5.4-stable review patch. If anyone has any objections, please let me know.
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From: Boqun Feng boqun.feng@gmail.com
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 --- kernel/locking/lockdep.c | 3 +++ 1 file changed, 3 insertions(+)
--- a/kernel/locking/lockdep.c +++ b/kernel/locking/lockdep.c @@ -5011,6 +5011,9 @@ static void zap_class(struct pending_fre 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); } else {