Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753270AbYKGUy0 (ORCPT ); Fri, 7 Nov 2008 15:54:26 -0500 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1751414AbYKGUyQ (ORCPT ); Fri, 7 Nov 2008 15:54:16 -0500 Received: from e2.ny.us.ibm.com ([32.97.182.142]:59117 "EHLO e2.ny.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750953AbYKGUyP (ORCPT ); Fri, 7 Nov 2008 15:54:15 -0500 Date: Fri, 7 Nov 2008 12:54:11 -0800 From: "Paul E. McKenney" To: Mathieu Desnoyers Cc: Peter Zijlstra , Steven Rostedt , David Howells , Linus Torvalds , akpm@linux-foundation.org, Ingo Molnar , linux-kernel@vger.kernel.org, Nicolas Pitre , Ralf Baechle , benh@kernel.crashing.org, paulus@samba.org, David Miller , Ingo Molnar , Thomas Gleixner , linux-arch@vger.kernel.org Subject: Re: [RFC patch 08/18] cnt32_to_63 should use smp_rmb() Message-ID: <20081107205411.GG6917@linux.vnet.ibm.com> Reply-To: paulmck@linux.vnet.ibm.com References: <20081107053349.861709786@polymtl.ca> <20081107052336.652868737@polymtl.ca> <25257.1226055312@redhat.com> <20081107170902.GD22134@Krystal> <20081107191833.GA31809@Krystal> <1226086322.31966.67.camel@lappy.programming.kicks-ass.net> <20081107200243.GB32761@Krystal> <20081107204546.GA3324@Krystal> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20081107204546.GA3324@Krystal> User-Agent: Mutt/1.5.15+20070412 (2007-04-11) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 4471 Lines: 132 On Fri, Nov 07, 2008 at 03:45:46PM -0500, Mathieu Desnoyers wrote: > * Mathieu Desnoyers (mathieu.desnoyers@polymtl.ca) wrote: > > * Peter Zijlstra (a.p.zijlstra@chello.nl) wrote: > > > On Fri, 2008-11-07 at 14:18 -0500, Mathieu Desnoyers wrote: > > > > * Steven Rostedt (rostedt@goodmis.org) wrote: > > > > > > > > > > On Fri, 7 Nov 2008, Mathieu Desnoyers wrote: > > > > > > > > > > > > __m_cnt_hi > > > > > > is read before > > > > > > mmio cnt_lo read > > > > > > > > > > > > for the detailed reasons explained in my previous discussion with > > > > > > Nicolas here : > > > > > > http://lkml.org/lkml/2008/10/21/1 > > > > > > > > > > > > I use smp_rmb() to do this on SMP systems (hrm, actually, a rmb() could > > > > > > be required so it works also on UP systems safely wrt interrupts). > > > > > > > > > > smp_rmb turns into a compiler barrier on UP and should prevent the below > > > > > description. > > > > > > > > > > > > > Ah, right, preserving program order on UP should be enough. smp_rmb() > > > > then. > > > > > > > > > I'm not quite sure I'm following here. Is this a global hardware clock > > > you're reading from multiple cpus, if so, are you sure smp_rmb() will > > > indeed be enough to sync the read? > > > > > > (In which case the smp_wmb() is provided by the hardware increasing the > > > clock?) > > > > > > If these are per-cpu clocks then even in the smp case we'd be good with > > > a plain barrier() because you'd only ever want to read your own cpu's > > > clock (and have a separate __m_cnt_hi per cpu). > > > > > > Or am I totally missing out on something? > > > > > > > This is the global hardware clock scenario. > > > > We have to order an uncached mmio read wrt a cached variable read/write. > > The uncached mmio read vs smp_rmb() barrier (e.g. lfence instruction) > > should be insured by program order because the read will skip the cache > > and go directly to the bus. Luckily we only do a mmio read and no mmio > > write, so mmiowb() is not required. > > > > You might be right in that it could require more barriers. > > > > Given adequate program order, we can assume the the mmio read will > > happen "on the spot", but that the cached read may be delayed. > > > > What we want is : > > > > readl(io_addr) > > read __m_cnt_hi > > write __m_cnt_hi > > > > With the two reads in the correct order. If we consider two consecutive > > executions on the same CPU : > > > > readl(io_addr) > > read __m_cnt_hi > > write __m_cnt_hi > > > > readl(io_addr) > > read __m_cnt_hi > > write __m_cnt_hi > > > > We might have to order the read/write pair wrt the following readl, such > > as : > > > > smp_rmb(); /* Waits for every cached memory reads to complete */ > > readl(io_addr); > > barrier(); /* Make sure the compiler leaves mmio read before cached read */ > > read __m_cnt_hi > > write __m_cnt_hi > > > > smp_rmb(); /* Waits for every cached memory reads to complete */ > > readl(io_addr) > > barrier(); /* Make sure the compiler leaves mmio read before cached read */ > > read __m_cnt_hi > > write __m_cnt_hi > > > > Would that make more sense ? > > > > Oh, actually, I got things reversed in this email : the readl(io_addr) > must be done _after_ the __m_cnt_hi read. > > Therefore, two consecutive executions would look like : > > barrier(); /* Make sure the compiler does not reorder __m_cnt_hi and > previous mmio read. */ > read __m_cnt_hi > smp_rmb(); /* Waits for every cached memory reads to complete */ If these are MMIO reads, then you need rmb() rather than smp_rmb(), at least on architectures that can reorder writes (Power, Itanium, and I believe also ARM, ...). Thanx, Paul > readl(io_addr); > write __m_cnt_hi > > > barrier(); /* Make sure the compiler does not reorder __m_cnt_hi and > previous mmio read. */ > read __m_cnt_hi > smp_rmb(); /* Waits for every cached memory reads to complete */ > readl(io_addr); > write __m_cnt_hi > > Mathieu > > > Mathieu > > > > -- > > Mathieu Desnoyers > > OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F BA06 3F25 A8FE 3BAE 9A68 > > -- > Mathieu Desnoyers > OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F BA06 3F25 A8FE 3BAE 9A68 -- 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/