From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1759937AbYDOIpa (ORCPT ); Tue, 15 Apr 2008 04:45:30 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1752302AbYDOIpW (ORCPT ); Tue, 15 Apr 2008 04:45:22 -0400 Received: from pentafluge.infradead.org ([213.146.154.40]:44394 "EHLO pentafluge.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752772AbYDOIpU (ORCPT ); Tue, 15 Apr 2008 04:45:20 -0400 Subject: Re: [PATCH] Replace completions with semaphores From: Peter Zijlstra To: Bart Van Assche Cc: Andi Kleen , Roland Dreier , Ingo Molnar , Matthew Wilcox , Ingo Oeser , Daniel Walker , linux-kernel@vger.kernel.org, Linus Torvalds In-Reply-To: References: <20080411210022.GJ11962@parisc-linux.org> <20080413125758.GQ11962@parisc-linux.org> <20080414153928.GA22259@elte.hu> <1208190749.7375.10.camel@twins> <87d4osuy6r.fsf@basil.nowhere.org> <1208195673.7164.2.camel@twins> <4803AD91.5020001@firstfloor.org> <1208242017.7053.4.camel@lappy> Content-Type: text/plain Date: Tue, 15 Apr 2008 10:44:48 +0200 Message-Id: <1208249088.7124.7.camel@twins> Mime-Version: 1.0 X-Mailer: Evolution 2.22.0 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, 2008-04-15 at 09:17 +0200, Bart Van Assche wrote: > On Tue, Apr 15, 2008 at 8:46 AM, Peter Zijlstra wrote: > > > > On Tue, 2008-04-15 at 08:18 +0200, Bart Van Assche wrote: > > > Would it really be a good idea to give a synchronization concept that > > > behaves exactly like a semaphore another name than "semaphore" ? The > > > semaphore concept is well known and is taught in every computer > > > science course. > > > > Are the ramifications wrt priority inversion taught? Is it made clear > > that its hard to validate because there is no clear resource owner? > > > > Afaik, non of these subjects are touched upon in the CS-101 courses and > > that is exactly the problem. So you can say they are not well know, they > > are just widely misunderstood. > > > > And yes, if there are more hand a very few such users it doesn't make > > sense to keep them open coded. > > Regarding semaphores and priority inversion: I have never recommended > the use of semaphores over mutexes, all I recommended is to keep the > name "semaphore" for something that behaves like a semaphore. There > might be better ways to discourage the use of the semaphore API, e.g. > letting the compiler print a warning every time a semaphore function > is called unless one or another #define has been enabled. That sounds horrible; I really prefer targeted replacements like completions that make it clear what they're supposed to be used for. > Regarding priority inheritance: does the above mean that you consider > priority inheritance as an optimal solution for realizing real-time > behavior in the kernel ? Are you aware of the fundamental problems > associated with priority inheritance ? These issues are well explained > in Victor Yodaiken's paper "Against priority inheritance". See also > http://www.linuxdevices.com/files/misc/yodaiken-july02.pdf . Priority inheritance isn't ideal, but comming from a general purpose kernel that wasn't build from scratch to accomodate hard realtime its basically the only option. Also things like lockdep are a real help to a lot of developers - loads of locking bugs never make it into the kernel because of it.