From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932668AbcESWa2 (ORCPT ); Thu, 19 May 2016 18:30:28 -0400 Received: from g2t4618.austin.hp.com ([15.73.212.83]:47446 "EHLO g2t4618.austin.hp.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755206AbcESWaX (ORCPT ); Thu, 19 May 2016 18:30:23 -0400 Message-ID: <1463696487.2587.93.camel@j-VirtualBox> Subject: Re: [PATCH v4 2/5] locking/rwsem: Protect all writes to owner by WRITE_ONCE From: Jason Low To: Waiman Long Cc: jason.low2@hp.com, Davidlohr Bueso , Peter Zijlstra , Ingo Molnar , linux-kernel@vger.kernel.org, "Paul E. McKenney" , Scott J Norton , Douglas Hatch Date: Thu, 19 May 2016 15:21:27 -0700 In-Reply-To: <1463601515.2587.24.camel@j-VirtualBox> References: <1463534783-38814-1-git-send-email-Waiman.Long@hpe.com> <1463534783-38814-3-git-send-email-Waiman.Long@hpe.com> <20160518140436.GA6273@linux-uzut.site> <1463592095.3369.10.camel@j-VirtualBox> <573CB496.4010707@hpe.com> <1463601515.2587.24.camel@j-VirtualBox> Content-Type: text/plain; charset="UTF-8" X-Mailer: Evolution 3.10.4-0ubuntu2 Mime-Version: 1.0 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, 2016-05-18 at 12:58 -0700, Jason Low wrote: > On Wed, 2016-05-18 at 14:29 -0400, Waiman Long wrote: > > On 05/18/2016 01:21 PM, Jason Low wrote: > > > On Wed, 2016-05-18 at 07:04 -0700, Davidlohr Bueso wrote: > > >> On Tue, 17 May 2016, Waiman Long wrote: > > >> > > >>> Without using WRITE_ONCE(), the compiler can potentially break a > > >>> write into multiple smaller ones (store tearing). So a read from the > > >>> same data by another task concurrently may return a partial result. > > >>> This can result in a kernel crash if the data is a memory address > > >>> that is being dereferenced. > > >>> > > >>> This patch changes all write to rwsem->owner to use WRITE_ONCE() > > >>> to make sure that store tearing will not happen. READ_ONCE() may > > >>> not be needed for rwsem->owner as long as the value is only used for > > >>> comparison and not dereferencing. > > > It might be okay to leave out READ_ONCE() for reading rwsem->owner, but > > > couldn't we include it to at least document that we're performing a > > > "special" lockless read? > > > > > > > Using READ_ONCE() does have a bit of cost as it limits compiler > > optimization. If we changes all access to rwsem->owner to READ_ONCE() > > and WRITE_ONCE(), we may as well change its type to volatile and be done > > with. > > Right, although there are still places like the init function where > WRITE_ONCE isn't necessary. > > > I am not against doing that, but it feels a bit over-reach for me. > > On the other hand, we may define a do-nothing macro that designates the > > owner as a special variable for documentation purpose, but don't need > > protection at that particular call site. > > It should be fine to use the standard READ_ONCE here, even if it's just > for documentation, as it's probably not going to cost anything in > practice. It would be better to avoid adding any special macros for this > which may just add more complexity. By the way, this potential "partial write" issue may also apply to mutexes as well, so we should also make a similar change to mutex_set_owner() and mutex_clear_owner(). Jason