[PATCH] drm: Do not call drm_dev_unregister twice on drm_unplug_dev

[Hans de Goede]

Since commit a39be606f99d ("drm: Do a full device unregister when
unplugging") drm_unplug_dev has been calling drm_dev_unregister followed
by a drm_put_dev when open_count reaches 0. This drm_put_dev calls
drm_dev_unregister again. Since drm_dev_unregister is not protected
against being called multiple times this leads to havoc.

This commit fixes this by calling drm_dev_unref instead of drm_put_dev.

This commit also moves the mutex_lock(&drm_global_mutex) to above the
drm_dev_unregister call to avoid a race between drm_unplug_dev and
drm_release still leading to drm_dev_unregister getting called twice.

Note that calling drm_dev_unregister with the drm_global_mutex locked is
fine, drm_release already did this (through drm_put_dev).

Fixes: a39be606f99d ("drm: Do a full device unregister when unplugging")
Cc: Chris Wilson <chris at chris-wilson.co.uk>
Cc: Marco Diego Aurélio Mesquita <marcodiegomesquita at gmail.com>
Reported-by: Marco Diego Aurélio Mesquita <marcodiegomesquita at gmail.com>
Signed-off-by: Hans de Goede <hdegoede at redhat.com>
---
Note I don't have any USB display devices at hand for testing atm so
this patch has only been compile tested.
---
 drivers/gpu/drm/drm_drv.c  | 10 +++++-----
 drivers/gpu/drm/drm_file.c |  2 +-
 2 files changed, 6 insertions(+), 6 deletions(-)

diff --git a/drivers/gpu/drm/drm_drv.c b/drivers/gpu/drm/drm_drv.c
index b5c6bb46a425..9f31d5d629d1 100644
--- a/drivers/gpu/drm/drm_drv.c
+++ b/drivers/gpu/drm/drm_drv.c
@@ -357,16 +357,16 @@ EXPORT_SYMBOL(drm_put_dev);
 
 void drm_unplug_dev(struct drm_device *dev)
 {
+	mutex_lock(&drm_global_mutex);
+
 	/* for a USB device */
 	drm_dev_unregister(dev);
 
-	mutex_lock(&drm_global_mutex);
-
 	drm_device_set_unplugged(dev);
 
-	if (dev->open_count == 0) {
-		drm_put_dev(dev);
-	}
+	if (dev->open_count == 0)
+		drm_dev_unref(dev);
+
 	mutex_unlock(&drm_global_mutex);
 }
 EXPORT_SYMBOL(drm_unplug_dev);
diff --git a/drivers/gpu/drm/drm_file.c b/drivers/gpu/drm/drm_file.c
index 3783b659cd38..edba71c8ccc3 100644
--- a/drivers/gpu/drm/drm_file.c
+++ b/drivers/gpu/drm/drm_file.c
@@ -424,7 +424,7 @@ int drm_release(struct inode *inode, struct file *filp)
 	if (!--dev->open_count) {
 		drm_lastclose(dev);
 		if (drm_device_is_unplugged(dev))
-			drm_put_dev(dev);
+			drm_dev_unref(dev);
 	}
 	mutex_unlock(&drm_global_mutex);
 
-- 
2.13.0