Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755261AbWKRWNS (ORCPT ); Sat, 18 Nov 2006 17:13:18 -0500 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1755268AbWKRWNS (ORCPT ); Sat, 18 Nov 2006 17:13:18 -0500 Received: from firewall.rowland.harvard.edu ([140.247.233.35]:9812 "HELO netrider.rowland.org") by vger.kernel.org with SMTP id S1755261AbWKRWNR (ORCPT ); Sat, 18 Nov 2006 17:13:17 -0500 Date: Sat, 18 Nov 2006 17:13:16 -0500 (EST) From: Alan Stern X-X-Sender: stern@netrider.rowland.org To: Oleg Nesterov cc: "Paul E. McKenney" , "Paul E. McKenney" , Kernel development list Subject: Re: [patch] cpufreq: mark cpufreq_tsc() as core_initcall_sync In-Reply-To: <20061118212542.GA235@oleg> Message-ID: MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1861 Lines: 43 On Sun, 19 Nov 2006, Oleg Nesterov wrote: > On 11/18, Alan Stern wrote: > > > > By the way, I think the fastpath for synchronize_srcu() should be safe, > > now that you have added the memory barriers into srcu_read_lock() and > > srcu_read_unlock(). You might as well try putting it in. > > I still think the fastpath should do mb() unconditionally to be correct. Yes, it definitely should. > > Although now that I look at it again, you have forgotten to put smp_mb() > > after the atomic_inc() call and before the atomic_dec(). > > As I see it, currently we don't need this barrier because synchronize_srcu() > does synchronize_sched() before reading ->hardluckref. > > But if we add the fastpath into synchronize_srcu() then yes, we need mb() > after atomic_inc(). > > Unless I totally confused :) Put it this way: If the missing memory barrier in srcu_read_lock() after the atomic_inc call isn't needed, then neither is the existing memory barrier after the per-cpu counter gets incremented. Likewise, if a memory barrier isn't needed before the atomic_dec in srcu_read_unlock(), then neither is the memory barrier before the per-cpu counter gets decremented. What you're ignoring is the synchronize_sched() call at the end of synchronize_srcu(), which has been replaced with smp_mb(). The smp_mb() needs to pair against a memory barrier on the read side, and that memory barrier has to occur after srcu_read_lock() has incremented the counter and before the read-side critical section begins. Otherwise code in the critical section might leak out to before the counter is incremented. Alan Stern - 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/