From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756989AbZA3V7g (ORCPT ); Fri, 30 Jan 2009 16:59:36 -0500 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1754467AbZA3V71 (ORCPT ); Fri, 30 Jan 2009 16:59:27 -0500 Received: from ozlabs.org ([203.10.76.45]:60489 "EHLO ozlabs.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752989AbZA3V70 (ORCPT ); Fri, 30 Jan 2009 16:59:26 -0500 From: Rusty Russell To: Andrew Morton Subject: Re: [PATCH 2/3] work_on_cpu: Use our own workqueue. Date: Sat, 31 Jan 2009 08:29:15 +1030 User-Agent: KMail/1.10.3 (Linux/2.6.27-9-generic; KDE/4.1.3; i686; ; ) Cc: Mike Travis , Ingo Molnar , Dave Jones , cpufreq@vger.kernel.org, linux-kernel@vger.kernel.org References: <20090116191108.135927000@polaris-admin.engr.sgi.com> <200901301633.54013.rusty@rustcorp.com.au> <20090129223042.47dc42a1.akpm@linux-foundation.org> In-Reply-To: <20090129223042.47dc42a1.akpm@linux-foundation.org> MIME-Version: 1.0 Content-Type: text/plain; charset="iso-8859-1" Content-Transfer-Encoding: 7bit Content-Disposition: inline Message-Id: <200901310829.17099.rusty@rustcorp.com.au> Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Friday 30 January 2009 17:00:42 Andrew Morton wrote: > On Fri, 30 Jan 2009 16:33:53 +1030 Rusty Russell wrote: > > > On Thursday 29 January 2009 12:42:05 Andrew Morton wrote: > > > On Thu, 29 Jan 2009 12:13:32 +1030 Rusty Russell wrote: > > > > > > > On Thursday 29 January 2009 06:14:40 Andrew Morton wrote: > > > > > It's vulnerable to the same deadlock, I think? Suppose we have: > > > > ... > > > > > - A calls work_on_cpu() and takes woc_mutex. > > > > > > > > > > - Before function_which_takes_L() has started to execute, task B takes L > > > > > then calls work_on_cpu() and task B blocks on woc_mutex. > > > > > > > > > > - Now function_which_takes_L() runs, and blocks on L > > > > > > > > Agreed, but now it's a fairly simple case. Both sides have to take lock L, and both have to call work_on_cpu. > > > > > > > > Workqueues are more generic and widespread, and an amazing amount of stuff gets called from them. That's why I felt uncomfortable with removing the one known problematic caller. > > > > > > > > > > hm. it's a bit of a timebomb. > > > > > > y'know, the original way in which acpi-cpufreq did this is starting to > > > look attractive. Migrate self to that CPU then just call the dang > > > function. Slow, but no deadlocks (I think)? > > > > Just buggy. What random thread was it mugging? If there's any path where > > it's not a kthread, what if userspace does the same thing at the same time? > > We risk running on the wrong cpu, *then* overriding userspace when we restore > > it. > > hm, Ok, not unficable but not pleasant. > > > In general these cpumask games are a bad idea. > > So we still don't have any non-buggy proposal. I disagree about the avoiding-workqueue one being buggy. The same logic applies to any simple callback function. Rusty.