linux-kernel - Re: [PATCH] device-dax: don't set kobj parent during cdev init

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Date:   Sat, 11 Feb 2017 10:58:28 -0800
From:   Dan Williams <dan.j.williams@...el.com>
To:     Logan Gunthorpe <logang@...tatee.com>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Johannes Thumshirn <jthumshirn@...e.de>,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        Sajjan Vikas C <vikas.cha.sajjan@....com>,
        Arnd Bergmann <arnd@...db.de>, Jan Kara <jack@...e.cz>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-nvdimm@...ts.01.org" <linux-nvdimm@...ts.01.org>
Subject: Re: [PATCH] device-dax: don't set kobj parent during cdev init

On Sat, Feb 11, 2017 at 10:55 AM, Dan Williams <dan.j.williams@...el.com> wrote:
> On Sat, Feb 11, 2017 at 10:43 AM, Logan Gunthorpe <logang@...tatee.com> wrote:
>>
>> On 11/02/17 11:27 AM, Dan Williams wrote:
>>> Why, when the lifetime of the cdev is already correct?
>>
>> Well, it's only correct if you use the kobj parent trick which Greg is
>> arguing against. As someone reviewing/copying code that trick is
>> unclear, undocumented and it looks rather odd messing with internal
>> kobjects. Taking the explicit reference would be very clear, very
>> standard and only net one additional line.
>>
>>> See commit ba09c01d2fa8 "dax: convert to the cdev api". I used to take
>>> explicit references like you suggest, but cdev made it cleaner.
>>
>> I agree that, on the whole, that patch makes things a good deal cleaner.
>> I'm not so sure that this one small aspect is an improvement.
>>
>> In any case, it's up to you. If you'd like I can certainly submit a v2
>> patch that adds the get/put.
>
> Can we meet in the middle and just add some comments about what is going on?
>
> It's a shame to add reference counts for something that is already
> properly reference counted.

Also when using an embedded cdev how would you recommend avoiding this problem?

https://lists.01.org/pipermail/linux-nvdimm/2016-August/006562.html