From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753186AbZA3Gbn (ORCPT ); Fri, 30 Jan 2009 01:31:43 -0500 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1751816AbZA3Gbf (ORCPT ); Fri, 30 Jan 2009 01:31:35 -0500 Received: from smtp1.linux-foundation.org ([140.211.169.13]:54408 "EHLO smtp1.linux-foundation.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751069AbZA3Gbe (ORCPT ); Fri, 30 Jan 2009 01:31:34 -0500 Date: Thu, 29 Jan 2009 22:30:42 -0800 From: Andrew Morton To: Rusty Russell Cc: Mike Travis , Ingo Molnar , Dave Jones , cpufreq@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH 2/3] work_on_cpu: Use our own workqueue. Message-Id: <20090129223042.47dc42a1.akpm@linux-foundation.org> In-Reply-To: <200901301633.54013.rusty@rustcorp.com.au> References: <20090116191108.135927000@polaris-admin.engr.sgi.com> <200901291213.32959.rusty@rustcorp.com.au> <20090128181205.3b15fa4a.akpm@linux-foundation.org> <200901301633.54013.rusty@rustcorp.com.au> X-Mailer: Sylpheed 2.4.8 (GTK+ 2.12.5; x86_64-redhat-linux-gnu) Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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.