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

[Waiman Long <longman@redhat.com>]

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?

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;