Re: Potential data race in psmouse_interrupt

[Dmitry Vyukov]

On Fri, Sep 4, 2015 at 6:56 PM, Dmitry Torokhov
<dmitry.torokhov@xxxxxxxxx> wrote:
> On Tue, Sep 1, 2015 at 11:46 AM, Dmitry Vyukov <dvyukov@xxxxxxxxxx> wrote:
>> On Fri, Aug 28, 2015 at 8:32 PM, Dmitry Torokhov
>> <dmitry.torokhov@xxxxxxxxx> wrote:
>>> On Fri, Aug 28, 2015 at 11:08 AM, Dmitry Vyukov <dvyukov@xxxxxxxxxx> wrote:
>>>> On Fri, Aug 28, 2015 at 7:51 PM, Dmitry Torokhov
>>>> <dmitry.torokhov@xxxxxxxxx> wrote:
>>>>> On Fri, Aug 28, 2015 at 10:34 AM, Dmitry Vyukov <dvyukov@xxxxxxxxxx> 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.
>>
>> Right.
>>
>>>>
>>>> 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.
>>
>> I don't know about serio_pause_rx, but copying of response should be
>> done after checking cmdcnt.
>
> It will stop the interrupt handler from running while we are examining
> the cmdcnt and copy out the data, thus removing the race.
>
>> Also you need to use smp_store_release/smp_load_acquire cmdcnt and
>> flags when they have dependent data. And READ_ONCE/WRITE_ONCE on
>> shared state otherwise is highly desirable.
>>
>>>> diff --git a/drivers/input/serio/libps2.c b/drivers/input/serio/libps2.c
>>>> index 7551699..51c747f 100644
>>>> --- a/drivers/input/serio/libps2.c
>>>> +++ b/drivers/input/serio/libps2.c
>>>> @@ -43,7 +43,7 @@ int ps2_sendbyte(struct ps2dev *ps2dev, unsigned
>>>> char byte, int timeout)
>>>>
>>>>          if (serio_write(ps2dev->serio, byte) == 0)
>>>>                  wait_event_timeout(ps2dev->wait,
>>>> -                                   !(ps2dev->flags & PS2_FLAG_ACK),
>>>> +                                   !(READ_ONCE(ps2dev->flags) & PS2_FLAG_ACK),
>>>>                                     msecs_to_jiffies(timeout));
>>>>
>>>>          serio_pause_rx(ps2dev->serio);
>>>> @@ -187,6 +187,7 @@ int __ps2_command(struct ps2dev *ps2dev, unsigned
>>>> char *param, int command)
>>>>          int receive = (command >> 8) & 0xf;
>>>>          int rc = -1;
>>>>          int i;
>>>> +        unsigned char cmdcnt;
>>>>
>>>>          if (receive > sizeof(ps2dev->cmdbuf)) {
>>>>                  WARN_ON(1);
>>>> @@ -225,23 +226,22 @@ int __ps2_command(struct ps2dev *ps2dev,
>>>> unsigned char *param, int command)
>>>>          timeout = msecs_to_jiffies(command == PS2_CMD_RESET_BAT ? 4000 : 500);
>>>>
>>>>          timeout = wait_event_timeout(ps2dev->wait,
>>>> -                                     !(ps2dev->flags &
>>>> PS2_FLAG_CMD1), timeout);
>>>> -
>>>> -        if (ps2dev->cmdcnt && !(ps2dev->flags & PS2_FLAG_CMD1)) {
>>>> +                !(READ_ONCE(ps2dev->flags) & PS2_FLAG_CMD1), timeout);
>>>>
>>>> +        if (READ_ONCE(&ps2dev->cmdcnt) &&
>>>> +                        !(READ_ONCE(&ps2dev->flags) & PS2_FLAG_CMD1)) {
>>>>                  timeout = ps2_adjust_timeout(ps2dev, command, timeout);
>>>>                  wait_event_timeout(ps2dev->wait,
>>>> -                                   !(ps2dev->flags & PS2_FLAG_CMD), timeout);
>>>> +                        !(READ_ONCE(&ps2dev->flags) & PS2_FLAG_CMD), timeout);
>>>
>>> What all these READ_ONCE()s give us?
>>
>> I've wrote up the response here:
>> https://github.com/google/ktsan/wiki/READ_ONCE-and-WRITE_ONCE
>
> I read it and I still do not understand what READ_ONCE() in
> wait_event* conditions will buy us.
>
> Also if the following is true:
>
>> As the consequence C compilers stopped guarantying that "word accesses are atomic".
>
> a lot of stuff will break in the kernel. Maybe compilers should stop
> moving towards the lala land?

It buys us:
1. More readable code but highlighting important aspects. Inter-thread
synchronization is important and complex, explicit is better than
implicit in such contexts.
2. Conformance to relevant standards and relieve you, me and everybody
else reading this code from spending time on proving that it cannot
break (which is not actually possible to do, "I don't see how it can
break" is not quite proof).
3. Allow tooling that finds undoubtedly harmful bugs, like this one.

I don't see any negative aspects to it. Do you see any? Because if you
see at least some value in at least on these points and don't see any
negative aspects, then it is worth doing.



-- 
Dmitry Vyukov, Software Engineer, dvyukov@xxxxxxxxxx
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