netdev - Re: Fwd: usbnet: Recursive Locking bug ?

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Date:	Tue, 7 Dec 2010 11:46:32 -0500 (EST)
From:	Alan Stern <stern@...land.harvard.edu>
To:	Neil Jones <neiljay@...il.com>
cc:	netdev@...r.kernel.org, <linux-usb@...r.kernel.org>
Subject: Re: Fwd: usbnet: Recursive Locking bug ?

On Tue, 7 Dec 2010, Neil Jones wrote:

> > But what makes sure that the URB unlinked in unlink_urbs() stays a valid pointer?
> > The bottom half may run and free the URB.
> 
> I think it may be safe to remove the lock as we are walking a list of
> SKBs not URBs,
> the BH can remove SKB's from the list but we are doing a safe list walk.
> + the BH takse the list lock when it does the remove.
> 
> I could be wrong though?

A simple answer to Oliver's question is to take a reference to the 
URB while still holding the lock, then release the lock before calling 
usb_unlink_urb(), then drop the reference to the URB.

Of course, this also requires you to restart the loop from the
beginning after each unlink, and it means you need to have a way to
recognize when an URB has already been unlinked.

Alan Stern

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