On Fri, Feb 4, 2022 at 8:28 AM Bjorn Helgaas <helgaas@xxxxxxxxxx> wrote: > > On Fri, Feb 04, 2022 at 02:51:16PM +0000, Jonathan Cameron wrote: > > On Thu, 3 Feb 2022 16:44:37 -0600 > > Bjorn Helgaas <helgaas@xxxxxxxxxx> wrote: > > > On Mon, Jan 31, 2022 at 11:19:46PM -0800, ira.weiny@xxxxxxxxx wrote: > > > > > + * pci_doe_create_doe_devices - Create auxiliary DOE devices for all DOE > > > > + * mailboxes found > > > > + * @pci_dev: The PCI device to scan for DOE mailboxes > > > > + * > > > > + * There is no coresponding destroy of these devices. This function associates > > > > + * the DOE auxiliary devices created with the pci_dev passed in. That > > > > + * association is device managed (devm_*) such that the DOE auxiliary device > > > > + * lifetime is always greater than or equal to the lifetime of the pci_dev. > > > > > > This seems backwards. What does it mean if the DOE aux dev > > > lifetime is *greater* than that of the pci_dev? Surely you can't > > > access a PCI DOE Capability if the pci_dev is gone? > > > > I think the description is inaccurate - the end of life is the same > > as that of the PCI driver binding to the pci_dev. It'll get cleared > > up if that is unbound etc. > > I don't know much about devm, but I *think* the devm things get > released by devres_release_all(), which is called by > __device_release_driver() after it calls the bus or driver's .remove() > method (pci_device_remove(), in this case). > > So in this case, I think the aux dev is created after the pci_dev and > released after the PCI driver and the PCI core are done with the > pci_dev. I assume some refcounting prevents the pci_dev from actually > being deallocated until the aux dev is done with it. > > I'm not confident that this is a robust situation. devm is a replacement for hand coding driver ->remove() handlers. Anything devm allocated at ->probe() will be freed in the proper reverse order by the driver core after it calls ->remove(). Ideally for pure devm usage the ->remove() handler can be elided altogether. I'll go read this patch to make sure it follows the expected pattern which is: 1/ Parent device driver performs kmalloc(), device_initialize(), and device_add() of a child device. 2/ Parent registers a devm handler for that child device that will trigger device_unregister() at remove During parent device unregister or unbind the devm action will complete device_unregister() for all children first. That process is independent of the device lifetime that can be arbitrarily extended by 3rd party get_device() or CONFIG_DEBUG_KOBJECT_RELEASE. The device core / kobject hierarchy guarantees that the parent device is pinned until after child-device final put event. I.e. final put_device() on a child also triggers a put_device() on the parent paired with the get_device() taken on the parent at device_add() time.
