From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754688AbbHJOYX (ORCPT ); Mon, 10 Aug 2015 10:24:23 -0400 Received: from casper.infradead.org ([85.118.1.10]:38487 "EHLO casper.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754612AbbHJOYU (ORCPT ); Mon, 10 Aug 2015 10:24:20 -0400 Date: Mon, 10 Aug 2015 16:24:17 +0200 From: Peter Zijlstra To: Boqun Feng Cc: Ingo Molnar , linux-kernel@vger.kernel.org Subject: Re: [Question] lockdep: Is nested lock handled correctly? Message-ID: <20150810142417.GC16853@twins.programming.kicks-ass.net> References: <20150810095247.GA4606@fixme-laptop.cn.ibm.com> <20150810114228.GY16853@twins.programming.kicks-ass.net> <20150810134924.GB4606@fixme-laptop.cn.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20150810134924.GB4606@fixme-laptop.cn.ibm.com> User-Agent: Mutt/1.5.21 (2012-12-30) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, Aug 10, 2015 at 09:49:24PM +0800, Boqun Feng wrote: > Hi Peter, > > On Mon, Aug 10, 2015 at 01:42:28PM +0200, Peter Zijlstra wrote: > > On Mon, Aug 10, 2015 at 05:52:47PM +0800, Boqun Feng wrote: > > > Hi Peter and Ingo, > > > > > > I'm now learning the code of lockdep and find that nested lock may not > > > be handled correctly because we fail to take held_lock merging into > > > consideration. I come up with an example and hope that could explain my > > > concern. > > > > > > Please consider this lock/unlock sequence, I also put a patch ading this > > > sequence as a test into locking-selftest: > > > > > > (lock_X1 and lock_X2 belong to the same lock class X, lock_Y1 belongs to > > > another lock class Y) > > > > > > spin_lock(&lock_X1); > > > spin_lock(&lock_Y1); > > > spin_lock_nested_lock(&lock_X2, &lock_X1); > > Sorry for the typo here.. should be spin_lock_nest_lock(). > > > > spin_unlock(&lock_Y1); > > > spin_unlock(&lock_X2); > > > spin_unlock(&lock_X1); > > > > > > > > > This is totally legal in current lockdep rules, right? > > > > Yuck, I'd say no. That's quite horrible. > > > > I admit that I didn't find this is horrible at first, but now I agree > with you, this is not a rational locking order. Thank you. > > > Why would you ever want to do that? > > Though I don't want to have a locking order like that either, we can't > stop others from using that order(maybe a good design review will) and > lockdep yells something -unrelated- in such an order. > > I think we can either let lockdep complain if some one uses this > locking order or clean up current code a little bit to tolarent this. > > If you really think we should do something about it, I can write the > patch and add test cases. Maybe something like the below in __lock_acquire(): /* Daft bugger, can't guard a nesting order with the same lock class */ if (DEBUG_LOCKS_WARN_ON(lock == nest_lock)) return 0; ?