netdev - Re: usbnet: driver_info->stop required to stop USB interrupts?

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Date:	Tue, 11 Mar 2014 10:10:19 +0100
From:	Oliver Neukum <oliver@...kum.org>
To:	Julius Werner <jwerner@...omium.org>
Cc:	Grant Grundler <grundler@...gle.com>,
	netdev <netdev@...r.kernel.org>, Freddy Xin <freddy@...x.com.tw>,
	"linux-usb@...r.kernel.org" <linux-usb@...r.kernel.org>,
	Allan Chou <allan@...x.com.tw>
Subject: Re: usbnet: driver_info->stop required to stop USB interrupts?

On Mon, 2014-03-10 at 19:53 -0700, Julius Werner wrote:
> I think usbnet_stop() raced with the dev->bh tasklet, which by itself
> might not be a problem (usbnet_stop() later kills the tasklet itself,
> so it should expect that it can be running before that). The issue is
> that it calls usbnet_terminate_urbs() before that, which temporarily
> installs a waitqueue in dev->wait in order to be able to wait on the
> tasklet to run and finish up some queues. The waiting itself looks
> okay, but the access to 'dev->wait' is totally unprotected and can
> race arbitrarily. I think in this case usbnet_bh() managed to succeed
> it's dev->wait check just before usbnet_terminate_urbs() sets it back
> to NULL. The latter then finishes and the waitqueue_t structure on its
> stack gets overwritten by other functions halfway through the
> wake_up() call in usbnet_bh().
> 
> I think the best solution would be to just make dev->wait a directly
> embedded structure inside struct usbnet instead of a pointer to
> something stack-allocated. usbnet_bh() could just call wake_up()
> unconditionally (if empty it will be a noop), and then one other check
> for !dev->wait could be replaced with a call to waitqueue_active().
> Then the waitqueue-internal locks should be enough to protect all
> accesses.

The diagnosis seems spot on. The fix is not quite so simple.
dev->wait is abused as a flag in resume(). The easiest fix is just
to make sure resume() is never called while stopping.

	Regards
		Oliver



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