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

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Message-ID: <87hatzhn0k.fsf@nemi.mork.no>
Date:	Mon, 25 Jun 2012 09:24:43 +0200
From:	Bjørn Mork <bjorn@...k.no>
To:	Ming Lei <tom.leiming@...il.com>
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

[I don't have anything more to add to the generic usbnet discussion, but
 want to comment on the qmi_wwan issues]

Ming Lei <tom.leiming@...il.com> writes:

> 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.

True, but irrelevant.  The pointer is either valid or NULL.  We don't
need to care about synchronizing the exact time it is set to NULL.

The locking in cdc-wdm will ensure that the pointer is valid while it is
in use by .manage_power, because usbnet_disconnect is prevented from
continuing with free_netdev() while any caller of .manage_power is
running.

> 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.

There is no problem wrt qmi_wwan and intfdata as long as the NULL test
is added to .manage_power.

Bjørn
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