reservation.h: build error with lockdep disabled

reservation.h: build error with lockdep disabled

[Russell King - ARM Linux]

As of 3c3b177a9369 ("reservation: add suppport for read-only access
using rcu") linux/reservation.h uses lockdep macros:

+#define reservation_object_held(obj) lockdep_is_held(&(obj)->lock.base)

This results in build errors when lockdep is disabled as lockdep_is_held()
is only available when lockdep is enabled.  This has been reported today
to break the etnaviv kernel driver, which we're hoping to submit for 4.5.

As this gets used with rcu_dereference_protected(), eg:

static inline struct reservation_object_list *
reservation_object_get_list(struct reservation_object *obj)
{
        return rcu_dereference_protected(obj->fence,
                                         reservation_object_held(obj));
}

I'm guessing that it's not going to be a simple case of making it always
return true or always return false.

Any ideas how to solve this?

-- 
FTTC broadband for 0.8mile line: currently at 9.6Mbps down 400kbps up
according to speedtest.net.

Re: reservation.h: build error with lockdep disabled

[Paul E. McKenney]

On Thu, Nov 26, 2015 at 01:43:40PM +0000, Russell King - ARM Linux wrote:
> As of 3c3b177a9369 ("reservation: add suppport for read-only access
> using rcu") linux/reservation.h uses lockdep macros:
> 
> +#define reservation_object_held(obj) lockdep_is_held(&(obj)->lock.base)
> 
> This results in build errors when lockdep is disabled as lockdep_is_held()
> is only available when lockdep is enabled.  This has been reported today
> to break the etnaviv kernel driver, which we're hoping to submit for 4.5.
> 
> As this gets used with rcu_dereference_protected(), eg:
> 
> static inline struct reservation_object_list *
> reservation_object_get_list(struct reservation_object *obj)
> {
>         return rcu_dereference_protected(obj->fence,
>                                          reservation_object_held(obj));
> }
> 
> I'm guessing that it's not going to be a simple case of making it always
> return true or always return false.
> 
> Any ideas how to solve this?

The usual approach is something like this:

#ifdef CONFIG_PROVE_LOCKING
#define reservation_object_held(obj) lockdep_is_held(&(obj)->lock.base)
#else
#define reservation_object_held(obj) true
#endif

							Thanx, Paul