On Wed, Aug 10, 2016 at 10:16 PM, Al Viro <[email protected]> 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__); >> put_device(dev); >> dax_dev_put(dax_dev); >> >> 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!
