Re: [PATCH] net/usbnet: avoid recursive locking in usbnet_stop()

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

quoted

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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