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

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Date:	Mon, 20 Feb 2012 15:31:07 -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:

> On 02/20/2012 05:21 PM, Oliver Neukum wrote:
> >> defer_bh() takes the lok which is hold during unlink_urbs(). The safe
> >> walk suggest that the skb will be removed from the list and this is done
> >> by defer_bh() so it seems to be okay to drop the lock here.
> >
> > I am afraid there's something wrong in the hcd driver. Async unlink must
> > be possible with a lock held. I cannot approve this patch.
> 
> Hmmm. The comment above unlink() says that. Looking through other hcds
> it seems that musb is not the only one doing it wrong. Oh well...

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.

Alan Stern

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