netdev - Re: [PATCH net-next 6/7] lockdep: provide dummy forward declaration of *_is

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Date:   Thu, 17 Sep 2020 17:14:29 +0200
From:   peterz@...radead.org
To:     Jakub Kicinski <kuba@...nel.org>
Cc:     davem@...emloft.net, paulmck@...nel.org, joel@...lfernandes.org,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
        rcu@...r.kernel.org, josh@...htriplett.org,
        christian.brauner@...ntu.com, mingo@...hat.com, will@...nel.org
Subject: Re: [PATCH net-next 6/7] lockdep: provide dummy forward declaration
 of *_is_held() helpers

On Wed, Sep 16, 2020 at 11:45:27AM -0700, Jakub Kicinski wrote:
> When CONFIG_LOCKDEP is not set, lock_is_held() and lockdep_is_held()
> are not declared or defined. This forces all callers to use ifdefs
> around these checks.
> 
> Recent RCU changes added a lot of lockdep_is_held() calls inside
> rcu_dereference_protected(). rcu_dereference_protected() hides
> its argument on !LOCKDEP builds, but this may lead to unused variable
> warnings.
> 
> Provide forward declarations of lock_is_held() and lockdep_is_held()
> but never define them. This way callers can keep them visible to
> the compiler on !LOCKDEP builds and instead depend on dead code
> elimination to remove the references before the linker barfs.
> 
> We need lock_is_held() for RCU.
> 
> Signed-off-by: Jakub Kicinski <kuba@...nel.org>
> --
> CC: peterz@...radead.org
> CC: mingo@...hat.com
> CC: will@...nel.org
> ---
>  include/linux/lockdep.h | 6 ++++++
>  1 file changed, 6 insertions(+)
> 
> diff --git a/include/linux/lockdep.h b/include/linux/lockdep.h
> index 6a584b3e5c74..6b5bbc536bf6 100644
> --- a/include/linux/lockdep.h
> +++ b/include/linux/lockdep.h
> @@ -371,6 +371,12 @@ static inline void lockdep_unregister_key(struct lock_class_key *key)
>  
>  #define lockdep_depth(tsk)	(0)
>  
> +/*
> + * Dummy forward declarations, allow users to write less ifdef-y code
> + * and depend on dead code elimination.
> + */
> +int lock_is_held(const void *);

extern int lock_is_held(const struct lockdep_map *);

> +int lockdep_is_held(const void *);

extern

I suppose we can't pull the lockdep_is_held() definition out from under
CONFIG_LOCKDEP because it does the ->dep_map dereference and many types
will simply not have that member.

>  #define lockdep_is_held_type(l, r)		(1)
>  
>  #define lockdep_assert_held(l)			do { (void)(l); } while (0)