locking/lockdep: Decrease nr_unused_locks if lock unused in zap_class()

commit 495f53d5cca0f939eaed9dca90b67e7e6fb0e30c upstream.

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.

Signed-off-by: Boqun Feng <boqun.feng@gmail.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Reviewed-by: Waiman Long <longman@redhat.com>
Cc: stable@vger.kernel.org
Link: https://lore.kernel.org/r/20250326180831.510348-1-boqun.feng@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/kernel/locking/lockdep.c b/kernel/locking/lockdep.c
index 3468d8230e5f75638fb74e8ba89a3841fbefba95..9419a79e8833ff418e43045eae6683cd4acf2438 100644 (file)
--- a/kernel/locking/lockdep.c
+++ b/kernel/locking/lockdep.c
@@ -6141,6 +6141,9 @@ 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)