From mboxrd@z Thu Jan 1 00:00:00 1970 From: Chase Douglas Subject: Re: [PATCH v2] Input: evdev - Add EVIOC mechanism to extract the MT slot state Date: Fri, 06 Jan 2012 10:56:46 -0800 Message-ID: <4F0743EE.6080302@canonical.com> References: <1325863019-2242-1-git-send-email-rydberg@euromail.se> <20120106181845.GA22653@core.coreip.homeip.net> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit Return-path: Received: from youngberry.canonical.com ([91.189.89.112]:39116 "EHLO youngberry.canonical.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754248Ab2AFSzW (ORCPT ); Fri, 6 Jan 2012 13:55:22 -0500 In-Reply-To: <20120106181845.GA22653@core.coreip.homeip.net> Sender: linux-input-owner@vger.kernel.org List-Id: linux-input@vger.kernel.org To: Dmitry Torokhov Cc: Benjamin Tissoires , Henrik Rydberg , linux-input@vger.kernel.org, linux-kernel@vger.kernel.org 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. -- Chase