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

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 <[e-mail]>
Signed-off-by: Ingo Molnar <[e-mail]>
Reviewed-by: Waiman Long <[e-mail]>
Cc: [e-mail]
Link: https://lore.kernel.org/r/[e-mail]

kernel/locking/lockdep.c

···
 		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)