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, 6 insertions, 16 deletions

diff --git a/include/drm/drm_vma_manager.h b/include/drm/drm_vma_manager.h
index 089cb734f6a3..2f63dd5e05eb 100644
--- a/include/drm/drm_vma_manager.h
+++ b/include/drm/drm_vma_manager.h
@@ -54,9 +54,6 @@ void drm_vma_offset_manager_init(struct drm_vma_offset_manager *mgr,
 				 unsigned long page_offset, unsigned long size);
 void drm_vma_offset_manager_destroy(struct drm_vma_offset_manager *mgr);
 
-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 *drm_vma_offset_lookup_locked(struct drm_vma_offset_manager *mgr,
 							   unsigned long start,
 							   unsigned long pages);
@@ -71,25 +68,25 @@ bool drm_vma_node_is_allowed(struct drm_vma_offset_node *node,
 			     struct file *filp);
 
 /**
- * drm_vma_offset_exact_lookup() - Look up node by exact address
+ * drm_vma_offset_exact_lookup_locked() - Look up node by exact address
  * @mgr: Manager object
  * @start: Start address (page-based, not byte-based)
  * @pages: Size of object (page-based)
  *
- * Same as drm_vma_offset_lookup() but does not allow any offset into the node.
+ * Same as drm_vma_offset_lookup_locked() but does not allow any offset into the node.
  * It only returns the exact object with the given start address.
  *
  * RETURNS:
  * Node at exact start address @start.
  */
 static inline struct drm_vma_offset_node *
-drm_vma_offset_exact_lookup(struct drm_vma_offset_manager *mgr,
-			    unsigned long start,
-			    unsigned long pages)
+drm_vma_offset_exact_lookup_locked(struct drm_vma_offset_manager *mgr,
+				   unsigned long start,
+				   unsigned long pages)
 {
 	struct drm_vma_offset_node *node;
 
-	node = drm_vma_offset_lookup(mgr, start, pages);
+	node = drm_vma_offset_lookup_locked(mgr, start, pages);
 	return (node && node->vm_node.start == start) ? node : NULL;
 }
 
@@ -108,13 +105,6 @@ drm_vma_offset_exact_lookup(struct drm_vma_offset_manager *mgr,
  * not call any other VMA helpers while holding this lock.
  *
  * Note: You're in atomic-context while holding this lock!
- *
- * 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);
  */
 static inline void drm_vma_offset_lock_lookup(struct drm_vma_offset_manager *mgr)
 {