Re: Potential data race in psmouse_interrupt

[Dmitry Torokhov <dmitry.torokhov@gmail.com>]

On Fri, Aug 28, 2015 at 11:32 AM, Dmitry Torokhov
<dmitry.torokhov@gmail.com> wrote:
> On Fri, Aug 28, 2015 at 11:08 AM, Dmitry Vyukov <dvyukov@google.com> wrote:
>> On Fri, Aug 28, 2015 at 7:51 PM, Dmitry Torokhov
>> <dmitry.torokhov@gmail.com> wrote:
>>> On Fri, Aug 28, 2015 at 10:34 AM, Dmitry Vyukov <dvyukov@google.com> wrote:
>>>> Hello,
>>>>
>>>> I am looking at this code in __ps2_command again:
>>>>
>>>> /*
>>>> * The reset command takes a long time to execute.
>>>> */
>>>> timeout = msecs_to_jiffies(command == PS2_CMD_RESET_BAT ? 4000 : 500);
>>>>
>>>> timeout = wait_event_timeout(ps2dev->wait,
>>>>     !(READ_ONCE(ps2dev->flags) & PS2_FLAG_CMD1), timeout);
>>>>
>>>> if (smp_load_acquire(&ps2dev->cmdcnt) &&
>>>>     !(smp_load_acquire(&ps2dev->flags) & PS2_FLAG_CMD1)) {
>>>>               timeout = ps2_adjust_timeout(ps2dev, command, timeout);
>>>>               wait_event_timeout(ps2dev->wait,
>>>>                         !(smp_load_acquire(&ps2dev->flags) &
>>>> PS2_FLAG_CMD), timeout);
>>>> }
>>>>
>>>> if (param)
>>>>     for (i = 0; i < receive; i++)
>>>>               param[i] = ps2dev->cmdbuf[(receive - 1) - i];
>>>>
>>>>
>>>> Here are two moments I don't understand:
>>>> 1. The last parameter of ps2_adjust_timeout is timeout in jiffies (it
>>>> is compared against 100ms). However, timeout is assigned to result of
>>>> wait_event_timeout, which returns 0 or 1. This does not make sense to
>>>> me. What am I missing?
>>>
>>> The fact that wait_event_timeout can return value greater than one:
>>>
>>>  * Returns:
>>>  * 0 if the @condition evaluated to %false after the @timeout elapsed,
>>>  * 1 if the @condition evaluated to %true after the @timeout elapsed,
>>>  * or the remaining jiffies (at least 1) if the @condition evaluated
>>>               ^^^^^^^^^^^^^^^^^^^^^^^^^
>>
>>
>> OK, makes sense now!
>>
>>>> 2. This code pays great attention to timeouts, but in the end I don't
>>>> see how it handles timeouts. That is, if a timeout is happened, we
>>>> still copyout (garbage) from cmdbuf. What am I missing here?
>>>
>>> Once upon a time wait_event() did not return positive value when
>>> timeout expired and then condition satisfied. So we just examine the
>>> final state (psmpouse->cmdcnt should be 0 if command actually
>>> succeeded) and even if we copy in garbage nobody should care since
>>> we'll return error in this case.
>>
>>
>> I see.
>> But the cmdcnt is re-read after copying out response. So it is
>> possible that we read garbage response, but then read cmdcnt==0 and
>> return OK to caller.
>
> That assumes that we actually timed out, and while we were copying the
> data the response finally came.
>
>>
>> So far I have something along the following lines to fix data races in libps2.c
>
> I don't know, maybe we should simply move call to
> serio_pause_rx(ps2dev->serio) higher, before we check ps2dev->cmdcnt,
> and move copying of the buffer down, after checking cmdcnt.

By the way, please either drop ktsan group from public postngs or open
it to post from public.

Thanks.

-- 
Dmitry