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

[Boqun Feng]

On Wed, Mar 26, 2025 at 02:26:53PM -0400, Waiman Long wrote:
> 
> 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@xxxxxxxxxxxxxxx
> > Signee-off-by: Boqun Feng <boqun.feng@xxxxxxxxx>
> 
> Typo: "Signee-off-by"?
> 

Oops, yeah.

> Other than that, LGTM
> 
> Reviewed-by: Waiman Long <longman@xxxxxxxxxx>
> 

Thanks!

Regards,
Boqun

> > ---
> >   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)
>