netdev - Re: [PATCH net] net: qmi_wwan: fix Oops while disconnecting

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Message-Id: <201206250815.39466.oliver@neukum.org>
Date:	Mon, 25 Jun 2012 08:15:39 +0200
From:	Oliver Neukum <oliver@...kum.org>
To:	Ming Lei <tom.leiming@...il.com>
Cc:	Bjørn Mork <bjorn@...k.no>,
	netdev@...r.kernel.org, linux-usb@...r.kernel.org,
	Marius Bjørnstad Kotsbak 
	<marius.kotsbak@...il.com>
Subject: Re: [PATCH net] net: qmi_wwan: fix Oops while disconnecting

Am Montag, 25. Juni 2012, 05:37:20 schrieb Ming Lei:
> On Mon, Jun 25, 2012 at 1:47 AM, Bjørn Mork <bjorn@...k.no> wrote:
> > Oliver Neukum <oliver@...kum.org> wrote:
> >>Am Sonntag, 24. Juni 2012, 11:34:19 schrieb Bjørn Mork:
> >>
> >>> Sorry, I did not understand what you meant we should do here.  The
> >>extra
> >>> usb_set_intfdata(, NULL) in usbnet_disconnect() won't make any
> >>> difference for that piece of code, will it?
> >>
> >>The point is that if it may be set to NULL, we always want it to be set
> >>to
> >>NULL, so we catch bugs.
> >>
> >>> And the USB core ensures that intfdata is set to NULL before any
> >>> reprobing, so that will never be a problem.  That's the reason why it
> >>> seems redundant setting it in usbnet_disconnect().
> >>
> >>The point is that if there is a problem because intfdata is set to
> >>NULL,
> >>there is very likely a problem in form of a race condition, if intfdata
> >>were not set to NULL in usbnet's disconnect handler.
> 
> I don't agree on the assumption.
> 
> The current problem is caused by the set to NULL without any
> protection or sync mechanism on it, and it is really a bug.

Minidrivers can test for NULL.
That may not be enough and locking may be needed.

> Also we didn't say the set to NULL will be cancelled, just delay
> the clear until it is safe to do it, eg. after complete of unregister_netdev()
> and driver_info->unbind, when the previous .open/.close has been
> completed already or the later ones will notice the disconnection
> early and won't touch usb_get_intfdata() any more.

We can move to after unregister_netdev()
I am unhappy with it going after unbind.

> > Thanks for explaining. Yes, that makes sense to me as well.
> >
> > So then the original patch against qmi_wwan should go in, and we should leave usbnet as it is. Are everyone comfortable with that?
> 
> I don't see any races caused by just removing usb_set_intfdata(, NULL)
> or moving it after driver_info->unbind simply in usbnet_disconnect.

They are not caused. They are harder to detect.

> In fact, suppose that .open/.close and .disconnect are run on different CPUs,
> there are no any guarantee that .open/.close can always see the clear action
> immediately. Also, the clear of intfdata may not be observed in .manage_power
> since usb_set_intfdata(, NULL) may be completed after the lock wdm_mutex
> operation.

Sure, it is a debugging aid. It has the drawback that minidrivers have
to be able to deal with intfdata being NULL. That is not hard to do.

	Regards
		Oliver
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