From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752994AbZF3GAa (ORCPT ); Tue, 30 Jun 2009 02:00:30 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1751488AbZF3GAX (ORCPT ); Tue, 30 Jun 2009 02:00:23 -0400 Received: from e9.ny.us.ibm.com ([32.97.182.139]:45017 "EHLO e9.ny.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751203AbZF3GAW (ORCPT ); Tue, 30 Jun 2009 02:00:22 -0400 Date: Mon, 29 Jun 2009 23:00:31 -0700 From: "Paul E. McKenney" To: Matt Mackall Cc: Christoph Lameter , linux-kernel@vger.kernel.org, linux-mm@kvack.org, penberg@cs.helsinki.fi, jdb@comx.dk Subject: Re: [PATCH RFC] fix RCU-callback-after-kmem_cache_destroy problem in sl[aou]b Message-ID: <20090630060031.GL7070@linux.vnet.ibm.com> Reply-To: paulmck@linux.vnet.ibm.com References: <20090625193137.GA16861@linux.vnet.ibm.com> <1246315553.21295.100.camel@calx> <1246320394.21295.105.camel@calx> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1246320394.21295.105.camel@calx> 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 On Mon, Jun 29, 2009 at 07:06:34PM -0500, Matt Mackall wrote: > On Mon, 2009-06-29 at 19:19 -0400, Christoph Lameter wrote: > > On Mon, 29 Jun 2009, Matt Mackall wrote: > > > > > This is a reasonable point, and in keeping with the design principle > > > 'callers should handle their own special cases'. However, I think it > > > would be more than a little surprising for kmem_cache_free() to do the > > > right thing, but not kmem_cache_destroy(). > > > > kmem_cache_free() must be used carefully when using SLAB_DESTROY_BY_RCU. > > The freed object can be accessed after free until the rcu interval > > expires (well sortof, it may even be reallocated within the interval). > > > > There are special RCU considerations coming already with the use of > > kmem_cache_free(). > > > > Adding RCU operations to the kmem_cache_destroy() logic may result in > > unnecessary RCU actions for slabs where the coder is ensuring that the > > RCU interval has passed by other means. > > Do we care? Cache destruction shouldn't be in anyone's fast path. > Correctness is more important and users are more liable to be correct > with this patch. I am with Matt on this one -- if we are going to hand the users of SLAB_DESTROY_BY_RCU a hand grenade, let's at least leave the pin in. Thanx, Paul