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

[Ming Lei <tom.leiming@gmail.com>]

On Mon, Jun 25, 2012 at 3:24 PM, Bjørn Mork <bjorn@mork.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