From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753381Ab2ALL65 (ORCPT ); Thu, 12 Jan 2012 06:58:57 -0500 Received: from relay3-d.mail.gandi.net ([217.70.183.195]:60410 "EHLO relay3-d.mail.gandi.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751481Ab2ALL64 (ORCPT ); Thu, 12 Jan 2012 06:58:56 -0500 X-Originating-IP: 217.70.178.135 X-Originating-IP: 50.43.15.19 Date: Thu, 12 Jan 2012 03:58:30 -0800 From: Josh Triplett To: Jan Engelhardt Cc: paulmck@linux.vnet.ibm.com, laijs@cn.fujitsu.com, manfred@colorfullife.com, dhowells@redhat.com, linux-kernel@vger.kernel.org Subject: Re: [PATCH] rcu: avoid checking for constant Message-ID: <20120112115830.GA3436@leaf> References: <1326345104-6919-1-git-send-email-jengelh@medozas.de> <20120112071431.GA1896@leaf> <20120112095257.GA2441@leaf> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.5.21 (2010-09-15) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Jan 12, 2012 at 11:34:23AM +0100, Jan Engelhardt wrote: > On Thursday 2012-01-12 10:52, Josh Triplett wrote: > >> +#define __kfree_rcu(head, offset) \ > >> + call_rcu(head, (void (*)(struct rcu_head *))(unsigned long)(offset) + \ > >> + BUILD_BUG_ON_ZERO((offset) >= 4096)) > >> + > > > >I had to stare at this for a while, and look up the definition of > >BUILD_BUG_ON_ZERO. Naturally I assumed that BUILD_BUG_ON_ZERO(arg) > >meant BUILT_BUG_ON((arg) == 0), which would have made the logic > >backwards here. However, per the definition it just provides a > >zero-returning version of BUILD_BUG_ON. Ow. > > Same impression here. BUILD_BUG_ON_ZERO was introduced by > > commit 4552d5dc08b79868829b4be8951b29b07284753f > Author: Jan Beulich > Date: Mon Jun 26 13:57:28 2006 +0200 > > while Rusty's CCAN archive calls it "BUILD_BUG_OR_ZERO" (since either > it's a bug, or returning neutral zero). Sounds like a good target for a fix at some point. > rcu: avoid checking for constant > > When compiling kernel or module code with -O0, "offset" is no longer > considered a constant, and therefore always triggers the build error > that BUILD_BUG_ON is defined to yield. > > Therefore, change the innards of kfree_rcu so that the offset is not > tunneled through a function argument before checking it. The commit message looks good now. > @@ -835,7 +817,20 @@ void __kfree_rcu(struct rcu_head *head, unsigned long offset) > * > * Note that the allowable offset might decrease in the future, for example, > * to allow something like kmem_cache_free_rcu(). > + * > + * The BUILD_BUG_ON check must not involve any function calls, hence the > + * checks are done in macros here. __is_kfree_rcu_offset is also used by > + * kernel/rcu.h. The first sentence of that paragraph seems like a worthwhile addition. Please drop the second, though, since it'll inevitably become outdated. > +#define __is_kfree_rcu_offset(offset) ((offset) < 4096) > + > +#define __kfree_rcu(head, offset) \ > + do { \ > + typedef void (*rcu_callback)(struct rcu_head *); \ > + BUILD_BUG_ON(!__is_kfree_rcu_offset(offset)); \ > + call_rcu(head, (rcu_callback)(unsigned long)(offset)); \ > + } while (0) No, you can't define that typedef here with that name. Unlike in the inline function, in a macro you could introduce a name conflict. - Josh Triplett