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Re: [PATCH v2] Input: evdev - Add EVIOC mechanism to extract the MT slot state

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On 01/06/2012 11:58 AM, Dmitry Torokhov wrote:
> On Fri, Jan 06, 2012 at 10:56:46AM -0800, Chase Douglas wrote:
>> On 01/06/2012 10:18 AM, Dmitry Torokhov wrote:
>>> Hi Benjamin,
>>>
>>> On Fri, Jan 06, 2012 at 07:00:22PM +0100, Benjamin Tissoires wrote:
>>>> Hi guys,
>>>> I read somewhere in the code of Android a comment in which they
>>>> complain about not being able to retrieve the slots states. So they
>>>> assume they are all at 0.
>>>> So this mechanism is good to have.
>>>> However, back in January 2011, Dmitry raised the problem that this
>>>> code was not thread safe.What happens if 2 applications ask for
>>>> different slots values (let say X.org and utouch-frame)?
>>>
>>> 2 different processes should be fine; the problem would be if 2 threads
>>> of the same process share the same file descriptor. So far the rest of
>>> evdev copes just fine with multiple threads using the same fd (all
>>> operations are atomic in this regard), setting ABS_MT_SLOT before
>>> fetching the state break this property.
>>
>> How is this any different than two threads trying to set a different
>> property, like the fuzz factor of an axis? This seems like something
>> that should be guarded by a lock in userspace, essentially.
> 
> From kernel POV both operations succeed and produce consistent reults.
> Consider EVIOCSABS when one thread using the same FD sets range 0-100
> and another 200-1000. At no time in the kernel we get to state of
> min = 200 and max = 1000. In the end we'll end up with either 0-100 or
> 200-1000 but not mix of both. So the kernle state is internally
> consistent.

I don't see how modifying the slot requested could ever get the kernel
into an inconsistent state.

> With proposed solution one client may request data for slot 2 but
> instead get info for slot 5 if another client manages to slide in.

You can do the same thing with EVIOCSABS. If you don't do proper locking
and handling, two threads can assume they wrote a value to evdev and it
was successful, when in reality only the second thread to make the call
has any effect.

I know there's a slight distinction between these two scenarios, but my
point is that if you are doing multithreaded evdev reading from the same
evdev fd, you are asking for trouble and you need to be careful. That
even goes for modifying any of the other state through EVIOCSABS from
multiple processes. And really, how many programs are out there reading
from the same evdev fd in multiple threads. I'd wager a fair amount of
money the answer is 0.

-- Chase
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