drm/gem: fix up flink name create race

This is the 2nd attempt, I've always been a bit dissatisified with the
tricky nature of the first one:

http://lists.freedesktop.org/archives/dri-devel/2012-July/025451.html

The issue is that the flink ioctl can race with calling gem_close on
the last gem handle. In that case we'll end up with a zero handle
count, but an flink name (and it's corresponding reference). Which
results in a neat space leak.

In my first attempt I've solved this by rechecking the handle count.
But fundamentally the issue is that ->handle_count isn't your usual
refcount - it can be resurrected from 0 among other things.

For those special beasts atomic_t often suggest way more ordering that
it actually guarantees. To prevent being tricked by those hairy
semantics take the easy way out and simply protect the handle with the
existing dev->object_name_lock.

With that change implemented it's dead easy to fix the flink vs. gem
close reace: When we try to create the name we simply have to check
whether there's still officially a gem handle around and if not refuse
to create the flink name. Since the handle count decrement and flink
name destruction is now also protected by that lock the reace is gone
and we can't ever leak the flink reference again.

Outside of the drm core only the exynos driver looks at the handle
count, and tbh I have no idea why (it's just for debug dmesg output
luckily).

I've considered inlining the drm_gem_object_handle_free, but I plan to
add more name-like things (like the exported dma_buf) to this scheme,
so it's clearer to leave the handle freeing in its own function.

This is exercised by the new gem_flink_race i-g-t testcase, which on
my snb leaks gem objects at a rate of roughly 1k objects/s.

v2: Fix up the error path handling in handle_create and make it more
robust by simply calling object_handle_unreference.

v3: Fix up the handle_unreference logic bug - atomic_dec_and_test
retursn 1 for 0. Oops.

v4: Squash in inlining of drm_gem_object_handle_reference as suggested
by Dave Airlie and add a note that we now have a testcase.

Cc: Dave Airlie <airlied@gmail.com>
Cc: Inki Dae <inki.dae@samsung.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Dave Airlie <airlied@redhat.com>

1 files changed, 10 insertions, 9 deletions

diff --git a/include/drm/drmP.h b/include/drm/drmP.h
index 1a7a78fdb4b7..57dce6081d73 100644
--- a/include/drm/drmP.h
+++ b/include/drm/drmP.h
@@ -615,8 +615,16 @@ struct drm_gem_object {
 	/** Reference count of this object */
 	struct kref refcount;
 
-	/** Handle count of this object. Each handle also holds a reference */
-	atomic_t handle_count; /* number of handles on this object */
+	/**
+	 * handle_count - gem file_priv handle count of this object
+	 *
+	 * Each handle also holds a reference. Note that when the handle_count
+	 * drops to 0 any global names (e.g. the id in the flink namespace) will
+	 * be cleared.
+	 *
+	 * Protected by dev->object_name_lock.
+	 * */
+	unsigned handle_count;
 
 	/** Related drm device */
 	struct drm_device *dev;
@@ -1572,13 +1580,6 @@ int drm_gem_handle_create(struct drm_file *file_priv,
 			  u32 *handlep);
 int drm_gem_handle_delete(struct drm_file *filp, u32 handle);
 
-static inline void
-drm_gem_object_handle_reference(struct drm_gem_object *obj)
-{
-	drm_gem_object_reference(obj);
-	atomic_inc(&obj->handle_count);
-}
-
 void drm_gem_object_handle_unreference_unlocked(struct drm_gem_object *obj);
 
 void drm_gem_free_mmap_offset(struct drm_gem_object *obj);