From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Paul E. McKenney" Subject: Re: [RFC PATCH 1/2] arm64: mm: Use READ_ONCE/WRITE_ONCE when accessing page tables Date: Thu, 28 Sep 2017 17:58:30 -0700 Message-ID: <20170929005830.GU3521@linux.vnet.ibm.com> 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> <20170928185909.kgwy36hkoqxqgyc3@tower> Reply-To: paulmck@linux.vnet.ibm.com Mime-Version: 1.0 Return-path: Content-Disposition: inline In-Reply-To: <20170928185909.kgwy36hkoqxqgyc3@tower> Sender: linux-alpha-owner@vger.kernel.org List-ID: Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit To: Michael Cree , 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 On Fri, Sep 29, 2017 at 07:59:09AM +1300, Michael Cree wrote: > 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). I salute your dedication to Alpha! ;-) Thanx, Paul