Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756011AbYCQVXm (ORCPT ); Mon, 17 Mar 2008 17:23:42 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1752727AbYCQVXd (ORCPT ); Mon, 17 Mar 2008 17:23:33 -0400 Received: from x346.tv-sign.ru ([89.108.83.215]:56203 "EHLO mail.screens.ru" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751801AbYCQVXc (ORCPT ); Mon, 17 Mar 2008 17:23:32 -0400 Date: Tue, 18 Mar 2008 00:23:04 +0300 From: Oleg Nesterov To: "Paul E. McKenney" Cc: linux-kernel@vger.kernel.org, rostedt@goodmis.org, linux-rt-users@vger.kernel.org, mingo@elte.hu, ego@in.ibm.com, dipankar@in.ibm.com, tytso@us.ibm.com, dvhltc@us.ibm.com, akpm@linux-foundation.org, josh@freedesktop.org, tglx@linutronix.de, niv@us.ibm.com, heiko.carstens@de.ibm.com Subject: Re: [PATCH] fix misplaced mb() in rcu_enter/exit_nohz() Message-ID: <20080317212304.GA118@tv-sign.ru> References: <20080317010821.GA29875@linux.vnet.ibm.com> <20080317183047.GA188@tv-sign.ru> <20080317190605.GG10955@linux.vnet.ibm.com> <20080317201741.GA92@tv-sign.ru> <20080317204357.GI10955@linux.vnet.ibm.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20080317204357.GI10955@linux.vnet.ibm.com> User-Agent: Mutt/1.5.11 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2270 Lines: 69 On 03/17, Paul E. McKenney wrote: > > On Mon, Mar 17, 2008 at 11:17:41PM +0300, Oleg Nesterov wrote: > > (to clarify: my question is completely offtopic to this patch) > > On 03/17, Paul E. McKenney wrote: > > > On Mon, Mar 17, 2008 at 09:30:47PM +0300, Oleg Nesterov wrote: > > > > I'm not sure the code below is up to date, but what I have in > > > > arch/s390/kernel/time.c is: > > > > > > > > stop_hz_timer: > > > > > > > > cpu_set(cpu, nohz_cpu_mask); > > > > > > > > if (rcu_needs_cpu(cpu) || local_softirq_pending()) { > > > > cpu_clear(cpu, nohz_cpu_mask); > > > > return; > > > > } > > > > > > > > Don't we need smp_mb() after cpu_set() ? > > > > > > S390's memory model is quite strong, so it might not be needed. > > > > OK, in that case we shouldn't worry. > > I don't know if I would go -that- far. ;-) > > > > In any > > > case, if needed, it goes -before- the cpu_set(), because the problems > > > would arise if prior RCU read-side critical sections were to be reordered > > > to follow this cpu_set(), right? > > > > No, but it is very possible I missed something. > > > > What if rcu_needs_cpu(cpu) is executed before cpu_set(cpu, nohz_cpu_mask)? > > It can miss rcu_start_batch() -> rcp->cur++ and return false, but at the > > same time rcu_start_batch() may see nohz_cpu_mask without this CPU. > > If you mean that the rcu_needs_cpu() executes before the cpu_set() in > the code fragment above, while the rcu_start_batch() executes on some > other CPU? Yes, and __rcu_pending() sees the old value of ->cur. IOW. Suppose that this CPU reads rcp->cur out of order. To simplify, let's suppose that stop_hz_timer() on CPU_0 in fact does xxx = rcu_needs_cpu(cpu); // false // ---- WINDOW ------ cpu_set(cpu, nohz_cpu_mask); if (xxx || local_softirq_pending()) { ... abort ... } ...proceed... Another CPU does rcu_start_batch() in the window above. In that case rcp->cpumask will include CPU_0, and the grace period can't be completed untill CPU_0 is "woken". Oleg. -- 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/