drm/gem: support BO freeing without dev->struct_mutex

Finally all the core gem and a lot of drivers are entirely free of
dev->struct_mutex depencies, and we can start to have an entirely
lockless unref path.

To make sure that no one who touches the core code accidentally breaks
existing drivers which still require dev->struct_mutex I've made the
might_lock check unconditional.

While at it de-inline the ref/unref functions, they've become a bit
too big.

v2: Make it not leak like a sieve.

v3: Review from Lucas:
- drop != NULL in pointer checks.
- fixup copypasted kerneldoc to actually match the functions.

v4:
Add __drm_gem_object_unreference as a fastpath helper for drivers who
abolished dev->struct_mutex, requested by Chris.

v5: Fix silly mistake in drm_gem_object_unreference_unlocked caught by
intel-gfx CI - I checked for gem_free_object instead of
gem_free_object_unlocked ...

Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Alex Deucher <alexdeucher@gmail.com>
Cc: Lucas Stach <l.stach@pengutronix.de>
Reviewed-by: Lucas Stach <l.stach@pengutronix.de> (v3)
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk> (v4)
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1462178451-1765-1-git-send-email-daniel.vetter@ffwll.ch

2 files changed, 27 insertions, 36 deletions

diff --git a/include/drm/drmP.h b/include/drm/drmP.h
index 7901768bb47c..360b2a74e1ef 100644
--- a/include/drm/drmP.h
+++ b/include/drm/drmP.h
@@ -580,12 +580,21 @@ struct drm_driver {
         void (*debugfs_cleanup)(struct drm_minor *minor);
 
         /**
-         * Driver-specific constructor for drm_gem_objects, to set up
-         * obj->driver_private.
+         * @gem_free_object: deconstructor for drm_gem_objects
          *
-         * Returns 0 on success.
+         * This is deprecated and should not be used by new drivers. Use
+         * @gem_free_object_unlocked instead.
          */
         void (*gem_free_object) (struct drm_gem_object *obj);
+
+        /**
+         * @gem_free_object_unlocked: deconstructor for drm_gem_objects
+         *
+         * This is for drivers which are not encumbered with dev->struct_mutex
+         * legacy locking schemes. Use this hook instead of @gem_free_object.
+         */
+        void (*gem_free_object_unlocked) (struct drm_gem_object *obj);
+
         int (*gem_open_object) (struct drm_gem_object *, struct drm_file *);
         void (*gem_close_object) (struct drm_gem_object *, struct drm_file *);
 
diff --git a/include/drm/drm_gem.h b/include/drm/drm_gem.h
index 0b3e11ab8757..408d6c47d98b 100644
--- a/include/drm/drm_gem.h
+++ b/include/drm/drm_gem.h
@@ -200,47 +200,29 @@ drm_gem_object_reference(struct drm_gem_object *obj)
 }
 
 /**
- * drm_gem_object_unreference - release a GEM BO reference
+ * __drm_gem_object_unreference - raw function to release a GEM BO reference
  * @obj: GEM buffer object
  *
- * This releases a reference to @obj. Callers must hold the dev->struct_mutex
- * lock when calling this function, even when the driver doesn't use
- * dev->struct_mutex for anything.
+ * This function is meant to be used by drivers which are not encumbered with
+ * dev->struct_mutex legacy locking and which are using the
+ * gem_free_object_unlocked callback. It avoids all the locking checks and
+ * locking overhead of drm_gem_object_unreference() and
+ * drm_gem_object_unreference_unlocked().
  *
- * For drivers not encumbered with legacy locking use
- * drm_gem_object_unreference_unlocked() instead.
+ * Drivers should never call this directly in their code. Instead they should
+ * wrap it up into a driver_gem_object_unreference(struct driver_gem_object
+ * *obj) wrapper function, and use that. Shared code should never call this, to
+ * avoid breaking drivers by accident which still depend upon dev->struct_mutex
+ * locking.
  */
 static inline void
-drm_gem_object_unreference(struct drm_gem_object *obj)
+__drm_gem_object_unreference(struct drm_gem_object *obj)
 {
-        if (obj != NULL) {
-                WARN_ON(!mutex_is_locked(&obj->dev->struct_mutex));
-
-                kref_put(&obj->refcount, drm_gem_object_free);
-        }
+        kref_put(&obj->refcount, drm_gem_object_free);
 }
 
-/**
- * drm_gem_object_unreference_unlocked - release a GEM BO reference
- * @obj: GEM buffer object
- *
- * This releases a reference to @obj. Callers must not hold the
- * dev->struct_mutex lock when calling this function.
- */
-static inline void
-drm_gem_object_unreference_unlocked(struct drm_gem_object *obj)
-{
-        struct drm_device *dev;
-
-        if (!obj)
-                return;
-
-        dev = obj->dev;
-        if (kref_put_mutex(&obj->refcount, drm_gem_object_free, &dev->struct_mutex))
-                mutex_unlock(&dev->struct_mutex);
-        else
-                might_lock(&dev->struct_mutex);
-}
+void drm_gem_object_unreference_unlocked(struct drm_gem_object *obj);
+void drm_gem_object_unreference(struct drm_gem_object *obj);
 
 int drm_gem_handle_create(struct drm_file *file_priv,
                           struct drm_gem_object *obj,