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

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

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.

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.

-- 
Dmitry