Re: [PATCH 02/13] driver core: Remove the link if there is no driver with AUTO flag

[Evan Green]

On Mon, Dec 31, 2018 at 8:52 PM Yong Wu <yong.wu@xxxxxxxxxxxx> wrote:
>
> DL_FLAG_AUTOREMOVE_CONSUMER/SUPPLIER means "Remove the link
> automatically on consumer/supplier driver unbind", that means we should
> remove whole the device_link when there is no this driver no matter what
> the ref_count of the link is.
>
> CC: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
> Signed-off-by: Yong Wu <yong.wu@xxxxxxxxxxxx>
> ---
> The ref_count of our device_link normally is over 1. When the consumer
> device driver is removed, whole the device_link should be removed.
> Thus, I add this patch.
> ---

I will admit to reading about device links for the first time while
reviewing this patch, but I don't really get this. Why use a kref at
all if we're just going to ignore its value? For instance, I see that
if you call device_link_add() with the same supplier and consumer, it
uses the kref to return the same link. That machinery is broken with
your change. Although I don't see any uses of it, you might also
expect a supplier or consumer could do a kref_get() on the link it got
back from device_link_add(), and have a reasonable expectation that
the link wouldn't be freed out from under it. This would also be
broken.

Can you explain why your device_links normally have a reference count
>1, and why those additional references can't be cleaned up in an
orderly fashion?

(To be honest, I don't really understand the case for the AUTOREMOVE
flags at all. Is there some case where the party that set up the link
can't tear it down? Or is this a way to devm_ify the link, where devm
itself doesn't work because the links themselves stall out that
mechanism?)