linux-kernel - Re: [PATCH 01/11] watchdog: WatchDog Timer Driver Core

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Date:	Fri, 29 Jul 2011 11:24:32 +0200
From:	Lars-Peter Clausen <lars@...afoo.de>
To:	Wim Van Sebroeck <wim@...ana.be>
CC:	LKML <linux-kernel@...r.kernel.org>,
	Linux Watchdog Mailing List <linux-watchdog@...r.kernel.org>,
	Alan Cox <alan@...rguk.ukuu.org.uk>
Subject: Re: [PATCH 01/11] watchdog: WatchDog Timer Driver Core - Add basic
 framework

On 07/27/2011 10:24 PM, Wim Van Sebroeck wrote:
> Hi Lars-Peter,
> 
>>>>> +int watchdog_dev_unregister(struct watchdog_device *watchdog)
>>>>> +{
>>>>> +	/* Check that a watchdog device was registered in the past */
>>>>> +	if (!test_bit(0, &watchdog_dev_busy) || !wdd)
>>>>> +		return -ENODEV;
>>>>> +
>>>>> +	/* We can only unregister the watchdog device that was registered */
>>>>> +	if (watchdog != wdd) {
>>>>> +		pr_err("%s: watchdog was not registered as /dev/watchdog.\n",
>>>>> +			watchdog->info->identity);
>>>>> +		return -ENODEV;
>>>>> +	}
>>>>> +
>>>>> +	/* Unregister the miscdevice */
>>>>> +	misc_deregister(&watchdog_miscdev);
>>>>> +	wdd = NULL;
>>>>> +	clear_bit(0, &watchdog_dev_busy);
>>>>> +	return 0;
>>>>> +}
>>>>
>>>> What happens if the watchdog gets unregistered if the device is still opened?
>>>> Even though if you'd check wdd for not being NULL in the file callbacks there
>>>> is still a chance for races if the devices is unregistered at the same time as
>>>> the callback is running. You'd either need a big lock to protect from having a
>>>> file callback and unregister running concurrently or add ref-counting to the
>>>> watchdog_device, the later best done by embedding a struct device and using the
>>>> device driver model.
>>>
>>> You cannot unload the watchdog-drivers module if /dev/watchdog is still open.
>>> So if the watchdog_unregister function is in the exit function of the module
>>> then we are safe. But I think you have a point if that is not the case.
>>> Solution would be to return an error when the watchdog_unregister_device routine
>>> is called and the WDOG_DEV_OPEN bit is set. Will create an extra patch for that.
>>>
>>
>> The problem is, that this doesn't fit nicely into the linux device driver
>> model, because it doesn't allow the removal of a device to fail. So you'll
>> still end of with undefined behavior.
> 
> It's not an issue now. But we will indeed have to tackle it, when we start using
> the device driver model.
> 

Well, the framework itself might not be using the device driver model yet, but
drivers using the framework do. For example for a driver using a platform
device there would be no correct solution for handling an error from
watchdog_unregister in the drivers remove callback.

- Lars
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