[PATCH] locking: lockdep: Decrease nr_unused_locks if lock unused in zap_class()

[Boqun Feng <boqun.feng@gmail.com>]

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>
---
 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)
-- 
2.47.1