Re: [PATCH] lockdep: constify usage of struct lock_class_key where possible

[Heiner Kallweit <hkallweit1@gmail.com>]

On 29.04.2026 19:04, Waiman Long wrote:
> On 4/28/26 5:05 PM, Heiner Kallweit wrote:
>> Constify usage of struct lock_class_key where possible. This requires
>> to constify also usage of struct lockdep_subclass_key in two places.
>>
>> Especially relevant is constification of lockdep_map.key, as it makes
>> clear that the key isn't changed during lifetime of struct lockdep_map.
> 
> Other than making clear that the lock_class_key isn't being modified, is there other benefit of making this change like helping compiler to generate better code?
> 
Doing an objdump -o kernel/locking/lockdep.o I see no difference in section
sizes. Hence at least with gcc it doesn't seem to affect code generation.
I didn't check whether it affects callers of e.g. lockdep_init_map_wait().

> Cheers,
> Longman
> 
>>
>> Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
>> ---
>>   include/linux/lockdep.h       | 18 ++++++++++--------
>>   include/linux/lockdep_types.h |  2 +-
>>   kernel/locking/lockdep.c      | 12 ++++++------
>>   3 files changed, 17 insertions(+), 15 deletions(-)
>>
>> diff --git a/include/linux/lockdep.h b/include/linux/lockdep.h
>> index 621566345..3fa2b62f2 100644
>> --- a/include/linux/lockdep.h
>> +++ b/include/linux/lockdep.h
>> @@ -125,25 +125,27 @@ extern void lockdep_unregister_key(struct lock_class_key *key);
>>    * to lockdep:
>>    */
>>   -extern void lockdep_init_map_type(struct lockdep_map *lock, const char *name,
>> -    struct lock_class_key *key, int subclass, u8 inner, u8 outer, u8 lock_type);
>> +void lockdep_init_map_type(struct lockdep_map *lock, const char *name,
>> +               const struct lock_class_key *key, int subclass,
>> +               u8 inner, u8 outer, u8 lock_type);
>>     static inline void
>>   lockdep_init_map_waits(struct lockdep_map *lock, const char *name,
>> -               struct lock_class_key *key, int subclass, u8 inner, u8 outer)
>> +               const struct lock_class_key *key, int subclass,
>> +               u8 inner, u8 outer)
>>   {
>>       lockdep_init_map_type(lock, name, key, subclass, inner, outer, LD_LOCK_NORMAL);
>>   }
>>     static inline void
>>   lockdep_init_map_wait(struct lockdep_map *lock, const char *name,
>> -              struct lock_class_key *key, int subclass, u8 inner)
>> +              const struct lock_class_key *key, int subclass, u8 inner)
>>   {
>>       lockdep_init_map_waits(lock, name, key, subclass, inner, LD_WAIT_INV);
>>   }
>>     static inline void lockdep_init_map(struct lockdep_map *lock, const char *name,
>> -                 struct lock_class_key *key, int subclass)
>> +                    const struct lock_class_key *key, int subclass)
>>   {
>>       lockdep_init_map_wait(lock, name, key, subclass, LD_WAIT_INV);
>>   }
>> @@ -252,9 +254,9 @@ static inline int lock_is_held(const struct lockdep_map *lock)
>>   #define lockdep_is_held(lock)        lock_is_held(&(lock)->dep_map)
>>   #define lockdep_is_held_type(lock, r)    lock_is_held_type(&(lock)->dep_map, (r))
>>   -extern void lock_set_class(struct lockdep_map *lock, const char *name,
>> -               struct lock_class_key *key, unsigned int subclass,
>> -               unsigned long ip);
>> +void lock_set_class(struct lockdep_map *lock, const char *name,
>> +            const struct lock_class_key *key, unsigned int subclass,
>> +            unsigned long ip);
>>     #define lock_set_novalidate_class(l, n, i) \
>>       lock_set_class(l, n, &__lockdep_no_validate__, 0, i)
>> diff --git a/include/linux/lockdep_types.h b/include/linux/lockdep_types.h
>> index eae115a26..0d3df9019 100644
>> --- a/include/linux/lockdep_types.h
>> +++ b/include/linux/lockdep_types.h
>> @@ -184,7 +184,7 @@ void clear_lock_stats(struct lock_class *class);
>>    * This is embedded into specific lock instances:
>>    */
>>   struct lockdep_map {
>> -    struct lock_class_key        *key;
>> +    const struct lock_class_key        *key;
>>       struct lock_class        *class_cache[NR_LOCKDEP_CACHING_CLASSES];
>>       const char            *name;
>>       u8                wait_type_outer; /* can be taken in this context */
>> diff --git a/kernel/locking/lockdep.c b/kernel/locking/lockdep.c
>> index f13883162..fe1415260 100644
>> --- a/kernel/locking/lockdep.c
>> +++ b/kernel/locking/lockdep.c
>> @@ -888,7 +888,7 @@ static int count_matching_names(struct lock_class *new_class)
>>   static noinstr struct lock_class *
>>   look_up_lock_class(const struct lockdep_map *lock, unsigned int subclass)
>>   {
>> -    struct lockdep_subclass_key *key;
>> +    const struct lockdep_subclass_key *key;
>>       struct hlist_head *hash_head;
>>       struct lock_class *class;
>>   @@ -1286,7 +1286,7 @@ static bool is_dynamic_key(const struct lock_class_key *key)
>>   static struct lock_class *
>>   register_lock_class(struct lockdep_map *lock, unsigned int subclass, int force)
>>   {
>> -    struct lockdep_subclass_key *key;
>> +    const struct lockdep_subclass_key *key;
>>       struct hlist_head *hash_head;
>>       struct lock_class *class;
>>       int idx;
>> @@ -4937,8 +4937,8 @@ static inline int check_wait_context(struct task_struct *curr,
>>    * Initialize a lock instance's lock-class mapping info:
>>    */
>>   void lockdep_init_map_type(struct lockdep_map *lock, const char *name,
>> -                struct lock_class_key *key, int subclass,
>> -                u8 inner, u8 outer, u8 lock_type)
>> +               const struct lock_class_key *key, int subclass,
>> +               u8 inner, u8 outer, u8 lock_type)
>>   {
>>       int i;
>>   @@ -5408,7 +5408,7 @@ static int reacquire_held_locks(struct task_struct *curr, unsigned int depth,
>>     static int
>>   __lock_set_class(struct lockdep_map *lock, const char *name,
>> -         struct lock_class_key *key, unsigned int subclass,
>> +         const struct lock_class_key *key, unsigned int subclass,
>>            unsigned long ip)
>>   {
>>       struct task_struct *curr = current;
>> @@ -5733,7 +5733,7 @@ static noinstr void check_flags(unsigned long flags)
>>   }
>>     void lock_set_class(struct lockdep_map *lock, const char *name,
>> -            struct lock_class_key *key, unsigned int subclass,
>> +            const struct lock_class_key *key, unsigned int subclass,
>>               unsigned long ip)
>>   {
>>       unsigned long flags;
>