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

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Message-ID: <20110804202533.GV4227@infomag.iguana.be>
Date:	Thu, 4 Aug 2011 22:25:33 +0200
From:	Wim Van Sebroeck <wim@...ana.be>
To:	Lars-Peter Clausen <lars@...afoo.de>
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

Hi Lars,

> >>>>> +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.

That is correct. we have nothing for handling the error. So I didn't introduce
a fix for it neither. What I wanted to tell was:
* a driver now registers a miscdevice in its init function and deregisters it
during the exit function. You cannot unload the module as long as the miscdevice
for the watchdog is open.
* a driver that uses the core will register the watchdog device during init and
will only unregister it during exit. Because we prevent the driver of being
unloaded (via try_module_get and module_put), you cannot unload the module as
long as /dev/watchdog is open. So the unregister will not take place.

So in it's current form, we still have no issue.

Kind regards,
Wim.


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