[PATCH] drm/gem: Use kref_get_unless_zero for the weak mmap references

[Daniel Vetter]

On Thu, Oct 15, 2015 at 11:06:59AM +0100, Chris Wilson wrote:
> On Thu, Oct 15, 2015 at 11:33:43AM +0200, Daniel Vetter wrote:
> > Compared to wrapping the final kref_put with dev->struct_mutex this
> > allows us to only acquire the offset manager look both in the final
> > cleanup and in the lookup. Which has the upside that no locks leak out
> > of the core abstractions. But it means that we need to hold a
> > temporary reference to the object while checking mmap constraints, to
> > make sure the object doesn't disappear. Extended the critical region
> > would have worked too, but would result in more leaky locking.
> > 
> > Also, this is the final bit which required dev->struct_mutex in gem
> > core, now modern drivers can be completely struct_mutex free!
> > 
> > This needs a new drm_vma_offset_exact_lookup_locked and makes both
> > drm_vma_offset_exact_lookup and drm_vma_offset_lookup unused.
> > 
> > v2: Don't leak object references in failure paths (David).
> > 
> > Cc: David Herrmann <dh.herrmann at gmail.com>
> > Reviewed-by: David Herrmann <dh.herrmann at gmail.com>
> > Signed-off-by: Daniel Vetter <daniel.vetter at intel.com>
> > ---
> >  drivers/gpu/drm/drm_gem.c         | 30 +++++++++++++++++------------
> >  drivers/gpu/drm/drm_vma_manager.c | 40 ++++++++++++---------------------------
> >  include/drm/drm_vma_manager.h     | 22 ++++++---------------
> >  3 files changed, 36 insertions(+), 56 deletions(-)
> > 
> > diff --git a/drivers/gpu/drm/drm_gem.c b/drivers/gpu/drm/drm_gem.c
> > index ab8ea42264f4..663fadbe979e 100644
> > --- a/drivers/gpu/drm/drm_gem.c
> > +++ b/drivers/gpu/drm/drm_gem.c
> > @@ -862,30 +862,36 @@ int drm_gem_mmap(struct file *filp, struct vm_area_struct *vma)
> >  {
> >  	struct drm_file *priv = filp->private_data;
> >  	struct drm_device *dev = priv->minor->dev;
> > -	struct drm_gem_object *obj;
> > +	struct drm_gem_object *obj = NULL;
> >  	struct drm_vma_offset_node *node;
> >  	int ret;
> >  
> >  	if (drm_device_is_unplugged(dev))
> >  		return -ENODEV;
> >  
> 
> /* bla bla bla
>  * When the object is being freed, after it hits 0-refcnt
>  * it acquires the struct_mutex and proceeds to tear down
>  * the object. In the process it will attempt to remove
>  * the VMA offset and so acquire this mgr->vm_lock.
>  * Therefore if we find an object with a 0-refcnt that matches
>  * our range, we know it is in the process of being destroyed
>  * and will be freed as soon as we release the lock - so
>  * we have to check for the 0-refcnted object and treat it as
>  * invalid.
>  */

As discussed on irc struct_mutex is just a digression, so I left that part
out when adding your comment.

> > -	mutex_lock(&dev->struct_mutex);
> > +	drm_vma_offset_lock_lookup(dev->vma_offset_manager);
> > +	node = drm_vma_offset_exact_lookup_locked(dev->vma_offset_manager,
> > +						  vma->vm_pgoff,
> > +						  vma_pages(vma));
> > +	if (likely(node)) {
> > +		obj = container_of(node, struct drm_gem_object, vma_node);
> > +		if (!kref_get_unless_zero(&obj->refcount))
> > +			obj = NULL;
> > +	}
> > +	drm_vma_offset_unlock_lookup(dev->vma_offset_manager);
> 
> Reviewed-by: Chris Wilson <chris at chris-wilson.co.uk>

All four core gem patches now merged in drm-misc, thanks for the review.
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch