Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755106AbZDVOJT (ORCPT ); Wed, 22 Apr 2009 10:09:19 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1751004AbZDVOJE (ORCPT ); Wed, 22 Apr 2009 10:09:04 -0400 Received: from e9.ny.us.ibm.com ([32.97.182.139]:60794 "EHLO e9.ny.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751099AbZDVOJC (ORCPT ); Wed, 22 Apr 2009 10:09:02 -0400 Date: Wed, 22 Apr 2009 07:08:57 -0700 From: "Paul E. McKenney" To: Jan Blunck Cc: Nick Piggin , Linux-Kernel Mailinglist Subject: Re: [PATCH] atomic: Only take lock when the counter drops to zero on UP as well Message-ID: <20090422140857.GB6760@linux.vnet.ibm.com> Reply-To: paulmck@linux.vnet.ibm.com References: <20090411141754.45F7B16080@e179.suse.de> <20090411174905.GH6822@linux.vnet.ibm.com> <627226E3-0AC6-4B25-A338-EA65F6C85BFF@suse.de> <20090413060243.GS6822@linux.vnet.ibm.com> <20090422125620.GF11220@bolzano.suse.de> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20090422125620.GF11220@bolzano.suse.de> User-Agent: Mutt/1.5.15+20070412 (2007-04-11) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1977 Lines: 45 On Wed, Apr 22, 2009 at 02:56:20PM +0200, Jan Blunck wrote: > On Sun, Apr 12, Paul E. McKenney wrote: > > > On Sun, Apr 12, 2009 at 01:32:54PM +0200, Jan Blunck wrote: > > > Am 11.04.2009 um 19:49 schrieb "Paul E. McKenney" > > > : > > > > > >> On Fri, Apr 10, 2009 at 06:13:57PM +0200, Jan Blunck wrote: > > >>> I think it is wrong to unconditionally take the lock before calling > > >>> atomic_dec_and_test() in _atomic_dec_and_lock(). This will deadlock in > > >>> situation where it is known that the counter will not reach zero (e.g. > > >>> holding > > >>> another reference to the same object) but the lock is already taken. > > >> > > >> The thought of calling _atomic_dec_and_lock() when you already hold the > > >> lock really really scares me. > > >> > > >> Could you please give an example where you need to do this? > > >> > > > > > > There is a part of the union mount patches that needs to do a union_put() > > > (which itself includes a path_put() that uses atomic_dec_and_lock() in > > > mntput() ). Since it is changing the namespace I need to hold the vfsmount > > > lock. I know that the mnt's count > 1 since it is a parent of the mnt I'm > > > changing in the mount tree. I could possibly delay the union_put(). > > > > > > In general this let's atomic_dec_and_lock() behave similar on SMP and UP. > > > Remember that this already works with CONFIG_SMP as before Nick's patch. > > > > I asked, I guess. ;-) > > > > There is some sort of common code path, so that you cannot simply call > > atomic_dec() when holding the lock? > > If it is possible I don't want to introduce another special mntput() variant > just for that code path. Fair enough!!! Thanx, Paul -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/