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

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Message-ID: <CACVXFVN3wJ3NWxSGj-yWCgtDE_sgJT5CZYHwYUWk1MxkphcsTg@mail.gmail.com>
Date:	Mon, 25 Jun 2012 11:37:20 +0800
From:	Ming Lei <tom.leiming@...il.com>
To:	Bjørn Mork <bjorn@...k.no>
Cc:	Oliver Neukum <oliver@...kum.org>, 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

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.

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.

> 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.

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.

So it is only the sync mechanism that  works on the race even the check is
added in the patch.  Putting usb_set_intfdata(, NULL) after driver_info->unbind
should be OK, and it is a general solution for the problem.

Thanks,
-- 
Ming Lei
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