From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754248AbZHPNTM (ORCPT ); Sun, 16 Aug 2009 09:19:12 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1753164AbZHPNTJ (ORCPT ); Sun, 16 Aug 2009 09:19:09 -0400 Received: from mail-bw0-f222.google.com ([209.85.218.222]:65345 "EHLO mail-bw0-f222.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752910AbZHPNTI (ORCPT ); Sun, 16 Aug 2009 09:19:08 -0400 Message-ID: <4A8807A3.3080604@colorfullife.com> Date: Sun, 16 Aug 2009 15:20:35 +0200 From: Manfred Spraul User-Agent: Mozilla/5.0 (X11; U; Linux x86_64; en-US; rv:1.9.1.1) Gecko/20090814 Fedora/3.0-2.6.b3.fc11 Thunderbird/3.0b3 MIME-Version: 1.0 To: npiggin@suse.de CC: Andrew Morton , Nadia Derbey , Pierre Peiffer , linux-kernel@vger.kernel.org Subject: Re: [patch 3/4] ipc: sem preempt improve References: <20090811110902.255877673@suse.de> <20090811111607.144736971@suse.de> In-Reply-To: <20090811111607.144736971@suse.de> Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 08/11/2009 01:09 PM, npiggin@suse.de wrote: > The strange sysv semaphore wakeup scheme has a kind of busy-wait lock > involved, which could deadlock if preemption is enabled during the > "lock". > > It is an implementation detail (due to a spinlock being held) that this > is actually the case. However if "spinlocks" are made preemptible, or if > the sem lock is changed to a sleeping lock for example, then the wakeup > would become buggy. So this might be a bugfix for -rt kernels. > > Imagine waker being preempted by wakee and never clearing IN_WAKEUP -- > if wakee has higher RT priority then there is a priority inversion deadlock. > Even if there is not a priority inversion to cause a deadlock, then there > is still time wasted spinning. > > Signed-off-by: Nick Piggin > Signed-off-by: Manfred Spraul