Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932410AbVLJPYV (ORCPT ); Sat, 10 Dec 2005 10:24:21 -0500 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S932476AbVLJPYV (ORCPT ); Sat, 10 Dec 2005 10:24:21 -0500 Received: from e3.ny.us.ibm.com ([32.97.182.143]:64932 "EHLO e3.ny.us.ibm.com") by vger.kernel.org with ESMTP id S932410AbVLJPYU (ORCPT ); Sat, 10 Dec 2005 10:24:20 -0500 Date: Sat, 10 Dec 2005 20:49:51 +0530 From: Srivatsa Vaddagiri To: Oleg Nesterov Cc: linux-kernel@vger.kernel.org, Dipankar Sarma , "Paul E. McKenney" , Andrew Morton Subject: Re: [PATCH] Fix RCU race in access of nohz_cpu_mask Message-ID: <20051210151951.GA2798@in.ibm.com> Reply-To: vatsa@in.ibm.com References: <439889FA.BB08E5E1@tv-sign.ru> <20051209024623.GA14844@in.ibm.com> <4399D852.47E0BB4E@tv-sign.ru> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <4399D852.47E0BB4E@tv-sign.ru> User-Agent: Mutt/1.4.1i Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2310 Lines: 68 On Fri, Dec 09, 2005 at 10:17:38PM +0300, Oleg Nesterov wrote: > Srivatsa Vaddagiri wrote: > > > > On Thu, Dec 08, 2005 at 10:31:06PM +0300, Oleg Nesterov wrote: > > > I can't see how this change can prevent idle cpus to be included in > > > ->cpumask, cpu can add itself to nohz_cpu_mask right after some other > > > cpu started new grace period. > > > > Yes that can happen, but if they check for rcu_pending right after that > > it will prevent them from going tickless atleast (which will prevent grace > > periods from being unnecessarily extended). Something like below: > > > > CPU0 CPU1 > > > > rcp->cur++; /* New GP */ > > > > smp_mb(); > > I think I need some education on memory barriers. > > Does this mb() garantees that the new value of ->cur will be visible > on other cpus immediately after smp_mb() (so that rcu_pending() will > notice it) ? AFAIK the new value of ->cur should be visible to other CPUs when smp_mb() returns. Here's a definition of smp_mb() from Paul's article: smp_mb(): "memory barrier" that orders both loads and stores. This means loads and stores preceding the memory barrier are committed to memory before any loads and stores following the memory barrier. [ http://www.linuxjournal.com/article/8211 ] > My understanding is that it only garantees that all stores before it > must be visible before any store after mb. (yes, mb implies rmb, but > I think it does not matter if CPU1 adds itself to nonhz mask after > CPU0 reads nohz_cpu_mask). This means that CPU1 can read the stale > value of ->cur. I guess I am wrong, but I can't prove it to myself. > > Could you please clarify this? > > Even simpler question: > > CPU0 > var = 1; > wmb(); > > after that CPU1 does rmb(). > > Does it garantees that CPU1 will see the new value of var? Again, going by the above article, I would expect CPU1 to see the new value of var. -- Thanks and Regards, Srivatsa Vaddagiri, Linux Technology Center, IBM Software Labs, Bangalore, INDIA - 560017 - 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/