From mboxrd@z Thu Jan 1 00:00:00 1970 From: Dmitry Torokhov Subject: Re: [PATCH v2] Input: evdev - Add EVIOC mechanism to extract the MT slot state Date: Fri, 6 Jan 2012 11:58:46 -0800 Message-ID: <20120106195846.GA23619@core.coreip.homeip.net> References: <1325863019-2242-1-git-send-email-rydberg@euromail.se> <20120106181845.GA22653@core.coreip.homeip.net> <4F0743EE.6080302@canonical.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Return-path: Received: from mail-tul01m020-f174.google.com ([209.85.214.174]:36153 "EHLO mail-tul01m020-f174.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1756116Ab2AFT6z (ORCPT ); Fri, 6 Jan 2012 14:58:55 -0500 Content-Disposition: inline In-Reply-To: <4F0743EE.6080302@canonical.com> Sender: linux-input-owner@vger.kernel.org List-Id: linux-input@vger.kernel.org To: Chase Douglas Cc: Benjamin Tissoires , Henrik Rydberg , linux-input@vger.kernel.org, linux-kernel@vger.kernel.org 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