[Intel-gfx] [PATCH v2] drm/i915: Gracefully handle obj not bound to GGTT in is_pin_display

[Chris Wilson]

On Fri, May 16, 2014 at 12:08:22PM +0100, oscar.mateo at intel.com wrote:
> From: Oscar Mateo <oscar.mateo at intel.com>
> 
> Otherwise, we do a NULL pointer dereference.
> 
> I've seen this happen while handling an error in
> i915_gem_object_pin_to_display_plane():
> 
> If i915_gem_object_set_cache_level() fails, we call is_pin_display()
> to handle the error. At this point, the object is still not pinned
> to GGTT and maybe not even bound, so we have to check before we
> dereference its GGTT vma.
> 
> v2: Chris Wilson says restoring the old value is easier, but that
> is_pin_display is useful as a theory of operation. Take the solomonic
> decision: at least this way is_pin_display is a little more robust
> (until Chris can kill it off).
> 
> Issue: VIZ-3772

I heard you wrote a testcase?

> Signed-off-by: Oscar Mateo <oscar.mateo at intel.com>
> ---
>  drivers/gpu/drm/i915/i915_gem.c | 16 ++++++++++++++--
>  1 file changed, 14 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/gpu/drm/i915/i915_gem.c b/drivers/gpu/drm/i915/i915_gem.c
> index 034ba2c..211b778 100644
> --- a/drivers/gpu/drm/i915/i915_gem.c
> +++ b/drivers/gpu/drm/i915/i915_gem.c
> @@ -3641,6 +3641,15 @@ unlock:
>  
>  static bool is_pin_display(struct drm_i915_gem_object *obj)
>  {
> +	struct i915_vma *vma;
> +
> +	if (list_empty(&obj->vma_list))
> +		return false;

Hmm, this is so that we don't trigger the WARN from inside
i915_gem_obj_to_ggtt(). I would say that means the WARN in the callee
has outlived its usefulness. Other callers WARN if they fail to find the
ggtt_vma they expect, so I think we can just drop the WARN and save the
duplication here.

> +
> +	vma = i915_gem_obj_to_ggtt(obj);
> +	if (!vma)
> +		return false;
> +
>  	/* There are 3 sources that pin objects:
>  	 *   1. The display engine (scanouts, sprites, cursors);
>  	 *   2. Reservations for execbuffer;
> @@ -3652,7 +3661,7 @@ static bool is_pin_display(struct drm_i915_gem_object *obj)
>  	 * subtracting the potential reference by the user, any pin_count
>  	 * remains, it must be due to another use by the display engine.
>  	 */
> -	return i915_gem_obj_to_ggtt(obj)->pin_count - !!obj->user_pin_count;
> +	return vma->pin_count - !!obj->user_pin_count;
>  }
>  
>  /*
> @@ -3666,6 +3675,7 @@ i915_gem_object_pin_to_display_plane(struct drm_i915_gem_object *obj,
>  				     struct intel_ring_buffer *pipelined)
>  {
>  	u32 old_read_domains, old_write_domain;
> +	bool was_pin_display;
>  	int ret;
>  
>  	if (pipelined != obj->ring) {
> @@ -3677,6 +3687,7 @@ i915_gem_object_pin_to_display_plane(struct drm_i915_gem_object *obj,
>  	/* Mark the pin_display early so that we account for the
>  	 * display coherency whilst setting up the cache domains.
>  	 */
> +	was_pin_display = obj->pin_display;
>  	obj->pin_display = true;
>  
>  	/* The display engine is not coherent with the LLC cache on gen6.  As
> @@ -3719,7 +3730,8 @@ i915_gem_object_pin_to_display_plane(struct drm_i915_gem_object *obj,
>  	return 0;
>  
>  err_unpin_display:
> -	obj->pin_display = is_pin_display(obj);
> +	WARN_ON(was_pin_display != is_pin_display(obj));
> +	obj->pin_display = was_pin_display;
>  	return ret;
>  }

Ok, this looks like a useful check.

Other than the debate over the placement of the WARN() in
i915_gem_obj_to_ggtt() (maybe leave a comment here to remind us to drop
the WARN and the check later?),
Reviewed-by: Chris Wilson <chris at chris-wilson.co.uk>
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre