From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754471AbYH0UXc (ORCPT ); Wed, 27 Aug 2008 16:23:32 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1752168AbYH0UXY (ORCPT ); Wed, 27 Aug 2008 16:23:24 -0400 Received: from e2.ny.us.ibm.com ([32.97.182.142]:37791 "EHLO e2.ny.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752151AbYH0UXX (ORCPT ); Wed, 27 Aug 2008 16:23:23 -0400 Subject: Re: [PATCH, RFC, tip/core/rcu] scalable classic RCU implementation From: Josh Triplett To: paulmck@linux.vnet.ibm.com Cc: linux-kernel@vger.kernel.org, cl@linux-foundation.org, mingo@elte.hu, akpm@linux-foundation.org, manfred@colorfullife.com, dipankar@in.ibm.com, schamp@sgi.com, niv@us.ibm.com, dvhltc@us.ibm.com, ego@in.ibm.com, laijs@cn.fujitsu.com, rostedt@goodmis.org In-Reply-To: <20080827183446.GL6711@linux.vnet.ibm.com> References: <20080821234318.GA1754@linux.vnet.ibm.com> <1219447772.5197.98.camel@josh-work.beaverton.ibm.com> <20080823015336.GP6744@linux.vnet.ibm.com> <1219701750.12334.29.camel@josh-work.beaverton.ibm.com> <20080826160530.GB6656@linux.vnet.ibm.com> <1219797516.6911.6.camel@josh-work.beaverton.ibm.com> <20080827183446.GL6711@linux.vnet.ibm.com> Content-Type: text/plain Date: Wed, 27 Aug 2008 13:23:28 -0700 Message-Id: <1219868608.10880.5.camel@josh-work.beaverton.ibm.com> Mime-Version: 1.0 X-Mailer: Evolution 2.22.3.1 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, 2008-08-27 at 11:34 -0700, Paul E. McKenney wrote: > On Tue, Aug 26, 2008 at 05:38:36PM -0700, Josh Triplett wrote: > > On Tue, 2008-08-26 at 09:05 -0700, Paul E. McKenney wrote: > > > On Mon, Aug 25, 2008 at 03:02:30PM -0700, Josh Triplett wrote: > > > > On Fri, 2008-08-22 at 18:53 -0700, Paul E. McKenney wrote: > > > > > On Fri, Aug 22, 2008 at 04:29:32PM -0700, Josh Triplett wrote: > > > > > > On Thu, 2008-08-21 at 16:43 -0700, Paul E. McKenney wrote: > > > > > > > @@ -658,14 +806,19 @@ int rcu_needs_cpu(int cpu) > > > > > > > struct rcu_data *rdp = &per_cpu(rcu_data, cpu); > > > > > > > struct rcu_data *rdp_bh = &per_cpu(rcu_bh_data, cpu); > > > > > > > > > > > > > > - return !!rdp->nxtlist || !!rdp_bh->nxtlist || rcu_pending(cpu); > > > > > > > + return !!*rdp->nxttail[RCU_DONE_TAIL] || > > > > > > > + !!*rdp_bh->nxttail[RCU_DONE_TAIL] || > > > > > > > + rcu_pending(cpu); > > > > > > > > > > > > !! seems unnecessary here. > > > > > > > > > > Someone once told me why this was necessary, but I forget. It was in the > > > > > original, and I didn't put it there. Some weirdness about conversion > > > > > to 32-bit integer when the lower 32 bits of the pointer was zero or > > > > > some such. So if your pointer value was 0x100000000, for example, > > > > > so that conversion to int gives zero. > > > > > > > > Good point! That doesn't apply if you use ||, though. If you just did > > > > "return somepointer" that could potentially cause the problem you > > > > describe. In any case, it can't *hurt* to have it; GCC should do the > > > > sane thing. > > > > > > OK. I will review this towards the end, leaving it there to remind me > > > in the meantime. > > > > > > So, would I need the !! on the left-hand operand of the first || due > > > to short-circuiting? > > > > No. || will always return 1 or 0. You only need the !! if you want to > > directly return the boolean value of a potentially 64-bit pointer. > > Even if one argument of || is long and the other int or some fool thing > like that? (What, me paranoid???) What, you don't know exactly how C behaves in every strange corner case? ;) || always produces a result of type int, and it compares each of its two arguments to 0 independently; to the best of my knowledge the size of those arguments never matters. - Josh Triplett