[alsa-devel] [PATCH v2 22/22] media: Ensure media device unregister is done only once

[Shuah Khan]

media_device_unregister() checks if the media device
is registered or not as the first step. However, the
MEDIA_FLAG_REGISTERED bit doesn't get cleared until
the end leaving a large window for two drivers to
attempt media device unregister.

The above leads to general protection faults when
device is removed.

Fix the problem with two phase media device unregister.
Add a new interface media_devnode_start_unregister()
to clear the MEDIA_FLAG_REGISTERED bit. Change
media_device_unregister() call this interface to mark
the start of unregister. This will ensure that media
device unregister is done only once.

Signed-off-by: Shuah Khan <shuahkh at osg.samsung.com>
---
 drivers/media/media-device.c  | 12 ++++++------
 drivers/media/media-devnode.c | 15 ++++++++++-----
 include/media/media-devnode.h | 17 +++++++++++++++++
 3 files changed, 33 insertions(+), 11 deletions(-)

diff --git a/drivers/media/media-device.c b/drivers/media/media-device.c
index 1f5d67e..584d46e 100644
--- a/drivers/media/media-device.c
+++ b/drivers/media/media-device.c
@@ -747,17 +747,17 @@ void media_device_unregister(struct media_device *mdev)
 	struct media_entity *next;
 	struct media_interface *intf, *tmp_intf;
 	struct media_entity_notify *notify, *nextp;
+	int ret;
 
 	if (mdev == NULL)
 		return;
 
-	spin_lock(&mdev->lock);
-
-	/* Check if mdev was ever registered at all */
-	if (!media_devnode_is_registered(&mdev->devnode)) {
-		spin_unlock(&mdev->lock);
+	/* Start unregister - continue if necessary */
+	ret = media_devnode_start_unregister(&mdev->devnode);
+	if (ret)
 		return;
-	}
+
+	spin_lock(&mdev->lock);
 
 	/* Remove all entities from the media device */
 	list_for_each_entry_safe(entity, next, &mdev->entities, graph_obj.list)
diff --git a/drivers/media/media-devnode.c b/drivers/media/media-devnode.c
index 29409f4..c27f9e7 100644
--- a/drivers/media/media-devnode.c
+++ b/drivers/media/media-devnode.c
@@ -272,15 +272,20 @@ error:
 	return ret;
 }
 
-void media_devnode_unregister(struct media_devnode *mdev)
+int __must_check media_devnode_start_unregister(struct media_devnode *mdev)
 {
-	/* Check if mdev was ever registered at all */
-	if (!media_devnode_is_registered(mdev))
-		return;
-
 	mutex_lock(&media_devnode_lock);
+	if (!media_devnode_is_registered(mdev)) {
+		mutex_unlock(&media_devnode_lock);
+		return -EINVAL;
+	}
 	clear_bit(MEDIA_FLAG_REGISTERED, &mdev->flags);
 	mutex_unlock(&media_devnode_lock);
+	return 0;
+}
+
+void media_devnode_unregister(struct media_devnode *mdev)
+{
 	device_unregister(&mdev->dev);
 }
 
diff --git a/include/media/media-devnode.h b/include/media/media-devnode.h
index fe42f08..6f08677 100644
--- a/include/media/media-devnode.h
+++ b/include/media/media-devnode.h
@@ -120,6 +120,23 @@ int __must_check media_devnode_register(struct media_devnode *mdev,
 					struct module *owner);
 
 /**
+ * media_devnode_start_unregister - start unregister on a media device node
+ * @mdev: the device node to start unregister
+ *
+ * This clears the MEDIA_FLAG_REGISTERED bit to indicate that unregister
+ * is in progress.
+ *
+ * This function can safely be called if the device node has never been
+ * registered or has already been unregistered.
+ *
+ * Zero is returned on success.
+ *
+ * -EINVAL is returned if the device node has never been
+ * registered or has already been unregistered.
+ */
+int __must_check media_devnode_start_unregister(struct media_devnode *mdev);
+
+/**
  * media_devnode_unregister - unregister a media device node
  * @mdev: the device node to unregister
  *
-- 
2.5.0