[PATCH] drm: Don't complain too much about struct_mutex.

[Daniel Vetter]

For modern drivers the DRM core doesn't use struct_mutex at all, which
means it's defacto a driver-private lock. But since we still need it
for legacy drivers we can't initialize it in drivers, which means all
the different instances share one lockdep key. Despite that they might
be placed in totally different places in the locking hierarchy.

This results in a lot of bogus lockdep splats when running stuff on
systems with multiple gpus. Partially remedy the situation by only
doing might_lock checks on drivers that do use struct_mutex still for
gem locking.

A more complete solution would be to do the mutex_init in the drm core
only for legacy drivers, plus add it to each modern driver that still
needs it, which would also give each its own lockdep key. Trying to do
that dynamically doesn't work, because lockdep requires it's keys to
be statically allocated.

Cc: Hans de Goede <hdegoede at redhat.com>
Cc: Ben Skeggs <bskeggs at redhat.com>
Cc: Jiri Slaby <jirislaby at gmail.com>
Cc: Peter Zijlstra <peterz at infradead.org>
Cc: Ingo Molnar <mingo at redhat.com>
Signed-off-by: Daniel Vetter <daniel.vetter at intel.com>
---
 drivers/gpu/drm/drm_gem.c | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/drivers/gpu/drm/drm_gem.c b/drivers/gpu/drm/drm_gem.c
index 8dc11064253d..9663a79dd363 100644
--- a/drivers/gpu/drm/drm_gem.c
+++ b/drivers/gpu/drm/drm_gem.c
@@ -826,13 +826,15 @@ drm_gem_object_put_unlocked(struct drm_gem_object *obj)
 		return;
 
 	dev = obj->dev;
-	might_lock(&dev->struct_mutex);
 
 	if (dev->driver->gem_free_object_unlocked)
 		kref_put(&obj->refcount, drm_gem_object_free);
-	else if (kref_put_mutex(&obj->refcount, drm_gem_object_free,
+	else {
+		might_lock(&dev->struct_mutex);
+		if (kref_put_mutex(&obj->refcount, drm_gem_object_free,
 				&dev->struct_mutex))
-		mutex_unlock(&dev->struct_mutex);
+			mutex_unlock(&dev->struct_mutex);
+	}
 }
 EXPORT_SYMBOL(drm_gem_object_put_unlocked);
 
-- 
2.13.2