linux-kernel - Re: How should an exit routine wait for release() callbacks?

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Message-ID: <461F6C8C.1020901@amd.com>
Date:	Fri, 13 Apr 2007 13:42:04 +0200
From:	"Markus Rechberger" <markus.rechberger@....com>
To:	"Cornelia Huck" <cornelia.huck@...ibm.com>
cc:	"Alan Stern" <stern@...land.harvard.edu>,
	"USB development list" <linux-usb-devel@...ts.sourceforge.net>,
	"Kernel development list" <linux-kernel@...r.kernel.org>
Subject: Re: How should an exit routine wait for release() callbacks?

Alan,

seems like you have the same problem as the dvb framework has/had.

http://mcentral.de/hg/~mrec/v4l-dvb-stable

The last 3 changesets do the trick to not oops, it will delay the 
deinitialization of the device till the last user closed the device node.

Markus

Cornelia Huck wrote:
> On Thu, 12 Apr 2007 17:23:18 -0400 (EDT),
> Alan Stern <stern@...land.harvard.edu> wrote:
>
>   
>> Here's a not-so-theoretical question.
>>
>> I've got a module which registers a struct device.  (It represents a
>> virtual device, not a real one, but that doesn't matter.)  Obviously the
>> module's exit routine has to wait until the release() routine for that
>> device has been invoked -- if it returned too early then the release()
>> call would oops.
>>
>> How should it wait?
>>     
>
> Device lifetime vs. module lifetime - that's a fun one...
>
>   
>> The most straightforward approach is to use a struct completion, like 
>> this:
>>
>> 	static struct {
>> 		struct device dev;
>> 		...
>> 	} my_dev;
>>
>> 	static DECLARE_COMPLETION(my_completion);
>>
>> 	static void my_release(struct device *dev)
>> 	{
>> 		complete(&my_completion);
>> 	}
>>
>> 	static void __exit my_exit(void)
>> 	{
>> 		device_unregister(&my_dev.dev);
>> 		wait_for_completion(&my_completion);
>> 	}
>>
>> The problem is that there is no guarantee a context switch won't take
>> place after my_release() has called complete() and before my_release()  
>> returns.  If that happens and my_exit() finishes running, then the module
>> will be unloaded and the next context switch back to finish off
>> my_release() will oops.
>>
>> Other approaches have similar defects.  So how can this problem be solved?
>>     
>
> What I see that a device driver may do now is the following:
> - disallow module unloading (duh)
> - move the release function outside the module
>
> To make the completion approach work, the complete() would need to be
> after the release function. This would imply an upper layer, but this
> upper layer would need to access the completion structure in the
> module...
>
> One could think about a owner field (for getting/putting the module
> reference) for the object (with a final module_put() after the release
> function has been called). The problem there would be that it would
> preclude unloading of the module if there isn't a "self destruct" knob
> for the object.
> -
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>
>
>
>   


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