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

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Date:	Thu, 12 Apr 2007 17:23:18 -0400 (EDT)
From:	Alan Stern <stern@...land.harvard.edu>
To:	USB development list <linux-usb-devel@...ts.sourceforge.net>
cc:	Kernel development list <linux-kernel@...r.kernel.org>
Subject: How should an exit routine wait for release() callbacks?

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?

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?

Alan Stern

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