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

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Message-ID: <4F42B4E4.5090100@linutronix.de>
Date:	Mon, 20 Feb 2012 22:02:28 +0100
From:	Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:	Alan Stern <stern@...land.harvard.edu>
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 02/20/2012 09:31 PM, Alan Stern wrote:
> 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.

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" ?

>
> Alan Stern
>

Sebastian
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