From mboxrd@z Thu Jan 1 00:00:00 1970 From: Mathieu Desnoyers Subject: Re: [RFC patch 08/18] cnt32_to_63 should use smp_rmb() Date: Fri, 7 Nov 2008 12:17:03 -0500 Message-ID: <20081107171703.GE22134@Krystal> References: <20081107164758.GB22134@Krystal> <20081107003816.9b0f947a.akpm@linux-foundation.org> <20081107052336.652868737@polymtl.ca> <20081107053349.861709786@polymtl.ca> <20081106220530.5b0e3a96.akpm@linux-foundation.org> <25363.1226056819@redhat.com> <8481.1226077497@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Transfer-Encoding: 7bit Return-path: Received: from tomts25-srv.bellnexxia.net ([209.226.175.188]:39123 "EHLO tomts25-srv.bellnexxia.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750929AbYKGRRG (ORCPT ); Fri, 7 Nov 2008 12:17:06 -0500 Content-Disposition: inline In-Reply-To: <8481.1226077497@redhat.com> Sender: linux-arch-owner@vger.kernel.org List-ID: To: David Howells Cc: Andrew Morton , Nicolas Pitre , Linus Torvalds , Ingo Molnar , Peter Zijlstra , linux-kernel@vger.kernel.org, Ralf Baechle , benh@kernel.crashing.org, paulus@samba.org, David Miller , Ingo Molnar , Thomas Gleixner , Steven Rostedt , linux-arch@vger.kernel.org * David Howells (dhowells@redhat.com) wrote: > Mathieu Desnoyers wrote: > > > First off, using this macro with get_cycles() is simply buggy, because the > > macro expects _perfect_ order of timestamps, no skew whatsoever, or > > otherwise time could jump. > > Erm... Why can't I pass it get_cycles()? Are you saying that sched_clock() > in MN10300 is wrong for it's use of get_cycles() with cnt32_to_63()? > Yes. Do you think the synchronization of the cycles counters is _perfect_ across CPUs so that there is no possible way whatsoever that two cycle counter values appear to go backward between CPUs ? (also taking in account delays in __m_cnt_hi write-back...) As I showed in my previous example, if you are unlucky enough to hit the spot where the cycle counters go backward at the time warp edge, time will jump of 2^32, so about 4.29s at 1GHz. > > __x.lo = readl(cnt_lo); /* mmio read */ > > readl() might insert an extra barrier instruction. Not only that, io_addr > must be unsigned long. If we expect the only correct use-case to be with readl(), I don't see the problem with added synchronization. > Ah, right, then the parameters should be updated accordingly. static inline u64 cnt32_to_63(unsigned long io_addr, u32 *__m_cnt_hi) { union cnt32_to_63 __x; __x.hi = *__m_cnt_hi; /* memory read for high bits internal state */ rmb(); /* * read high bits before low bits insures time * does not go backward. Sync across * CPUs and for interrupts. */ __x.lo = readl(io_addr); /* mmio read */ if (unlikely((s32)(__x.hi ^ __x.lo) < 0)) *__m_cnt_hi = __x.hi = (__x.hi ^ 0x80000000) + (__x.hi >> 31); return __x.val; } Mathieu > David -- Mathieu Desnoyers OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F BA06 3F25 A8FE 3BAE 9A68