From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1750753AbVLTRoO (ORCPT ); Tue, 20 Dec 2005 12:44:14 -0500 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1750755AbVLTRoO (ORCPT ); Tue, 20 Dec 2005 12:44:14 -0500 Received: from lirs02.phys.au.dk ([130.225.28.43]:6808 "EHLO lirs02.phys.au.dk") by vger.kernel.org with ESMTP id S1750753AbVLTRoO (ORCPT ); Tue, 20 Dec 2005 12:44:14 -0500 Date: Tue, 20 Dec 2005 18:43:03 +0100 (MET) From: Esben Nielsen To: Dinakar Guniguntala Cc: Ingo Molnar , robustmutexes@lists.osdl.org, linux-kernel@vger.kernel.org Subject: Re: Recursion bug in -rt In-Reply-To: <20051220155004.GA3906@in.ibm.com> Message-Id: Mime-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org On Tue, 20 Dec 2005, Dinakar Guniguntala wrote: > On Tue, Dec 20, 2005 at 02:19:56PM +0100, Ingo Molnar wrote: > > > > hm, i'm looking at -rf4 - these changes look fishy: > > > > - _raw_spin_lock(&lock_owner(lock)->task->pi_lock); > > + if (current != lock_owner(lock)->task) > > + _raw_spin_lock(&lock_owner(lock)->task->pi_lock); > > > > why is this done? > > > > Ingo, this is to prevent a kernel hang due to application error. > > Basically when an application does a pthread_mutex_lock twice on a > _nonrecursive_ mutex with robust/PI attributes the whole system hangs. > Ofcourse the application clearly should not be doing anything like > that, but it should not end up hanging the system either > Hmm, reading the comment on the function, wouldn't it be more natural to use if(task != lock_owner(lock)->task) as it assumes that task->pi_lock is locked, not that current->pi_lock is locked. By the way: task->pi_lock is taken. lock_owner(lock)->task->pi_lock will be taken. What if the task lock_owner(lock)->task tries to lock another futex, (lock2) with which has lock_owner(lock2)->task==task. Can't you promote a user space futex deadlock into a kernel spin deadlock this way? Esben > -Dinakar > >