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

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Date:	Mon, 25 Jun 2012 15:15:21 +0800
From:	Ming Lei <tom.leiming@...il.com>
To:	Oliver Neukum <oliver@...kum.org>
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

On Mon, Jun 25, 2012 at 2:15 PM, Oliver Neukum <oliver@...kum.org> wrote:
> Am Montag, 25. Juni 2012, 05:37:20 schrieb Ming Lei:

>> 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.

Any locking isn't needed if the set to NULL is put after
driver_info->unbind,  since ->unbind will call subdriver->disconnect,
which will hold the open/disconnect lock of wdm_mutex.

>
>> 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.
>

Could you let us know the reason? I think it may let the
patch not necessary.

>> 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.

The check isn't needed if the set to NULL is put after  driver_info->unbind
in usbnet_disconnect.

I think we can add the comment on it to avoid the unnecessary check.

Thanks,
-- 
Ming Lei
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