* Re: [PATCH] locking: lockdep: Decrease nr_unused_locks if lock unused in zap_class()
2025-03-26 18:08 [PATCH] locking: lockdep: Decrease nr_unused_locks if lock unused in zap_class() Boqun Feng
@ 2025-03-26 18:26 ` Waiman Long
2025-03-26 18:31 ` Boqun Feng
2025-03-27 7:24 ` Ingo Molnar
2025-03-27 7:29 ` [tip: locking/urgent] locking/lockdep: " tip-bot2 for Boqun Feng
2 siblings, 1 reply; 5+ messages in thread
From: Waiman Long @ 2025-03-26 18:26 UTC (permalink / raw)
To: Boqun Feng, Ingo Molnar, Peter Zijlstra
Cc: Will Deacon, linux-kernel, stable, Ingo Molnar
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)
^ permalink raw reply [flat|nested] 5+ messages in thread
* Re: [PATCH] locking: lockdep: Decrease nr_unused_locks if lock unused in zap_class()
2025-03-26 18:26 ` Waiman Long
@ 2025-03-26 18:31 ` Boqun Feng
0 siblings, 0 replies; 5+ messages in thread
From: Boqun Feng @ 2025-03-26 18:31 UTC (permalink / raw)
To: Waiman Long
Cc: Ingo Molnar, Peter Zijlstra, Will Deacon, linux-kernel, stable,
Ingo Molnar
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)
>
^ permalink raw reply [flat|nested] 5+ messages in thread
* Re: [PATCH] locking: lockdep: Decrease nr_unused_locks if lock unused in zap_class()
2025-03-26 18:08 [PATCH] locking: lockdep: Decrease nr_unused_locks if lock unused in zap_class() Boqun Feng
2025-03-26 18:26 ` Waiman Long
@ 2025-03-27 7:24 ` Ingo Molnar
2025-03-27 7:29 ` [tip: locking/urgent] locking/lockdep: " tip-bot2 for Boqun Feng
2 siblings, 0 replies; 5+ messages in thread
From: Ingo Molnar @ 2025-03-27 7:24 UTC (permalink / raw)
To: Boqun Feng
Cc: Peter Zijlstra, Will Deacon, Waiman Long, linux-kernel, stable,
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--;
^ permalink raw reply related [flat|nested] 5+ messages in thread
* [tip: locking/urgent] locking/lockdep: Decrease nr_unused_locks if lock unused in zap_class()
2025-03-26 18:08 [PATCH] locking: lockdep: Decrease nr_unused_locks if lock unused in zap_class() Boqun Feng
2025-03-26 18:26 ` Waiman Long
2025-03-27 7:24 ` Ingo Molnar
@ 2025-03-27 7:29 ` tip-bot2 for Boqun Feng
2 siblings, 0 replies; 5+ messages in thread
From: tip-bot2 for Boqun Feng @ 2025-03-27 7:29 UTC (permalink / raw)
To: linux-tip-commits
Cc: Boqun Feng, Ingo Molnar, Waiman Long, stable, x86, linux-kernel
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)
^ permalink raw reply related [flat|nested] 5+ messages in thread