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

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Message-ID: <87hau2hwna.fsf@nemi.mork.no>
Date:	Sat, 23 Jun 2012 17:32:09 +0200
From:	Bjørn Mork <bjorn@...k.no>
To:	Ming Lei <tom.leiming@...il.com>
Cc:	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

Ming Lei <tom.leiming@...il.com> writes:
> On Sat, Jun 23, 2012 at 4:45 PM, Bjørn Mork <bjorn@...k.no> wrote:
>
>> usbnet_disconnect() cannot continue to the point where it frees the
>> netdev before wdm_open/wdm_close has completed, because it waits for
>> qmi_wwan_unbind which waits for wdm_disconnect.  And wdm_disconnect
>> takes the both read and write mutexes.
>
> Thanks for your explanation.
>
> I see, and your patch is correct, but it might not be generic enough.
>
> So driver_info->unbind will sync with .open/.close of the subdriver,
> just like unregister_netdev will sync with .open/.close of usbnet interface.
>
> IMO, the general solution is to keep intfdata's lifetime after ->unbind,
> which can guarantee that intfdata can be accessed safely in .open/.close
> of usbnet interface and the other subdrivers' device.
>
> Suppose there will be another usbnet driver which has its own subdriver
> too, the same trick of checking need to be added again if not taking the
> general way of simply removing 'usb_set_intfdata(intf, NULL);' in
> usbnet_disconnect.

Yes, I guess so.

I am just worrying (maybe too much) about the unknown consequences of
removing that code in usbnet, not fully understanding why it was there
in the first place.  And I do not want to take the blame and cleanup
work if anything goes wrong :-) Fixing it in qmi_wwan feels much safer.

But if you want to remove the 'usb_set_intfdata(intf, NULL)', and the
other USB gurus agree, then I'm of course not going to object to that
alternative solution to this bug.

I still think it's unlikely this will ever hit another driver though. 


Bjørn
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