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 -- To unsubscribe from this list: send the line "unsubscribe linux-input" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
