On Wed, Aug 10, 2016 at 10:16 PM, Al Viro <viro(a)zeniv.linux.org.uk> wrote:
On Wed, Aug 10, 2016 at 09:49:22PM -0700, Dan Williams wrote:
> Where dax_dev_release() is the f_op->release() method, and is
> implemented to simply drop the final references on our driver objects:
> struct dax_dev *dax_dev = filp->private_data;
> struct device *dev = dax_dev->dev;
> dev_dbg(dax_dev->dev, "%s\n", __func__);
> The dax_dev object embeds a 'struct cdev' which means f_op->release()
> may free cdev, so __fput() needs to drop the cdev reference before
> calling f_op->release().
NAK. You *can't* free a structure that contains kobj with currently
positive refcount. Ever. If you embed a struct kobj into something,
you must use the refcount of that kobj (or one of its ancestors) to
control the lifetime of containing object. If your dax_dev_put() can
trigger freeing of dax_dev despite the still-positive refcount of
embedded cdev.kobj, it is fundamentally broken.
Ah, ok. I missed that cdev_put() drops a parent kobj ref, NULL in my
case. So that "put_device(dev)" above can just be delegated to
cdev_put() and I can remove the kref behind dax_dev_put(). Thank you
for straightening me out!