linux-kernel - Re: [PATCH v2] staging: gdm72xx: fix reference counting in gdm_wimax_event

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Date:	Wed, 8 Aug 2012 12:50:38 -0700
From:	Ben Chan <benchan@...omium.org>
To:	"devendra.aaru" <devendra.aaru@...il.com>
Cc:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	devel@...verdev.osuosl.org, linux-kernel@...r.kernel.org,
	Dan Carpenter <dan.carpenter@...cle.com>
Subject: Re: [PATCH v2] staging: gdm72xx: fix reference counting in gdm_wimax_event_init

Hi,

Does patch v2 make sense?

Thanks,
Ben

On Wed, Jul 25, 2012 at 6:53 AM, Ben Chan <benchan@...omium.org> wrote:
> Hi Devendra,
>
> Thanks for cleaning up the driver.  If I understand the code
> correctly, the original author wanted to initialize wm_event once and
> reuse it for multiple devices, and thus reference counted it with
> ref_cnt.
>
> For instance, each time gdm_usb_probe() is called, it may call
> register_wimax_device() / gdm_wimax_event_init(). wm_event is
> initialized the first time when wm_event.ref_cnt == 0 (alternatively,
> the code could check !wm_event.sock). Subsequent calls to
> gdm_wimax_event_init() will simply increase the ref count. Similarly,
> gdm_usb_disconnect() calls unregister_wimax_device() /
> gdm_wimax_event_exit(), which decreases the ref count and disposes
> wm_event when ref_cnt becomes zero.
>
> The code change in commit 8df858ea76b76dde9a39d4edd9aaded983582cfe
> only prevents ref_cnt from increasing beyond one. So the code no
> longer work when there are multiple devices (i.e. wm_event could be
> disposed even when there is an active device).
>
> Thanks,
> Ben
>
>
> On Tue, Jul 24, 2012 at 9:50 PM, devendra.aaru <devendra.aaru@...il.com> wrote:
>> On Tue, Jul 24, 2012 at 8:34 PM, Ben Chan <benchan@...omium.org> wrote:
>>> This patch fixes the commit "staging/gdm72xx: cleanup little at
>>> gdm_wimax_event_rcv" (8df858ea76b76dde9a39d4edd9aaded983582cfe),
>>> which mishandles the reference counting of wm_event.
>>>
>>> Signed-off-by: Ben Chan <benchan@...omium.org>
>>> ---
>>> Fixed the commit message as suggested by Dan Carpenter.
>>>
>>>  drivers/staging/gdm72xx/gdm_wimax.c |   16 ++++++++++------
>>>  1 files changed, 10 insertions(+), 6 deletions(-)
>>>
>>> diff --git a/drivers/staging/gdm72xx/gdm_wimax.c b/drivers/staging/gdm72xx/gdm_wimax.c
>>> index 0716efc..6cb8107 100644
>>> --- a/drivers/staging/gdm72xx/gdm_wimax.c
>>> +++ b/drivers/staging/gdm72xx/gdm_wimax.c
>>> @@ -258,12 +258,16 @@ static int gdm_wimax_event_init(void)
>>>         if (!wm_event.ref_cnt) {
>>>                 wm_event.sock = netlink_init(NETLINK_WIMAX,
>>>                                                 gdm_wimax_event_rcv);
>>> -               if (wm_event.sock)
>>> -                       wm_event.ref_cnt++;
>>> -               INIT_LIST_HEAD(&wm_event.evtq);
>>> -               INIT_LIST_HEAD(&wm_event.freeq);
>>> -               INIT_WORK(&wm_event.ws, __gdm_wimax_event_send);
>>> -               spin_lock_init(&wm_event.evt_lock);
>>> +               if (wm_event.sock) {
>>> +                       INIT_LIST_HEAD(&wm_event.evtq);
>>> +                       INIT_LIST_HEAD(&wm_event.freeq);
>>> +                       INIT_WORK(&wm_event.ws, __gdm_wimax_event_send);
>>> +                       spin_lock_init(&wm_event.evt_lock);
>>> +               }
>>> +       }
>>> +
>>> +       if (wm_event.sock) {
>>> +               wm_event.ref_cnt++;
>>>                 return 0;
>>>         }
>>>
>>> --
>>> 1.7.7.3
>>>
>>
>> Hi Ben,
>>
>> I have some basic understanding about this flow of the function,
>>
>> Here is my understanding of the thing i have done when i was doing
>> some cleanups to this driver.
>>
>> The ref_cnt will be 0 at first time. if so the sock creation happens
>> only once, and register_wimax_device only calls this function.
>> so i think doing the ref_cnt++ inside the if (!wm_event.ref_cnt) is valid.
>>
>> Please suggest me if i am wrong.
>>
>> Thanks,
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