From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-qt0-f200.google.com (mail-qt0-f200.google.com [209.85.216.200]) by kanga.kvack.org (Postfix) with ESMTP id 040056B0023 for ; Thu, 22 Mar 2018 20:50:21 -0400 (EDT) Received: by mail-qt0-f200.google.com with SMTP id h89so6792039qtd.18 for ; Thu, 22 Mar 2018 17:50:20 -0700 (PDT) Received: from mx1.redhat.com (mx3-rdu2.redhat.com. [66.187.233.73]) by mx.google.com with ESMTPS id 129si5186624qki.431.2018.03.22.17.50.19 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Thu, 22 Mar 2018 17:50:19 -0700 (PDT) Date: Thu, 22 Mar 2018 20:50:17 -0400 From: Jerome Glisse Subject: Re: [PATCH 04/15] mm/hmm: unregister mmu_notifier when last HMM client quit v2 Message-ID: <20180323005017.GB5011@redhat.com> References: <20180320020038.3360-5-jglisse@redhat.com> <20180321181614.9968-1-jglisse@redhat.com> <20180321234110.GK3214@redhat.com> <20180322233715.GA5011@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: Sender: owner-linux-mm@kvack.org List-ID: To: John Hubbard Cc: linux-mm@kvack.org, Andrew Morton , linux-kernel@vger.kernel.org, Evgeny Baskakov , Ralph Campbell , Mark Hairgrove On Thu, Mar 22, 2018 at 05:13:14PM -0700, John Hubbard wrote: > On 03/22/2018 04:37 PM, Jerome Glisse wrote: > > On Thu, Mar 22, 2018 at 03:47:16PM -0700, John Hubbard wrote: > >> On 03/21/2018 04:41 PM, Jerome Glisse wrote: > >>> On Wed, Mar 21, 2018 at 04:22:49PM -0700, John Hubbard wrote: > >>>> On 03/21/2018 11:16 AM, jglisse@redhat.com wrote: > >>>>> From: Jerome Glisse > > > > >>> > >>> No this code is correct. hmm->mm is set after hmm struct is allocated > >>> and before it is public so no one can race with that. It is clear in > >>> hmm_mirror_unregister() under the write lock hence checking it here > >>> under that same lock is correct. > >> > >> Are you implying that code that calls hmm_mirror_register() should do > >> it's own locking, to prevent simultaneous calls to that function? Because > >> as things are right now, multiple threads can arrive at this point. The > >> fact that mirror->hmm is not "public" is irrelevant; what matters is that > >>> 1 thread can change it simultaneously. > > > > The content of struct hmm_mirror should not be modified by code outside > > HMM after hmm_mirror_register() and before hmm_mirror_unregister(). This > > is a private structure to HMM and the driver should not touch it, ie it > > should be considered as read only/const from driver code point of view. > > Yes, that point is clear and obvious. > > > > > It is also expected (which was obvious to me) that driver only call once > > and only once hmm_mirror_register(), and only once hmm_mirror_unregister() > > for any given hmm_mirror struct. Note that driver can register multiple > > _different_ mirror struct to same mm or differents mm. > > > > There is no need of locking on the driver side whatsoever as long as the > > above rules are respected. I am puzzle if they were not obvious :) > > Those rules were not obvious. It's unusual to claim that register and unregister > can run concurrently, but regiser and register cannot. Let's please document > the rules a bit in the comments. I am really surprise this was not obvious. All existing _register API in the kernel follow this. You register something once only and doing it twice for same structure (ie unique struct hmm_mirror *mirror pointer value) leads to serious bugs (doing so concurently or not). For instance if you call mmu_notifier_register() twice (concurrently or not) with same pointer value for struct mmu_notifier *mn then bad thing will happen. Same for driver_register() but this one actualy have sanity check and complain loudly if that happens. I doubt there is any single *_register/unregister() in the kernel that does not follow this. Note that doing register/unregister concurrently for the same unique hmm_mirror struct is also illegal. However concurrent register and unregister of different hmm_mirror struct is legal and this is the reasons for races we were discussing. Cheers, Jerome