[PATCH 09/35] drm/gem: support BO freeing without dev->struct_mutex

[Chris Wilson]

On Tue, Apr 26, 2016 at 07:29:42PM +0200, Daniel Vetter wrote:
> Finally all the core gem and a lot of drivers are entirely free of
> dev->struct_mutex depencies, and we can start to have an entirely
> lockless unref path.
> 
> To make sure that no one who touches the core code accidentally breaks
> existing drivers which still require dev->struct_mutex I've made the
> might_lock check unconditional.
> 
> While at it de-inline the ref/unref functions, they've become a bit
> too big.
> 
> v2: Make it not leak like a sieve.
> 
> Signed-off-by: Daniel Vetter <daniel.vetter at intel.com>
> ---
>  drivers/gpu/drm/drm_gem.c | 64 ++++++++++++++++++++++++++++++++++++++---------
>  include/drm/drmP.h        | 12 ++++++++-
>  include/drm/drm_gem.h     | 45 ++-------------------------------
>  3 files changed, 65 insertions(+), 56 deletions(-)
> 
> diff --git a/drivers/gpu/drm/drm_gem.c b/drivers/gpu/drm/drm_gem.c
> index 25dac31eef37..8f2eff448bb5 100644
> --- a/drivers/gpu/drm/drm_gem.c
> +++ b/drivers/gpu/drm/drm_gem.c
> @@ -788,16 +788,7 @@ drm_gem_object_release(struct drm_gem_object *obj)
>  }
>  EXPORT_SYMBOL(drm_gem_object_release);
>  
> -/**
> - * drm_gem_object_free - free a GEM object
> - * @kref: kref of the object to free
> - *
> - * Called after the last reference to the object has been lost.
> - * Must be called holding struct_ mutex
> - *
> - * Frees the object
> - */
> -void
> +static void
>  drm_gem_object_free(struct kref *kref)
>  {
>  	struct drm_gem_object *obj =
> @@ -806,10 +797,59 @@ drm_gem_object_free(struct kref *kref)
>  
>  	WARN_ON(!mutex_is_locked(&dev->struct_mutex));
>  
> -	if (dev->driver->gem_free_object != NULL)
> +	if (dev->driver->gem_free_object_unlocked != NULL)
> +		dev->driver->gem_free_object_unlocked(obj);
> +	else if (dev->driver->gem_free_object != NULL)
>  		dev->driver->gem_free_object(obj);
>  }
> -EXPORT_SYMBOL(drm_gem_object_free);
> +
> +/**
> + * drm_gem_object_unreference_unlocked - release a GEM BO reference
> + * @obj: GEM buffer object
> + *
> + * This releases a reference to @obj. Callers must not hold the
> + * dev->struct_mutex lock when calling this function.
> + */
> +void
> +drm_gem_object_unreference_unlocked(struct drm_gem_object *obj)
> +{

I have i915.ko in a state where we could use this as well. I would like
to have our inlines back though. I would add

static inline void
i915_gem_object_put(struct drm_i915_gem_object *obj)
{
	kref_put(&obj->base.refcount, drm_gem_object_free);
}

Hmm, considering how simple that is, maybe I won't ask for

static inline void
__drm_gem_object_unreference_unlocked(struct drm_gem_object *obj)
{
	BUG_ON(obj->dev->driver->gem_free_object == NULL);
	kref_put(&obj->refcount, drm_gem_object_free);
}

after all!
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre