drm: Fix drm_release() and device unplug

If userspace has open fd(s) when drm_dev_unplug() is run, it will result
in drm_dev_unregister() being called twice. First in drm_dev_unplug() and
then later in drm_release() through the call to drm_put_dev().

Since userspace already holds a ref on drm_device through the drm_minor,
it's not necessary to add extra ref counting based on no open file
handles. Instead just drm_dev_put() unconditionally in drm_dev_unplug().

We now have this:
- Userpace holds a ref on drm_device as long as there's open fd(s)
- The driver holds a ref on drm_device as long as it's bound to the
  struct device

When both sides are done with drm_device, it is released.

Signed-off-by: Noralf Trønnes <noralf@tronnes.org>
Reviewed-by: Oleksandr Andrushchenko <oleksandr_andrushchenko@epam.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Sean Paul <sean@poorly.run>
Link: https://patchwork.freedesktop.org/patch/msgid/20190208140103.28919-2-noralf@tronnes.org

2 files changed, 3 insertions, 9 deletions

diff --git a/drivers/gpu/drm/drm_drv.c b/drivers/gpu/drm/drm_drv.c
index 4fec551eb781..f2d5a41b61f7 100644
--- a/drivers/gpu/drm/drm_drv.c
+++ b/drivers/gpu/drm/drm_drv.c
@@ -376,11 +376,7 @@ void drm_dev_unplug(struct drm_device *dev)
         synchronize_srcu(&drm_unplug_srcu);
 
         drm_dev_unregister(dev);
-
-        mutex_lock(&drm_global_mutex);
-        if (dev->open_count == 0)
-                drm_dev_put(dev);
-        mutex_unlock(&drm_global_mutex);
+        drm_dev_put(dev);
 }
 EXPORT_SYMBOL(drm_dev_unplug);
 
diff --git a/drivers/gpu/drm/drm_file.c b/drivers/gpu/drm/drm_file.c
index b1838a41ad43..ee4df8f8a62d 100644
--- a/drivers/gpu/drm/drm_file.c
+++ b/drivers/gpu/drm/drm_file.c
@@ -479,11 +479,9 @@ int drm_release(struct inode *inode, struct file *filp)
 
         drm_file_free(file_priv);
 
-        if (!--dev->open_count) {
+        if (!--dev->open_count)
                 drm_lastclose(dev);
-                if (drm_dev_is_unplugged(dev))
-                        drm_put_dev(dev);
-        }
+
         mutex_unlock(&drm_global_mutex);
 
         drm_minor_release(minor);