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

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Date:	Mon, 25 Jun 2012 16:08:52 +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 3:24 PM, Bjørn Mork <bjorn@...k.no> wrote:

>
> 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.

What I mean is that the situation is just what moving the set to NULL
is doing.

>> 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.

It depends on the ARCH or compiler.

Considered there is not any locking/memory barrier between the set to NULL
and read the pointer, also no ACCESS_ONCE on read or store the pointer,
reading in .manage_power may see a invalid pointer if the CPU doesn't
support Store Atomicity or the compiler does a byte-at-a-time optimization
on the store[1].

So why not take the correct way in theory? also it is a general solution,
and we can document its usage.

[1], Paul mentioned it in the previous discussion
http://lkml.org/lkml/2012/6/6/280

Thanks,
-- 
Ming Lei
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