linux-kernel - Re: [RFC PATCH v3 1/9] software_node: Add helper function to unregister arrays of software

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Date:   Wed, 21 Oct 2020 10:54:15 +0100
From:   Dan Scally <djrscally@...il.com>
To:     Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
Cc:     Sakari Ailus <sakari.ailus@...ux.intel.com>,
        linux-kernel@...r.kernel.org, linux-media@...r.kernel.org,
        linux.walleij@...aro.org, prabhakar.mahadev-lad.rj@...renesas.com,
        heikki.krogerus@...ux.intel.com, dmitry.torokhov@...il.com,
        laurent.pinchart+renesas@...asonboard.com,
        kieran.bingham+renesas@...asonboard.com, jacopo+renesas@...ndi.org,
        robh@...nel.org, davem@...emloft.net, linux@...musvillemoes.dk,
        sergey.senozhatsky@...il.com, rostedt@...dmis.org,
        pmladek@...e.com, mchehab@...nel.org, tian.shu.qiu@...el.com,
        bingbu.cao@...el.com, yong.zhi@...el.com, rafael@...nel.org,
        gregkh@...uxfoundation.org, kitakar@...il.com,
        dan.carpenter@...cle.org
Subject: Re: [RFC PATCH v3 1/9] software_node: Add helper function to
 unregister arrays of software_nodes ordered parent to child

On 21/10/2020 10:40, Andy Shevchenko wrote:
> On Tue, Oct 20, 2020 at 11:52:56PM +0100, Dan Scally wrote:
>> On 20/10/2020 11:05, Sakari Ailus wrote:
>>> On Mon, Oct 19, 2020 at 11:58:55PM +0100, Daniel Scally wrote:
>>>> Software nodes that are children of another software node should be
>>>> unregistered before their parent. To allow easy unregistering of an array
>>>> of software_nodes ordered parent to child, add a helper function to loop
>>>> over and unregister nodes in such an array in reverse order.
> ...
>
>>>> + * software_node_unregister_nodes_reverse - Unregister an array of software
>>>> + * nodes in reverse order.
>>>> + * @nodes: Array of software nodes to be unregistered.
>>>> + *
>>>> + * NOTE: The same warning applies as with software_node_unregister_nodes.
>>>> + * Unless you are _sure_ that the array of nodes is ordered parent to child
>>>> + * it is wiser to remove them individually in the correct order.
>>> Could the default order in software_node_unregister_nodes() be reversed
>>> instead? There are no users so this should be easy to change.
>>>
>>> Doing this only one way may require enforcing the registration order in
>>> software_node_register_nodes(), but the end result would be safer.
>>>
>>> What do you think?
>> Yeah fine by me. We can just use software_node_to_swnode(node->parent)
>> within software_node_unregister_nodes() to check that children come
>> after their parents have already been processed. I'll add a patch to do
>> that in the next version of this series, and another changing the
>> ordering of software_node_unregister_node_group() as Andy suggests for
>> consistency.
> I remember it was a big discussion between Rafael, Heikki and Greg KH about
> child-parent release in kobjects. That ended up with few patches to device
> object handling along with one that reversed the order of swnode unregistering
> in test_printf.c. So here is the question who is maintaining the order: a kref
> (via kobject) or a caller?
I would expect the caller to maintain the order correctly, and just have
the register() function validate that the ordering is good and complain
if not.