Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751356AbdI1Ta0 (ORCPT ); Thu, 28 Sep 2017 15:30:26 -0400 Received: from smtp-3.orcon.net.nz ([60.234.4.44]:59996 "EHLO smtp-3.orcon.net.nz" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750759AbdI1TaY (ORCPT ); Thu, 28 Sep 2017 15:30:24 -0400 X-Greylist: delayed 1805 seconds by postgrey-1.27 at vger.kernel.org; Thu, 28 Sep 2017 15:30:24 EDT Date: Fri, 29 Sep 2017 07:59:09 +1300 From: Michael Cree To: "Paul E. McKenney" Cc: Will Deacon , Peter Zijlstra , kirill.shutemov@linux.intel.com, linux-kernel@vger.kernel.org, ynorov@caviumnetworks.com, rruigrok@codeaurora.org, linux-arch@vger.kernel.org, akpm@linux-foundation.org, catalin.marinas@arm.com, rth@twiddle.net, ink@jurassic.park.msu.ru, mattst88@gmail.com, linux-alpha@vger.kernel.org Subject: Re: [RFC PATCH 1/2] arm64: mm: Use READ_ONCE/WRITE_ONCE when accessing page tables Message-ID: <20170928185909.kgwy36hkoqxqgyc3@tower> Mail-Followup-To: Michael Cree , "Paul E. McKenney" , Will Deacon , Peter Zijlstra , kirill.shutemov@linux.intel.com, linux-kernel@vger.kernel.org, ynorov@caviumnetworks.com, rruigrok@codeaurora.org, linux-arch@vger.kernel.org, akpm@linux-foundation.org, catalin.marinas@arm.com, rth@twiddle.net, ink@jurassic.park.msu.ru, mattst88@gmail.com, linux-alpha@vger.kernel.org References: <1506527369-19535-1-git-send-email-will.deacon@arm.com> <1506527369-19535-2-git-send-email-will.deacon@arm.com> <20170928083801.m6rb4frbbgzgam2o@hirez.programming.kicks-ass.net> <20170928084535.GA19060@arm.com> <20170928154354.GK3521@linux.vnet.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20170928154354.GK3521@linux.vnet.ibm.com> User-Agent: NeoMutt/20170113 (1.7.2) X-GeoIP: NZ X-Spam_score: -2.9 X-Spam_score_int: -28 X-Spam_bar: -- Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1802 Lines: 34 On Thu, Sep 28, 2017 at 08:43:54AM -0700, Paul E. McKenney wrote: > On Thu, Sep 28, 2017 at 09:45:35AM +0100, Will Deacon wrote: > > On Thu, Sep 28, 2017 at 10:38:01AM +0200, Peter Zijlstra wrote: > > > On Wed, Sep 27, 2017 at 04:49:28PM +0100, Will Deacon wrote: > > > > In many cases, page tables can be accessed concurrently by either another > > > > CPU (due to things like fast gup) or by the hardware page table walker > > > > itself, which may set access/dirty bits. In such cases, it is important > > > > to use READ_ONCE/WRITE_ONCE when accessing page table entries so that > > > > entries cannot be torn, merged or subject to apparent loss of coherence. > > > > > > In fact, we should use lockless_dereference() for many of them. Yes > > > Alpha is the only one that cares about the difference between that and > > > READ_ONCE() and they do have the extra barrier, but if we're going to do > > > this, we might as well do it 'right' :-) > > > > I know this sounds daft, but I think one of the big reasons why > > lockless_dereference() doesn't get an awful lot of use is because it's > > such a mouthful! Why don't we just move the smp_read_barrier_depends() > > into READ_ONCE? Would anybody actually care about the potential impact on > > Alpha (which, frankly, is treading on thin ice given the low adoption of > > lockless_dereference())? > > This is my cue to ask my usual question... ;-) > > Are people still running mainline kernels on Alpha? (Added Alpha folks.) Yes. I run two Alpha build daemons that build the unofficial debian-alpha port. Debian popcon reports nine machines running Alpha, which are likely to be running the 4.12.y kernel which is currently in debian-alpha, (and presumably soon to be 4.13.y which is now built on Alpha in experimental). Cheers Michael