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fbqIngo Molnar
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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
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kernel/locking/lockdep.c

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@@ -6264,6 +6264,9 @@ static void zap_class(struct pending_free *pf, struct lock_class *class)
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hlist_del_rcu(&class->hash_entry);
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WRITE_ONCE(class->key, NULL);
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WRITE_ONCE(class->name, NULL);
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/* Class allocated but not used, -1 in nr_unused_locks */
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if (class->usage_mask == 0)
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debug_atomic_dec(nr_unused_locks);
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nr_lock_classes--;
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__clear_bit(class - lock_classes, lock_classes_in_use);
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if (class - lock_classes == max_lock_class_idx)

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