On Wed, Jul 08, 2015 at 11:10:26AM -0600, Jason Gunthorpe wrote:

On Wed, Jul 08, 2015 at 12:16:30PM +0200, Jens Wiklander wrote:

quoted

+static void tee_device_complete_unused(struct kref *kref)
+{
+	struct tee_device *teedev;
+
+	teedev = container_of(kref, struct tee_device, users);
+	/* When the mutex is released, no other tee_device_get() will succeed */
+	teedev->desc = NULL;
+	complete(&teedev->c_no_users);
+}
+
+void tee_device_put(struct tee_device *teedev)
+{
+	mutex_lock(&teedev->mutex);
+	/* Shouldn't put in this state */
+	if (!WARN_ON(!teedev->desc))
+		kref_put(&teedev->users, tee_device_complete_unused);
+	mutex_unlock(&teedev->mutex);
+}
+
+bool tee_device_get(struct tee_device *teedev)
+{
+	mutex_lock(&teedev->mutex);
+	if (!teedev->desc) {
+		mutex_unlock(&teedev->mutex);
+		return false;
+	}
+	kref_get(&teedev->users);
+	mutex_unlock(&teedev->mutex);
+	return true;
+}

If you are holding the mutex then you don't really need a kref, just a
simple active count counter.

I've been a bit learly lately about seeing krefs used for something
other than kfree, I've seen a few subtle mistakes in those schemes -
yours looks OK, only because of the lock, and the lock makes the kref
redundant..

quoted

+       cdev_init(&teedev->cdev, &tee_fops);
+       teedev->cdev.owner = teedesc->owner;

This also needs to set teedev->cdev.kobj.parent.
I'm guessing:

 teedev->cdev.kobj.parent = &teedev->dev.kobj;

TPM had the same mistake..

Really?  As of a few years ago, A cdev's kobject should not be touched
by anything other than the cdev core.  It's not a "real" kobject in that
it is never registered in sysfs, and no one sees it.  I keep meaning to
just use something else one of these days for that structure, as lots of
people get it wrong.  Or has things changed there?

thanks,

greg k-h