Re: Potential data race in psmouse_interrupt
From: Dmitry Torokhov
Date: Fri Sep 04 2015 - 12:57:04 EST
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?
Thanks.
--
Dmitry
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