From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752452AbaEAVCu (ORCPT ); Thu, 1 May 2014 17:02:50 -0400 Received: from mail-qg0-f41.google.com ([209.85.192.41]:46028 "EHLO mail-qg0-f41.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752351AbaEAVCq (ORCPT ); Thu, 1 May 2014 17:02:46 -0400 Date: Thu, 1 May 2014 17:02:42 -0400 From: Tejun Heo To: Jiri Kosina Cc: Jiri Slaby , linux-kernel@vger.kernel.org, jirislaby@gmail.com, Vojtech Pavlik , Michael Matz , Steven Rostedt , Frederic Weisbecker , Ingo Molnar , Greg Kroah-Hartman , "Theodore Ts'o" , Dipankar Sarma , "Paul E. McKenney" Subject: Re: [RFC 09/16] kgr: mark task_safe in some kthreads Message-ID: <20140501210242.GA28948@mtj.dyndns.org> References: <1398868249-26169-1-git-send-email-jslaby@suse.cz> <1398868249-26169-10-git-send-email-jslaby@suse.cz> <20140501142414.GA31611@htj.dyndns.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: 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 Hello, Jiri. On Thu, May 01, 2014 at 10:17:44PM +0200, Jiri Kosina wrote: > I agree that this expectation might really somewhat implicit and is not > probably properly documented anywhere. The basic observation is "whenever > kthread_should_stop() is being called, all data structures are in a > consistent state and don't need any further updates in order to achieve > consistency, because we can exit the loop immediately here", as > kthread_should_stop() is the very last thing every freezable kernel thread But kthread_should_stop() doesn't necessarily imply that "we can exit the loop *immediately*" at all. It just indicates that it should terminate in finite amount of time. I don't think it'd be too difficult to find cases where kthreads do some stuff before returning after testing kthread_should_stop(). e.g. after pending changes, workqueue rescuers do one final loop over pending work items after kthread_should_stop() tests positive to ensure empty queue on exit. Please note that there's no expectation of discontinuity over the test. The users may carry over any state across the test as they see fit. > is calling before starting a new iteration. > > For the sake of collecting data points -- do you happen to have any > counter-example to the assumption? Just grep for kthread_should_stop() and look for the ones which doesn't immediately perform return? I think there are more which don't return *immediately*. You'd have to audit each and everyone to determine that they don't carry over states across the test. Most will hopefully be trivial but not all. More importantly, sounds like a maintenance nightmare to me without any means to guarantee, or even reasonably increase, correctness. Thanks. -- tejun