drm/gem: Use kref_get_unless_zero for the weak mmap references

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).

v3: Add a comment from Chris explaining how the ordering works, with
the slight adjustment that I dropped any mention of struct_mutex since
with this patch it's now immaterial ot core gem.

Cc: David Herrmann <dh.herrmann@gmail.com>
Reviewed-by: David Herrmann <dh.herrmann@gmail.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: http://mid.gmane.org/1444901623-18918-1-git-send-email-daniel.vetter@ffwll.ch
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>

1 files changed, 12 insertions, 28 deletions

diff --git a/drivers/gpu/drm/drm_vma_manager.c b/drivers/gpu/drm/drm_vma_manager.c
index 68c1f32fb086..2f2ecde8285b 100644
--- a/drivers/gpu/drm/drm_vma_manager.c
+++ b/drivers/gpu/drm/drm_vma_manager.c
@@ -112,7 +112,7 @@ void drm_vma_offset_manager_destroy(struct drm_vma_offset_manager *mgr)
 EXPORT_SYMBOL(drm_vma_offset_manager_destroy);
 
 /**
- * drm_vma_offset_lookup() - Find node in offset space
+ * drm_vma_offset_lookup_locked() - Find node in offset space
  * @mgr: Manager object
  * @start: Start address for object (page-based)
  * @pages: Size of object (page-based)
@@ -122,37 +122,21 @@ EXPORT_SYMBOL(drm_vma_offset_manager_destroy);
  * region and the given node will be returned, as long as the node spans the
  * whole requested area (given the size in number of pages as @pages).
  *
- * RETURNS:
- * Returns NULL if no suitable node can be found. Otherwise, the best match
- * is returned. It's the caller's responsibility to make sure the node doesn't
- * get destroyed before the caller can access it.
- */
-struct drm_vma_offset_node *drm_vma_offset_lookup(struct drm_vma_offset_manager *mgr,
-						  unsigned long start,
-						  unsigned long pages)
-{
-	struct drm_vma_offset_node *node;
-
-	read_lock(&mgr->vm_lock);
-	node = drm_vma_offset_lookup_locked(mgr, start, pages);
-	read_unlock(&mgr->vm_lock);
-
-	return node;
-}
-EXPORT_SYMBOL(drm_vma_offset_lookup);
-
-/**
- * drm_vma_offset_lookup_locked() - Find node in offset space
- * @mgr: Manager object
- * @start: Start address for object (page-based)
- * @pages: Size of object (page-based)
+ * Note that before lookup the vma offset manager lookup lock must be acquired
+ * with drm_vma_offset_lock_lookup(). See there for an example. This can then be
+ * used to implement weakly referenced lookups using kref_get_unless_zero().
  *
- * Same as drm_vma_offset_lookup() but requires the caller to lock offset lookup
- * manually. See drm_vma_offset_lock_lookup() for an example.
+ * Example:
+ *     drm_vma_offset_lock_lookup(mgr);
+ *     node = drm_vma_offset_lookup_locked(mgr);
+ *     if (node)
+ *         kref_get_unless_zero(container_of(node, sth, entr));
+ *     drm_vma_offset_unlock_lookup(mgr);
  *
  * RETURNS:
  * Returns NULL if no suitable node can be found. Otherwise, the best match
- * is returned.
+ * is returned. It's the caller's responsibility to make sure the node doesn't
+ * get destroyed before the caller can access it.
  */
 struct drm_vma_offset_node *drm_vma_offset_lookup_locked(struct drm_vma_offset_manager *mgr,
 							 unsigned long start,