[PATCH v2] drm: Release driver references to handle before making it available again

[Chris Wilson]

On Mon, Jan 11, 2016 at 07:51:03PM +0200, Ville Syrjälä wrote:
> On Fri, Jan 08, 2016 at 11:27:05PM +0000, Chris Wilson wrote:
> > When userspace closes a handle, we remove it from the file->object_idr
> > and then tell the driver to drop its references to that file/handle.
> > However, as the file/handle is already available again for reuse, it may
> > be reallocated back to userspace and active on a new object before the
> > driver has had a chance to drop the old file/handle references.
> 
> Hmm. What's the problem with another object starting to reuse the same
> handle while we're still deleting the old one? So far I didn't spot
> anything in the code that would go boom if there's another object
> already around with the same handle.

Imagine a driver storing a hashtable to contract the handle->object->vma
lookup into just handle->vma, like the old idea of replacing the
object_idr with an ida plus hashtable of objects. (This saves the double
step lookup that caused a regression with the ppgtt work, and the linear
walk of object->vma_list which is a major slowdown in various full-pggtt
OpenGL tests). In such a scheme, where the driver has a parallel lut to
the core, the driver needs to be notified before the handle is then
accessible again to userspace. Or in any other scenario where the driver
is using the handle, as would be implied by having the open/close
callbacks.
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre