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

[Lucas Stach <l.stach@pengutronix.de>]

Am Dienstag, den 26.04.2016, 19:29 +0200 schrieb Daniel Vetter:
> 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@intel.com>

Two mostly cosmetic comments below, otherwise looks good:

Reviewed-by: Lucas Stach <l.stach@pengutronix.de>

> ---
>  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)

Why those explicit != NULL checks? The style mostly used in this file is
to omit those, like "if (dev->driver->gem_free_object_unlocked)". Same
comment applies to several hunks of this patch.

> +		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)
> +{
> +	struct drm_device *dev;
> +
> +	if (!obj)
> +		return;
> +
> +	dev = obj->dev;
> +	might_lock(&dev->struct_mutex);
> +
> +	if (dev->driver->gem_free_object != NULL)
> +		kref_put(&obj->refcount, drm_gem_object_free);
> +	else if (kref_put_mutex(&obj->refcount, drm_gem_object_free,
> +				&dev->struct_mutex))
> +		mutex_unlock(&dev->struct_mutex);
> +}
> +EXPORT_SYMBOL(drm_gem_object_unreference_unlocked);
> +
> +/**
> + * drm_gem_object_unreference - release a GEM BO reference
> + * @obj: GEM buffer object
> + *
> + * This releases a reference to @obj. Callers must hold the dev->struct_mutex
> + * lock when calling this function, even when the driver doesn't use
> + * dev->struct_mutex for anything.
> + *
> + * For drivers not encumbered with legacy locking use
> + * drm_gem_object_unreference_unlocked() instead.
> + */
> +void
> +drm_gem_object_unreference(struct drm_gem_object *obj)
> +{
> +	if (obj != NULL) {
> +		WARN_ON(!mutex_is_locked(&obj->dev->struct_mutex));
> +
> +		kref_put(&obj->refcount, drm_gem_object_free);
> +	}
> +}
> +EXPORT_SYMBOL(drm_gem_object_unreference);
>  
>  /**
>   * drm_gem_vm_open - vma->ops->open implementation for GEM
> diff --git a/include/drm/drmP.h b/include/drm/drmP.h
> index c81dd2250fc6..7e30b3d2b25c 100644
> --- a/include/drm/drmP.h
> +++ b/include/drm/drmP.h
> @@ -583,9 +583,19 @@ struct drm_driver {
>  	 * Driver-specific constructor for drm_gem_objects, to set up
>  	 * obj->driver_private.
>  	 *
> -	 * Returns 0 on success.
> +	 * This is deprecated and should not be used by new drivers. Use
> +	 * @gem_free_object_unlocked instead.
>  	 */
>  	void (*gem_free_object) (struct drm_gem_object *obj);
> +
> +	/**
> +	 * Driver-specific constructor for drm_gem_objects, to set up
> +	 * obj->driver_private.

This part of the comment is really off. I see it's just copy&paste from
the comment above gem_free_object, but I think it should have been
placed above gem_open_object and has nothing to do with the free*
callbacks.

>  This is for drivers which are not encumbered
> +	 * with dev->struct_mutex legacy locking schemes. Use this hook instead
> +	 * of @gem_free_object.
> +	 */
> +	void (*gem_free_object_unlocked) (struct drm_gem_object *obj);
> +
>  	int (*gem_open_object) (struct drm_gem_object *, struct drm_file *);
>  	void (*gem_close_object) (struct drm_gem_object *, struct drm_file *);
>  
> diff --git a/include/drm/drm_gem.h b/include/drm/drm_gem.h
> index 0b3e11ab8757..ae1c7f18eec0 100644
> --- a/include/drm/drm_gem.h
> +++ b/include/drm/drm_gem.h
> @@ -175,7 +175,6 @@ struct drm_gem_object {
>  };
>  
>  void drm_gem_object_release(struct drm_gem_object *obj);
> -void drm_gem_object_free(struct kref *kref);
>  int drm_gem_object_init(struct drm_device *dev,
>  			struct drm_gem_object *obj, size_t size);
>  void drm_gem_private_object_init(struct drm_device *dev,
> @@ -199,48 +198,8 @@ drm_gem_object_reference(struct drm_gem_object *obj)
>  	kref_get(&obj->refcount);
>  }
>  
> -/**
> - * drm_gem_object_unreference - release a GEM BO reference
> - * @obj: GEM buffer object
> - *
> - * This releases a reference to @obj. Callers must hold the dev->struct_mutex
> - * lock when calling this function, even when the driver doesn't use
> - * dev->struct_mutex for anything.
> - *
> - * For drivers not encumbered with legacy locking use
> - * drm_gem_object_unreference_unlocked() instead.
> - */
> -static inline void
> -drm_gem_object_unreference(struct drm_gem_object *obj)
> -{
> -	if (obj != NULL) {
> -		WARN_ON(!mutex_is_locked(&obj->dev->struct_mutex));
> -
> -		kref_put(&obj->refcount, 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.
> - */
> -static inline void
> -drm_gem_object_unreference_unlocked(struct drm_gem_object *obj)
> -{
> -	struct drm_device *dev;
> -
> -	if (!obj)
> -		return;
> -
> -	dev = obj->dev;
> -	if (kref_put_mutex(&obj->refcount, drm_gem_object_free, &dev->struct_mutex))
> -		mutex_unlock(&dev->struct_mutex);
> -	else
> -		might_lock(&dev->struct_mutex);
> -}
> +void drm_gem_object_unreference_unlocked(struct drm_gem_object *obj);
> +void drm_gem_object_unreference(struct drm_gem_object *obj);
>  
>  int drm_gem_handle_create(struct drm_file *file_priv,
>  			  struct drm_gem_object *obj,


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