linux-kernel - Re: [PATCH 0/4] treewide: add missing put

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Date:	Mon, 16 Dec 2013 18:18:53 +0100
From:	Levente Kurusa <levex@...ux.com>
To:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>
CC:	Bjorn Helgaas <bhelgaas@...gle.com>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 0/4] treewide: add missing put_device calls

On 12/15/2013 06:03 PM, Greg Kroah-Hartman wrote:
> On Sun, Dec 15, 2013 at 08:55:27AM +0100, Levente Kurusa wrote:
>> On 12/14/2013 06:24 PM, Greg Kroah-Hartman wrote:
>>> On Fri, Dec 13, 2013 at 01:42:05PM -0700, Bjorn Helgaas wrote:
>>>> [+cc Greg]
>>>>
>>>> On Fri, Dec 13, 2013 at 12:22 PM, Levente Kurusa <levex@...ux.com> wrote:
>>>>> Hi,
>>>>>
>>>>> This is just the beginning of patchset-set that aims to fix possible
>>>>> problems caused by not calling put_device() if device_register() fails.
>>>>>
>>>>> The root cause for the need to call put_device() is that the underlying
>>>>> kobject still has a reference count of 1. Thus, device.release() will not
>>>>> be called and the device will just sit there waiting for a put_device().
>>>>> Adding the put_device() also removes the need for the call to kfree() as most
>>>>> release functions already call kfree() on the container of the device.
>>>>>
>>>>> While these have not been experienced, they are potential issues and thus
>>>>> they need to be fixed. Also, they are a few more files that have the same
>>>>> kind of issue, those will be fixed if these are accepted.
>>>>
>>>> Thanks for doing this.  This is the sort of mistake that just gets
>>>> copied everywhere, so fixing the examples in the tree will help
>>>> prevent the problem from spreading more.
>>>>
>>>> I don't know if there's really value in having device_register()
>>>> return an error but rely on the caller to do the put_device().  Are
>>>> there cases where the caller still needs the struct device even if
>>>> device_register() fails?  E.g., could we do something like this
>>>> instead (I know some callers would also require corresponding changes
>>>> to avoid double puts):
>>
>> There are cases where it is needed. There are quite a few files which
>> when device_register() fails, the driver print an error messages.
> 
> That shouldn't be needed, and can be removed.
Yes, we could put a pr_warn() when device_register() fails.


> 
>> IIRC, there are also a few where the device is also unregistered from
>> the specific subsystem's core.
> 
> Do you have a specific example of this?  This should happen in the
> release function of the device already, not in some other code.
> 
Character drivers who register with device_register() call cdev_del() when device_register()
fails. cdev_del() in turn calls kobject_put on the kobject of the device. Of course, this could also
be replaced. Anyways, I have another set of these patches (approx 40) that I will post in a day or so.
With that most (if not all) should be fixed.


-- 
Regards,
Levente Kurusa
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