On Fri, Jan 06, 2012 at 12:09:36PM -0800, Chase Douglas wrote: > 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. It may cause client get data that it did not request. In other worse it kernel may supply wrong data to the caller. > > > 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. As with pretty much any other resource; but there is a reason we have atomic variables and operations. The distinction is that both operations carried out completely and consistently. > > 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. I am really not concerned about what userspace might do - I've looked at enough code to see all kinds of weird stuff. My task is to make sure that kernel interface is sane and since it is userspace ABI matter I want to be extra careful. Thanks. -- 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
