[PATCH v2] v4l: async: Protect against double notifier registrations

[Kieran Bingham]

From: Kieran Bingham <kieran.bingham+rene...@ideasonboard.com>

It can be easy to attempt to register the same notifier twice
in mis-handled error cases such as working with -EPROBE_DEFER.

This results in odd kernel crashes where the notifier_list becomes
corrupted due to adding the same entry twice.

Protect against this so that a developer has some sense of the pending
failure, and use a WARN_ON to identify the fault.

Signed-off-by: Kieran Bingham <kieran.bingham+rene...@ideasonboard.com>

---
v2:
 - Reduce verbosity
 - use WARN_ON()
 - Move notifier list initialisation after registration check

 drivers/media/v4l2-core/v4l2-async.c | 13 +++++++++++--
 1 file changed, 11 insertions(+), 2 deletions(-)

diff --git a/drivers/media/v4l2-core/v4l2-async.c 
b/drivers/media/v4l2-core/v4l2-async.c
index 2b08d03b251d..17a779440ae3 100644
--- a/drivers/media/v4l2-core/v4l2-async.c
+++ b/drivers/media/v4l2-core/v4l2-async.c
@@ -374,17 +374,26 @@ static int __v4l2_async_notifier_register(struct 
v4l2_async_notifier *notifier)
        struct device *dev =
                notifier->v4l2_dev ? notifier->v4l2_dev->dev : NULL;
        struct v4l2_async_subdev *asd;
+       struct v4l2_async_notifier *n;
        int ret;
        int i;
 
        if (notifier->num_subdevs > V4L2_MAX_SUBDEVS)
                return -EINVAL;
 
+       mutex_lock(&list_lock);
+
+       /* Avoid re-registering a notifier. */
+       list_for_each_entry(n, &notifier_list, list) {
+               if (WARN_ON(n == notifier)) {
+                       ret = -EEXIST;
+                       goto err_unlock;
+               }
+       }
+
        INIT_LIST_HEAD(&notifier->waiting);
        INIT_LIST_HEAD(&notifier->done);
 
-       mutex_lock(&list_lock);
-
        for (i = 0; i < notifier->num_subdevs; i++) {
                asd = notifier->subdevs[i];
 
-- 
2.7.4