linux-kernel - Re: [PATCH RESEND] char_dev: add cdev->release() and convert cdev

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Message-ID: <48B6E417.5030605@kernel.org>
Date:	Thu, 28 Aug 2008 19:44:55 +0200
From:	Tejun Heo <tj@...nel.org>
To:	Greg KH <greg@...ah.com>
CC:	Andrew Morton <akpm@...ux-foundation.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH RESEND] char_dev: add cdev->release() and convert	cdev_alloc()
 to use it

Greg KH wrote:
> On Thu, Aug 28, 2008 at 06:56:48PM +0200, Tejun Heo wrote:
>> Greg KH wrote:
>>> Ick, I really don't want struct cdev to be used for lifecycle
>>> management, as it is only for major:minor stuff.  Why do you want to
>>> make this change?
>> Well, as cdev can be referenced from userspace, ->release is required
>> for most purposes.  The reason why devices have been getting by without
>> it is because most chardevs are created on module load and destroyed on
>> module unload and in the meantime cdev refcount virtually equals module
>> refcnt, but I'm fairly sure we have cases where cdev can be destroyed
>> for other reasons then module unloading and it's very likely those cases
>> are buggy in the current code (backing structure gone bug cdev still
>> hanging around).
> 
> Hm, I thought Al covered that when he created the cdev interface, I
> would be a bit supprised if this was the case.

Hmmm.... I've never actually audited the code so... it could be that no
chardev is created and destroyed that way, I guess.

>> As CUSE can create and destroy devices regardless of module reference
>> count, it falls in the second category and needs cdev->release() to make
>> sure the backing structure doesn't go away till cdev is released.
> 
> But CUSE should be it's own module, right?  And it would "own" the cdev,
> so the module and cdev count should be fine and matching.  The userspace
> code could go away but the CUSE code should handle that with a different
> reference count.  This is the way that hardware drivers handle the
> issue.

The problem is not the device to talk to CUSE (/dev/cuse as in
/dev/fuse), for which module refcount and device refcount match fine.
But the whole point of CUSE is allowing CUSE clients to create arbitrary
character devices, so in addition to /dev/cuse which clients use to talk
to CUSE, CUSE hosts character devices for its clients and they come and
go dynamically and thus requires proper lifetime management.

Thanks.

-- 
tejun
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