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

[Chris Wilson]

On Thu, Sep 08, 2016 at 03:22:48PM +0800, Huang Rui wrote:
> 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.
> :-)

The object is owned by item. ref is just a pointer to it.
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre