From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1760984AbYEXRYa (ORCPT ); Sat, 24 May 2008 13:24:30 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1757675AbYEXRYX (ORCPT ); Sat, 24 May 2008 13:24:23 -0400 Received: from gateway-1237.mvista.com ([63.81.120.158]:8311 "EHLO gateway-1237.mvista.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755626AbYEXRYW (ORCPT ); Sat, 24 May 2008 13:24:22 -0400 Subject: Re: [PATCH 3/3] futex: fix miss ordered wakeups From: Daniel Walker To: Thomas Gleixner Cc: linux-kernel@vger.kernel.org In-Reply-To: References: <20080523042923.594531457@mvista.com> <20080523042923.916202203@mvista.com> <1211643165.18130.166.camel@localhost.localdomain> Content-Type: text/plain Date: Sat, 24 May 2008 10:24:20 -0700 Message-Id: <1211649860.18130.182.camel@localhost.localdomain> Mime-Version: 1.0 X-Mailer: Evolution 2.12.3 (2.12.3-3.fc8) Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Sat, 2008-05-24 at 19:03 +0200, Thomas Gleixner wrote: > On Sat, 24 May 2008, Daniel Walker wrote: > > On Sat, 2008-05-24 at 10:55 +0200, Thomas Gleixner wrote: > > > > > Normal futexes have no ordering guarantees at all. There is no > > > mechanism to prevent lock stealing from lower priority tasks. So why > > > should we care about the once a year case, where a sleepers priority > > > is modified ? > > > > Lock stealing? > > Do you have the faintest idea how the futex code works at all ? There > is no guarantee that the task which is woken up first gets the futex. Thomas if you want to be abusive, talk to someone else. > A) A task on another CPU can get it independent of its priority > B) In case of multiple waiters wakeup there is no guarantee either This is how I would imagine the pre-plist code would work. > > > If you need ordering guarantees then use PI futexes. > > > > There are degree's of overhead with each step.. Someone may not need or > > want priority inheritance. > > Then there is no need to add this artifical "correctness" at all. huh? > > > There are more issues vs. pi futexes as well. The simple case of > > > futex_wait() vs. futex_adjust_waiters will just upset lockdep, but > > > there are real dealocks vs. unqueue_me_pi waiting. > > > > You mean the lock ordering would cause the deadlock vs. unqueue_me_pi , > > or are you talking about something else? > > Do I write Chinese or what ? I guess so .. Daniel