Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752657AbbBSRTc (ORCPT ); Thu, 19 Feb 2015 12:19:32 -0500 Received: from cantor2.suse.de ([195.135.220.15]:36376 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751647AbbBSRTb (ORCPT ); Thu, 19 Feb 2015 12:19:31 -0500 Date: Thu, 19 Feb 2015 18:19:29 +0100 From: Vojtech Pavlik To: Josh Poimboeuf Cc: Peter Zijlstra , Andrew Morton , Ingo Molnar , Jiri Kosina , Seth Jennings , linux-kernel@vger.kernel.org Subject: Re: [PATCH 1/3] sched: add sched_task_call() Message-ID: <20150219171929.GA13178@suse.cz> References: <20150217181541.GP5029@twins.programming.kicks-ass.net> <20150217212532.GJ11861@treble.redhat.com> <20150218152100.GZ5029@twins.programming.kicks-ass.net> <20150218171256.GA28553@treble.hsd1.ky.comcast.net> <20150219002058.GD5029@twins.programming.kicks-ass.net> <20150219041753.GA13423@treble.redhat.com> <20150219101607.GG5029@twins.programming.kicks-ass.net> <20150219162429.GA15980@treble.redhat.com> <20150219163359.GA25438@suse.cz> <20150219170353.GB15980@treble.redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20150219170353.GB15980@treble.redhat.com> X-Bounce-Cookie: It's a lemon tree, dear Watson! User-Agent: Mutt/1.5.21 (2010-09-15) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1297 Lines: 31 On Thu, Feb 19, 2015 at 11:03:53AM -0600, Josh Poimboeuf wrote: > On Thu, Feb 19, 2015 at 05:33:59PM +0100, Vojtech Pavlik wrote: > > On Thu, Feb 19, 2015 at 10:24:29AM -0600, Josh Poimboeuf wrote: > > > > > > No, these tasks will _never_ make syscalls. So you need to guarantee > > > > they don't accidentally enter the kernel while you flip them. Something > > > > like so should do. > > > > > > > > You set TIF_ENTER_WAIT on them, check they're still in userspace, flip > > > > them then clear TIF_ENTER_WAIT. > > > > > > Ah, that's a good idea. But how do we check if they're in user space? > > > > I don't see the benefit in holding them in a loop - you can just as well > > flip them from the syscall code as kGraft does. > > But we were talking specifically about HPC tasks which never make > syscalls. Yes. I'm saying that rather than guaranteeing they don't enter the kernel (by having them spin) you can flip them in case they try to do that instead. That solves the race condition just as well. -- Vojtech Pavlik Director SUSE Labs -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/