From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.1 (2015-04-28) on archive.lwn.net X-Spam-Level: X-Spam-Status: No, score=-5.8 required=5.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,RCVD_IN_DNSWL_HI autolearn=ham autolearn_force=no version=3.4.1 Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by archive.lwn.net (Postfix) with ESMTP id 5B9DA7D082 for ; Thu, 11 Oct 2018 07:43:49 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726893AbeJKPJv (ORCPT ); Thu, 11 Oct 2018 11:09:51 -0400 Received: from mx1.redhat.com ([209.132.183.28]:59104 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726219AbeJKPJv (ORCPT ); Thu, 11 Oct 2018 11:09:51 -0400 Received: from smtp.corp.redhat.com (int-mx03.intmail.prod.int.phx2.redhat.com [10.5.11.13]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id EE4A09D426; Thu, 11 Oct 2018 07:43:47 +0000 (UTC) Received: from [10.36.117.105] (ovpn-117-105.ams2.redhat.com [10.36.117.105]) by smtp.corp.redhat.com (Postfix) with ESMTP id 72B7E608E7; Thu, 11 Oct 2018 07:43:46 +0000 (UTC) Subject: Re: [PATCH 2/2] docs/core-api: memory-hotplug: add some details about locking internals To: Mike Rapoport , Jonathan Corbet Cc: Andrew Morton , Stephen Rothwell , linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org References: <1539233897-10207-1-git-send-email-rppt@linux.ibm.com> <1539233897-10207-3-git-send-email-rppt@linux.ibm.com> From: David Hildenbrand Openpgp: preference=signencrypt Autocrypt: addr=david@redhat.com; prefer-encrypt=mutual; keydata= xsFNBFXLn5EBEAC+zYvAFJxCBY9Tr1xZgcESmxVNI/0ffzE/ZQOiHJl6mGkmA1R7/uUpiCjJ dBrn+lhhOYjjNefFQou6478faXE6o2AhmebqT4KiQoUQFV4R7y1KMEKoSyy8hQaK1umALTdL QZLQMzNE74ap+GDK0wnacPQFpcG1AE9RMq3aeErY5tujekBS32jfC/7AnH7I0v1v1TbbK3Gp XNeiN4QroO+5qaSr0ID2sz5jtBLRb15RMre27E1ImpaIv2Jw8NJgW0k/D1RyKCwaTsgRdwuK Kx/Y91XuSBdz0uOyU/S8kM1+ag0wvsGlpBVxRR/xw/E8M7TEwuCZQArqqTCmkG6HGcXFT0V9 PXFNNgV5jXMQRwU0O/ztJIQqsE5LsUomE//bLwzj9IVsaQpKDqW6TAPjcdBDPLHvriq7kGjt WhVhdl0qEYB8lkBEU7V2Yb+SYhmhpDrti9Fq1EsmhiHSkxJcGREoMK/63r9WLZYI3+4W2rAc UucZa4OT27U5ZISjNg3Ev0rxU5UH2/pT4wJCfxwocmqaRr6UYmrtZmND89X0KigoFD/XSeVv jwBRNjPAubK9/k5NoRrYqztM9W6sJqrH8+UWZ1Idd/DdmogJh0gNC0+N42Za9yBRURfIdKSb B3JfpUqcWwE7vUaYrHG1nw54pLUoPG6sAA7Mehl3nd4pZUALHwARAQABzSREYXZpZCBIaWxk ZW5icmFuZCA8ZGF2aWRAcmVkaGF0LmNvbT7CwX4EEwECACgFAljj9eoCGwMFCQlmAYAGCwkI BwMCBhUIAgkKCwQWAgMBAh4BAheAAAoJEE3eEPcA/4Na5IIP/3T/FIQMxIfNzZshIq687qgG 8UbspuE/YSUDdv7r5szYTK6KPTlqN8NAcSfheywbuYD9A4ZeSBWD3/NAVUdrCaRP2IvFyELj xoMvfJccbq45BxzgEspg/bVahNbyuBpLBVjVWwRtFCUEXkyazksSv8pdTMAs9IucChvFmmq3 jJ2vlaz9lYt/lxN246fIVceckPMiUveimngvXZw21VOAhfQ+/sofXF8JCFv2mFcBDoa7eYob s0FLpmqFaeNRHAlzMWgSsP80qx5nWWEvRLdKWi533N2vC/EyunN3HcBwVrXH4hxRBMco3jvM m8VKLKao9wKj82qSivUnkPIwsAGNPdFoPbgghCQiBjBe6A75Z2xHFrzo7t1jg7nQfIyNC7ez MZBJ59sqA9EDMEJPlLNIeJmqslXPjmMFnE7Mby/+335WJYDulsRybN+W5rLT5aMvhC6x6POK z55fMNKrMASCzBJum2Fwjf/VnuGRYkhKCqqZ8gJ3OvmR50tInDV2jZ1DQgc3i550T5JDpToh dPBxZocIhzg+MBSRDXcJmHOx/7nQm3iQ6iLuwmXsRC6f5FbFefk9EjuTKcLMvBsEx+2DEx0E UnmJ4hVg7u1PQ+2Oy+Lh/opK/BDiqlQ8Pz2jiXv5xkECvr/3Sv59hlOCZMOaiLTTjtOIU7Tq 7ut6OL64oAq+zsFNBFXLn5EBEADn1959INH2cwYJv0tsxf5MUCghCj/CA/lc/LMthqQ773ga uB9mN+F1rE9cyyXb6jyOGn+GUjMbnq1o121Vm0+neKHUCBtHyseBfDXHA6m4B3mUTWo13nid 0e4AM71r0DS8+KYh6zvweLX/LL5kQS9GQeT+QNroXcC1NzWbitts6TZ+IrPOwT1hfB4WNC+X 2n4AzDqp3+ILiVST2DT4VBc11Gz6jijpC/KI5Al8ZDhRwG47LUiuQmt3yqrmN63V9wzaPhC+ xbwIsNZlLUvuRnmBPkTJwwrFRZvwu5GPHNndBjVpAfaSTOfppyKBTccu2AXJXWAE1Xjh6GOC 8mlFjZwLxWFqdPHR1n2aPVgoiTLk34LR/bXO+e0GpzFXT7enwyvFFFyAS0Nk1q/7EChPcbRb hJqEBpRNZemxmg55zC3GLvgLKd5A09MOM2BrMea+l0FUR+PuTenh2YmnmLRTro6eZ/qYwWkC u8FFIw4pT0OUDMyLgi+GI1aMpVogTZJ70FgV0pUAlpmrzk/bLbRkF3TwgucpyPtcpmQtTkWS gDS50QG9DR/1As3LLLcNkwJBZzBG6PWbvcOyrwMQUF1nl4SSPV0LLH63+BrrHasfJzxKXzqg rW28CTAE2x8qi7e/6M/+XXhrsMYG+uaViM7n2je3qKe7ofum3s4vq7oFCPsOgwARAQABwsFl BBgBAgAPBQJVy5+RAhsMBQkJZgGAAAoJEE3eEPcA/4NagOsP/jPoIBb/iXVbM+fmSHOjEshl KMwEl/m5iLj3iHnHPVLBUWrXPdS7iQijJA/VLxjnFknhaS60hkUNWexDMxVVP/6lbOrs4bDZ NEWDMktAeqJaFtxackPszlcpRVkAs6Msn9tu8hlvB517pyUgvuD7ZS9gGOMmYwFQDyytpepo YApVV00P0u3AaE0Cj/o71STqGJKZxcVhPaZ+LR+UCBZOyKfEyq+ZN311VpOJZ1IvTExf+S/5 lqnciDtbO3I4Wq0ArLX1gs1q1XlXLaVaA3yVqeC8E7kOchDNinD3hJS4OX0e1gdsx/e6COvy qNg5aL5n0Kl4fcVqM0LdIhsubVs4eiNCa5XMSYpXmVi3HAuFyg9dN+x8thSwI836FoMASwOl C7tHsTjnSGufB+D7F7ZBT61BffNBBIm1KdMxcxqLUVXpBQHHlGkbwI+3Ye+nE6HmZH7IwLwV W+Ajl7oYF+jeKaH4DZFtgLYGLtZ1LDwKPjX7VAsa4Yx7S5+EBAaZGxK510MjIx6SGrZWBrrV TEvdV00F2MnQoeXKzD7O4WFbL55hhyGgfWTHwZ457iN9SgYi1JLPqWkZB0JRXIEtjd4JEQcx +8Umfre0Xt4713VxMygW0PnQt5aSQdMD58jHFxTk092mU+yIHj5LeYgvwSgZN4airXk5yRXl SE+xAvmumFBY Organization: Red Hat GmbH Message-ID: Date: Thu, 11 Oct 2018 09:43:45 +0200 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.0 MIME-Version: 1.0 In-Reply-To: <1539233897-10207-3-git-send-email-rppt@linux.ibm.com> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit X-Scanned-By: MIMEDefang 2.79 on 10.5.11.13 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.39]); Thu, 11 Oct 2018 07:43:48 +0000 (UTC) Sender: linux-doc-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-doc@vger.kernel.org On 11/10/2018 06:58, Mike Rapoport wrote: > From: David Hildenbrand > > Let's document the magic a bit, especially why device_hotplug_lock is > required when adding/removing memory and how it all play together with > requests to online/offline memory from user space. > > [ rppt: moved the text to Documentation/core-api/memory-hotplug.rst ] > > Link: http://lkml.kernel.org/r/20180925091457.28651-7-david@redhat.com > Signed-off-by: David Hildenbrand > Reviewed-by: Pavel Tatashin > Reviewed-by: Rashmica Gupta > Cc: Jonathan Corbet > Cc: Michal Hocko > Cc: Balbir Singh > Cc: Benjamin Herrenschmidt > Cc: Boris Ostrovsky > Cc: Dan Williams > Cc: Greg Kroah-Hartman > Cc: Haiyang Zhang > Cc: Heiko Carstens > Cc: John Allen > Cc: Joonsoo Kim > Cc: Juergen Gross > Cc: Kate Stewart > Cc: "K. Y. Srinivasan" > Cc: Len Brown > Cc: Martin Schwidefsky > Cc: Mathieu Malaterre > Cc: Michael Ellerman > Cc: Michael Neuling > Cc: Nathan Fontenot > Cc: Oscar Salvador > Cc: Paul Mackerras > Cc: Philippe Ombredanne > Cc: Rafael J. Wysocki > Cc: "Rafael J. Wysocki" > Cc: Stephen Hemminger > Cc: Thomas Gleixner > Cc: Vlastimil Babka > Cc: YASUAKI ISHIMATSU > Signed-off-by: Andrew Morton > Signed-off-by: Mike Rapoport > --- > Documentation/core-api/memory-hotplug.rst | 38 +++++++++++++++++++++++++++++++ > 1 file changed, 38 insertions(+) > > diff --git a/Documentation/core-api/memory-hotplug.rst b/Documentation/core-api/memory-hotplug.rst > index a99f2f2..de7467e 100644 > --- a/Documentation/core-api/memory-hotplug.rst > +++ b/Documentation/core-api/memory-hotplug.rst > @@ -85,3 +85,41 @@ MEM_ONLINE, or MEM_OFFLINE action to cancel hotplugging. It stops > further processing of the notification queue. > > NOTIFY_STOP stops further processing of the notification queue. > + > +Locking Internals > +================= > + > +When adding/removing memory that uses memory block devices (i.e. ordinary RAM), > +the device_hotplug_lock should be held to: > + > +- synchronize against online/offline requests (e.g. via sysfs). This way, memory > + block devices can only be accessed (.online/.state attributes) by user > + space once memory has been fully added. And when removing memory, we > + know nobody is in critical sections. > +- synchronize against CPU hotplug and similar (e.g. relevant for ACPI and PPC) > + > +Especially, there is a possible lock inversion that is avoided using > +device_hotplug_lock when adding memory and user space tries to online that > +memory faster than expected: > + > +- device_online() will first take the device_lock(), followed by > + mem_hotplug_lock > +- add_memory_resource() will first take the mem_hotplug_lock, followed by > + the device_lock() (while creating the devices, during bus_add_device()). > + > +As the device is visible to user space before taking the device_lock(), this > +can result in a lock inversion. > + > +onlining/offlining of memory should be done via device_online()/ > +device_offline() - to make sure it is properly synchronized to actions > +via sysfs. Holding device_hotplug_lock is advised (to e.g. protect online_type) > + > +When adding/removing/onlining/offlining memory or adding/removing > +heterogeneous/device memory, we should always hold the mem_hotplug_lock in > +write mode to serialise memory hotplug (e.g. access to global/zone > +variables). > + > +In addition, mem_hotplug_lock (in contrast to device_hotplug_lock) in read > +mode allows for a quite efficient get_online_mems/put_online_mems > +implementation, so code accessing memory can protect from that memory > +vanishing. > Looks good to me. -- Thanks, David / dhildenb