netdev - Re: [PATCH] usbnet: remove generic hard_header

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Date:	Thu, 13 Feb 2014 12:36:30 +0100
From:	Bjørn Mork <bjorn@...k.no>
To:	Emil Goode <emilgoode@...il.com>
Cc:	Steve Glendinning <steve.glendinning@...well.net>,
	Oliver Neukum <oneukum@...e.de>,
	"David S. Miller" <davem@...emloft.net>,
	Freddy Xin <freddy@...x.com.tw>,
	Eric Dumazet <edumazet@...gle.com>,
	Ming Lei <ming.lei@...onical.com>,
	Paul Gortmaker <paul.gortmaker@...driver.com>,
	Jeff Kirsher <jeffrey.t.kirsher@...el.com>,
	Liu Junliang <liujunliang_ljl@....com>,
	Octavian Purdila <octavian.purdila@...el.com>,
	linux-usb@...r.kernel.org, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] usbnet: remove generic hard_header_len check

Emil Goode <emilgoode@...il.com> writes:

> Yes I should have put a comment in the changelog about this. All skbs that
> are passed to rx_process have their state set to rx_cleanup and just because
> the skb was cloned doesn't mean that we should free the original in a
> different way. As it is I think we are acctually missing a call to
> usb_free_urb that is called on the common rx_cleanup path.

Yes, I wondered about that as well, thinking that this part actually
should be a separate patch for net+stable.

But then I wondered how we could possibly have had a bug like that
living here for so long, and the answer is we haven't.  You don't want
to free the URB.  It is already resubmitted with a different skb as its
buffer.  The call to usb_free_urb in the rx_cleanup path is there only
for the cases where the URB is not resubmitted. The rx_complete callback
is controlling this by updating entry->urb as appropriate.  So it will
always be NULL if rx_process is called, and the call to usb_free_urb has
no effect.

Rearranging this code to always take the same cleanup path is still a
very nice cleanup IMHO.

Bjørn
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