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

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Message-ID: <CAPcyv4hFBqVOJeWW0bqqV+vMVzTWnz3jOFi+kzDi=_xXBkTD1w@mail.gmail.com>
Date:   Sat, 11 Feb 2017 10:27:37 -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 9:59 AM, Logan Gunthorpe <logang@...tatee.com> wrote:
> On 11/02/17 01:56 AM, Dan Williams wrote:
>>
>> When the device is unregistered it invalidates all existing mappings,
>> but the driver may continue to service vm fault requests until the
>> final put of the cdev. Until that time the fault handler needs to be
>> able to check dax_dev->alive. Since the final cdev put is handled by
>> the vfs I use the cdev's kobject to keep the struct dax_dev instance
>> alive.
>
>
> I'm just taking a wild stab at this, but would it not make sense to just
> take a reference to the dax_dev device in dax_open and put it back it in
> dax_release? (Or perhaps, in the open/close of the vm_ops.) That way the
> structure won't be free'd until there are no users and alive will always be
> accessible.
>
> It would also be a bit more clear as to what's going on because you are
> actually making a reference in filp->private_data.
>

Why, when the lifetime of the cdev is already correct?

See commit ba09c01d2fa8 "dax: convert to the cdev api". I used to take
explicit references like you suggest, but cdev made it cleaner.