linux-kernel - Re: [PATCH v2] kset- and class-registration cleanups

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Message-ID: <87wppvlbh2.fsf@gmail.com>
Date:	Tue, 23 Feb 2016 15:58:49 +0100
From:	Nicolai Stange <nicstange@...il.com>
To:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:	Nicolai Stange <nicstange@...il.com>,
	Theodore Ts'o <tytso@....edu>,
	Andreas Dilger <adilger.kernel@...ger.ca>,
	Mark Fasheh <mfasheh@...e.com>,
	Joel Becker <jlbec@...lplan.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Joe Perches <joe@...ches.com>, linux-kernel@...r.kernel.org,
	linux-ext4@...r.kernel.org, ocfs2-devel@....oracle.com
Subject: Re: [PATCH v2] kset- and class-registration cleanups

Nicolai Stange <nicstange@...il.com> writes:

> In order to have something to base further discussion on, here comes the
> second version.
>

Hi Greg,

I'd like to ask whether this is still in your review queue or did it
slip through?

Should I resend?

Thank you,

Nicolai

> In addition to some changes to the inital patch (now [1/3]), two
> additional ones have been introduced.
>
> The three patches depend on one another in sequence.
>
> For a detailed changelist, see the end of this mail.
>
> Greg Kroah-Hartman <gregkh@...uxfoundation.org> writes:
>> On Tue, Nov 17, 2015 at 01:04:19AM +0100, Nicolai Stange wrote:
>> Yes, but those calls all succeed, so this isn't a problem in the "real"
>> world :)
>
> I added a word about non-impact in the commit message of [1/3].
>
>>> +static void glue_dirs_release_dummy(struct kobject *kobj)
>>> +{
>>> +	/*
>>> +	 * The glue_dirs kset member of struct subsys_private is never
>>> +	 * registered and thus, never unregistered.
>>> +	 * This release function is a dummy to make kset_init() happy.
>>> +	 */
>>> +	pr_err(
>>> +	"class (%p): unexpected kset_put() on glue_dirs, something is broken.",
>>> +		container_of(kobj, struct subsys_private,
>>> +				glue_dirs.kobj)->class);
>>> +	dump_stack();
>>
>> How can this ever happen?
>
> Not at all. I'm not sure I understand you correctly here.
> Do you strictly want to abandon the dummy releaser, or more specifically,
> the dummy kobj_type?
> For the moment, I turned the pr_err()/dump_stack() into a WARN() for the
> sake of better style.
>
>>>  	if (error) {
>>> -		kfree(cp);
>>> +		/*
>>> +		 * class->release() would be called by cp->subsys'
>>> +		 * release function. Prevent this from happening in
>>> +		 * the error case by zeroing cp->class out.
>>> +		 */
>>> +		cp->class = NULL;
>>> +		cls->p = NULL;
>>> +		kset_put(&cp->subsys);
>>
>> That seems pretty messy, why can't we allow the release to be called?  I
>> don't think this is really correct :(
>
> At the moment, it is necessary. I've added [3/3] for the case that you
> want cls->class_release() to get called from __class_register() on error.
> In [3/3] you will also find the (few) callers expecting the behaviour as
> it currently is.
>
>
> Detailed changes from initial version to v2:
> [1/3] lib/kobject: fix memory leak in error path of kset_create_and_add()
> This one is the original patch with a few changes:
> - added a word of non-impact to commit message
> - use WARN() instead of open coded pr_error()/dump_stack() pair in
>   struct class' glue_dirs' dummy class releaser.
> - follow my own advice in the docstring of kset_register() and
>   use kset_put() instead of kset_unregister() on error of
>   kset_register() in ext4's ext4_init_sysfs() and ocfs2's mlog_sys_init().
>
> [2/3] drivers/base/class: __class_register(): make error behaviour consistent
> This one is new and quite unrelated to the original patch.
> Following the sidenote made in my last mail, it makes __class_register()
> properly clean up after itself on error.
>
> [3/3] drivers/base/class: __class_register(): invoke class' releaser on failure
> This one is new.
> It addresses Greg K-H's comment on the initial version about the messiness
> of avoiding to call class->class_release() from __class_register() on
> error. Given the fact that I had to introduce an explicit
> cls->class_release() in the early part when there is no kset object
> available yet, I'm by no means sure that it is much less messy now and
> that this patch is worth the trouble.
> -> It is up to you to judge.
> As there are enough people bothered already, I did not CC the people
> suggested for this one by get_maintainer.pl: all of them are maintainers
> of external subsystems and probably not particularly interested in
> __class_register() and friends. I will do this as soon as
> a decision for this patch has been made, perhaps in a separate thread.