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

[Alexei Starovoitov <alexei.starovoitov@gmail.com>]

On Mon, May 12, 2025 at 6:26 AM Sebastian Andrzej Siewior
<bigeasy@linutronix.de> wrote:
>
> On 2025-04-30 20:27:14 [-0700], Alexei Starovoitov wrote:
> > From: Alexei Starovoitov <ast@kernel.org>
> >
> > 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@kernel.org>
> > ---
> >  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

I don't like it. It obfuscates the layout.