used, -1 in nr_unused_locks */ + /* Class allocated but not used, -1 in nr_unused_locks */ if (class->usage_mask == 0) debug_atomic_dec(nr_unused_locks); nr_lock_classes--;