Re: [PATCH 2/6] locking/local_lock: Expose dep_map in local_trylock_t.

[Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx>]

On 2025-04-30 20:27:14 [-0700], Alexei Starovoitov wrote:
> From: Alexei Starovoitov <ast@xxxxxxxxxx>
> 
> lockdep_is_held() macro assumes that "struct lockdep_map dep_map;"
> is a top level field of any lock that participates in LOCKDEP.
> Make it so for local_trylock_t.
> 
> Signed-off-by: Alexei Starovoitov <ast@xxxxxxxxxx>
> ---
>  include/linux/local_lock_internal.h | 9 ++++++---
>  1 file changed, 6 insertions(+), 3 deletions(-)
> 
> diff --git a/include/linux/local_lock_internal.h b/include/linux/local_lock_internal.h
> index bf2bf40d7b18..29df45f95843 100644
> --- a/include/linux/local_lock_internal.h
> +++ b/include/linux/local_lock_internal.h
> @@ -17,7 +17,10 @@ typedef struct {
>  
>  /* local_trylock() and local_trylock_irqsave() only work with local_trylock_t */
>  typedef struct {
> -	local_lock_t	llock;
> +#ifdef CONFIG_DEBUG_LOCK_ALLOC
> +	struct lockdep_map	dep_map;
> +	struct task_struct	*owner;
> +#endif
>  	u8		acquired;
>  } local_trylock_t;

So this trick should make it work. I am not sure it is worth it. It
would avoid the cast down the road…

diff --git a/include/linux/local_lock_internal.h b/include/linux/local_lock_internal.h
--- a/include/linux/local_lock_internal.h
+++ b/include/linux/local_lock_internal.h
@@ -17,10 +17,17 @@ typedef struct {
 
 /* local_trylock() and local_trylock_irqsave() only work with local_trylock_t */
 typedef struct {
+	union	{
+		local_lock_t		llock;
 #ifdef CONFIG_DEBUG_LOCK_ALLOC
-	struct lockdep_map	dep_map;
-	struct task_struct	*owner;
+# define LOCK_PAD_SIZE (offsetof(local_lock_t, dep_map))
+		struct {
+			u8 __padding[LOCK_PAD_SIZE];
+			struct lockdep_map	dep_map;
+		};
+#undef LOCK_PAD_SIZE
 #endif
+	};
 	u8		acquired;
 } local_trylock_t;
 
@@ -34,7 +41,7 @@ typedef struct {
 	.owner = NULL,
 
 # define LOCAL_TRYLOCK_DEBUG_INIT(lockname)		\
-	LOCAL_LOCK_DEBUG_INIT(lockname)
+	.llock	= { LOCAL_LOCK_DEBUG_INIT(llock.lockname) }
 
 static inline void local_lock_acquire(local_lock_t *l)
 {


Sebastian