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

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

[Boqun Feng]

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

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

[Waiman Long]

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@vger.kernel.org
> Signee-off-by: Boqun Feng <boqun.feng@gmail.com>

Typo: "Signee-off-by"?

Other than that, LGTM

Reviewed-by: Waiman Long <longman@redhat.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)

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@vger.kernel.org
> > Signee-off-by: Boqun Feng <boqun.feng@gmail.com>
> 
> Typo: "Signee-off-by"?
> 

Oops, yeah.

> Other than that, LGTM
> 
> Reviewed-by: Waiman Long <longman@redhat.com>
> 

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)
> 

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

[Ingo Molnar]

* Boqun Feng <boqun.feng@gmail.com> wrote:

> 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);

Nit: capitalization in comments should follow the style of the 
surrounding code - ie. I did the change below.

Thanks,

	Ingo

======================>
 kernel/locking/lockdep.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/locking/lockdep.c b/kernel/locking/lockdep.c
index 686546d52337..58d78a33ac65 100644
--- a/kernel/locking/lockdep.c
+++ b/kernel/locking/lockdep.c
@@ -6264,7 +6264,7 @@ 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 */
+		/* Class allocated but not used, -1 in nr_unused_locks */
 		if (class->usage_mask == 0)
 			debug_atomic_dec(nr_unused_locks);
 		nr_lock_classes--;

[tip: locking/urgent] locking/lockdep: Decrease nr_unused_locks if lock unused in zap_class()

[tip-bot2 for Boqun Feng]

The following commit has been merged into the locking/urgent branch of tip:

Commit-ID:     495f53d5cca0f939eaed9dca90b67e7e6fb0e30c
Gitweb:        https://git.kernel.org/tip/495f53d5cca0f939eaed9dca90b67e7e6fb0e30c
Author:        Boqun Feng <boqun.feng@gmail.com>
AuthorDate:    Wed, 26 Mar 2025 11:08:30 -07:00
Committer:     Ingo Molnar <mingo@kernel.org>
CommitterDate: Thu, 27 Mar 2025 08:23:17 +01:00

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 <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
---
 kernel/locking/lockdep.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/kernel/locking/lockdep.c b/kernel/locking/lockdep.c
index b15757e..58d78a3 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)