From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-qt0-f198.google.com (mail-qt0-f198.google.com [209.85.216.198]) by kanga.kvack.org (Postfix) with ESMTP id 6A9086B0023 for ; Thu, 22 Mar 2018 20:13:19 -0400 (EDT) Received: by mail-qt0-f198.google.com with SMTP id x35so6783214qtx.5 for ; Thu, 22 Mar 2018 17:13:19 -0700 (PDT) Received: from hqemgate14.nvidia.com (hqemgate14.nvidia.com. [216.228.121.143]) by mx.google.com with ESMTPS id j14si3221369qkk.201.2018.03.22.17.13.17 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Thu, 22 Mar 2018 17:13:17 -0700 (PDT) Subject: Re: [PATCH 04/15] mm/hmm: unregister mmu_notifier when last HMM client quit v2 References: <20180320020038.3360-5-jglisse@redhat.com> <20180321181614.9968-1-jglisse@redhat.com> <20180321234110.GK3214@redhat.com> <20180322233715.GA5011@redhat.com> From: John Hubbard Message-ID: Date: Thu, 22 Mar 2018 17:13:14 -0700 MIME-Version: 1.0 In-Reply-To: <20180322233715.GA5011@redhat.com> Content-Type: text/plain; charset="utf-8" Content-Language: en-US Content-Transfer-Encoding: quoted-printable Sender: owner-linux-mm@kvack.org List-ID: To: Jerome Glisse Cc: linux-mm@kvack.org, Andrew Morton , linux-kernel@vger.kernel.org, Evgeny Baskakov , Ralph Campbell , Mark Hairgrove 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: J=C3=A9r=C3=B4me 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=20 >> it's own locking, to prevent simultaneous calls to that function? Becaus= e >> 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 tha= t >>> 1 thread can change it simultaneously. >=20 > 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. >=20 > 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. >=20 > 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 unreg= ister can run concurrently, but regiser and register cannot. Let's please documen= t the rules a bit in the comments. >=20 > Note that the above rule means that for any given struct hmm_mirror their > can only be one and only one call to hmm_mirror_register() happening, no > concurrent call. If you are doing the latter then something is seriously > wrong in your design. >=20 > So to be clear on what variable are you claiming race ? > mirror->hmm ? > mirror->hmm->mm which is really hmm->mm (mirror part does not matter) ? >=20 > I will hold resending v4 until tomorrow morning (eastern time) so that > you can convince yourself that this code is right or prove me wrong. No need to wait. The documentation request above is a minor point, and we're OK with you resending v4 whenever you're ready. thanks, --=20 John Hubbard NVIDIA