netdev - Re: [PATCH] net/usbnet: avoid recursive locking in usbnet

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Date:	Mon, 20 Feb 2012 16:40:49 -0500 (EST)
From:	Alan Stern <stern@...land.harvard.edu>
To:	Sebastian Andrzej Siewior <bigeasy@...utronix.de>
cc:	Oliver Neukum <oliver@...kum.org>, <netdev@...r.kernel.org>,
	<linux-usb@...r.kernel.org>
Subject: Re: [PATCH] net/usbnet: avoid recursive locking in usbnet_stop()

On Mon, 20 Feb 2012, Sebastian Andrzej Siewior wrote:

> > What's the issue here?
> >
> > If a driver calls usb_unlink_urb() while holding a lock, and the
> > completion routine tries to acquire the same lock, then deadlock is
> > possible.  The fact that usb_unlink_urb() is asynchronous is not a
> > guarantee of anything; the HCD is allowed to call the completion
> > handler from within usb_unlink_urb().
> >
> > It's true that the kerneldoc for usb_unlink_urb() says "This request is
> > always asynchronous".  It might be a good idea to remove the word
> > "always", because it seems to give people the wrong idea.
> 
> I see. So you approve that patch and suggest to remove the "always"
> wording plus adding something like "the hcd might call complete routine
> during unlink" ?

Well, I haven't read the patch and don't really understand what issue 
it tries to solve.  But if that issue is the one I talked about above 
then yes, it makes sense.  And changing the documentation as you 
suggest would be a good thing to do in any case.

Alan Stern

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