[PATCH v3] drm: modify drm_global_item_ref to avoid two times of writing ref->object

[Huang Rui]

On Thu, Sep 08, 2016 at 02:36:06PM +0800, Chris Wilson wrote:
> On Wed, Sep 07, 2016 at 10:07:57PM -0400, Huang Rui wrote:
> > In previous drm_global_item_ref, there are two times of writing
> > ref->object if item->refcount is 0. So this patch does a minor update
> > to put alloc and init ref firstly, and then to modify the item of glob
> > array. Use "else" to avoid two times of writing ref->object. It can
> > make the code logic more clearly.
> > 
> > Signed-off-by: Huang Rui <ray.huang at amd.com>
> > ---
> > 
> > Changes from V2 -> V3:
> > - Use duplicate mutex release to avoid "goto" in non-error patch.
> > - Rename error label
> > 
> > ---
> >  drivers/gpu/drm/drm_global.c | 24 ++++++++++++++----------
> >  1 file changed, 14 insertions(+), 10 deletions(-)
> > 
> > diff --git a/drivers/gpu/drm/drm_global.c b/drivers/gpu/drm/drm_global.c
> > index 3d2e91c..b404287 100644
> > --- a/drivers/gpu/drm/drm_global.c
> > +++ b/drivers/gpu/drm/drm_global.c
> > @@ -65,30 +65,34 @@ void drm_global_release(void)
> >  
> >  int drm_global_item_ref(struct drm_global_reference *ref)
> >  {
> > -	int ret;
> > +	int ret = 0;
> >  	struct drm_global_item *item = &glob[ref->global_type];
> >  
> >  	mutex_lock(&item->mutex);
> >  	if (item->refcount == 0) {
> > -		item->object = kzalloc(ref->size, GFP_KERNEL);
> > -		if (unlikely(item->object == NULL)) {
> > +		ref->object = kzalloc(ref->size, GFP_KERNEL);
> 
> So the item refcount is 0, we operate on ref, whereas previous we
> inspected item and operated on item. Not an improvement.

Hmm, when item refcount is 0, we operate on ref to create object and init
it for item, so although item->object and ref->object here should point the
same thing, but we should alloc and init ref firstly and pass the
ref->object address to item (item actually points a static area). This make
the code logic more clearly and readable. So I updated it to "ref" here.
:-)

Thanks,
Rui