linux-kernel - Re: [RFC PATCH 2/3] usbnet: Avoid potential races in usbnet_deferred

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Date:   Tue, 19 Sep 2017 13:53:39 -0700
From:   Doug Anderson <dianders@...omium.org>
To:     Oliver Neukum <oneukum@...e.com>
Cc:     Guenter Roeck <groeck@...omium.org>,
        Grant Grundler <grundler@...omium.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-usb@...r.kernel.org" <linux-usb@...r.kernel.org>,
        netdev@...r.kernel.org
Subject: Re: [RFC PATCH 2/3] usbnet: Avoid potential races in usbnet_deferred_kevent()

Hi,

On Tue, Sep 19, 2017 at 1:37 PM, Oliver Neukum <oneukum@...e.com> wrote:
> Am Dienstag, den 19.09.2017, 09:15 -0700 schrieb Douglas Anderson:
>> In general when you've got a flag communicating that "something needs
>> to be done" you want to clear that flag _before_ doing the task.  If
>> you clear the flag _after_ doing the task you end up with the risk
>> that this will happen:
>>
>> 1. Requester sets flag saying task A needs to be done.
>> 2. Worker comes and stars doing task A.
>> 3. Worker finishes task A but hasn't yet cleared the flag.
>> 4. Requester wants to set flag saying task A needs to be done again.
>> 5. Worker clears the flag without doing anything.
>>
>> Let's make the usbnet codebase consistently clear the flag _before_ it
>> does the requested work.  That way if there's another request to do
>> the work while the work is already in progress it won't be lost.
>>
>> NOTES:
>> - No known bugs are fixed by this; it's just found by code inspection.
>
> Hi,
>
> unfortunately the patch is wrong. The flags must be cleared only
> in case the handler is successful. That is not guaranteed.
>
>         Regards
>                 Oliver
>
> NACK

OK, thanks for reviewing!  I definitely wasn't super confident about
the patch (hence the RFC).

Do you think that the races I identified are possible to hit?  In
other words: should I try to rework the patch somehow or just drop it?
 Originally I had the patch setting the flags back to true in the
failure cases, but then I convinced myself that wasn't needed.  I can
certainly go back and try it that way...

-Doug