From patchwork Tue Sep 18 11:48:22 2018 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: David Hildenbrand X-Patchwork-Id: 10604229 Return-Path: Received: from mail.wl.linuxfoundation.org (pdx-wl-mail.web.codeaurora.org [172.30.200.125]) by pdx-korg-patchwork-2.web.codeaurora.org (Postfix) with ESMTP id 171A615A6 for ; Tue, 18 Sep 2018 11:49:15 +0000 (UTC) Received: from mail.wl.linuxfoundation.org (localhost [127.0.0.1]) by mail.wl.linuxfoundation.org (Postfix) with ESMTP id 078EB2A6F6 for ; Tue, 18 Sep 2018 11:49:15 +0000 (UTC) Received: by mail.wl.linuxfoundation.org (Postfix, from userid 486) id EFB9E2A704; Tue, 18 Sep 2018 11:49:14 +0000 (UTC) X-Spam-Checker-Version: SpamAssassin 3.3.1 (2010-03-16) on pdx-wl-mail.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-7.9 required=2.0 tests=BAYES_00,MAILING_LIST_MULTI, RCVD_IN_DNSWL_HI autolearn=unavailable version=3.3.1 Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.wl.linuxfoundation.org (Postfix) with ESMTP id 966282A6F6 for ; Tue, 18 Sep 2018 11:49:14 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1729740AbeIRRVV (ORCPT ); Tue, 18 Sep 2018 13:21:21 -0400 Received: from mx1.redhat.com ([209.132.183.28]:58356 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727667AbeIRRVU (ORCPT ); Tue, 18 Sep 2018 13:21:20 -0400 Received: from smtp.corp.redhat.com (int-mx06.intmail.prod.int.phx2.redhat.com [10.5.11.16]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id 21F7B80E7E; Tue, 18 Sep 2018 11:49:07 +0000 (UTC) Received: from t460s.redhat.com (ovpn-116-248.ams2.redhat.com [10.36.116.248]) by smtp.corp.redhat.com (Postfix) with ESMTP id B43B417101; Tue, 18 Sep 2018 11:49:04 +0000 (UTC) From: David Hildenbrand To: linux-mm@kvack.org Cc: linux-kernel@vger.kernel.org, linux-doc@vger.kernel.org, linuxppc-dev@lists.ozlabs.org, linux-acpi@vger.kernel.org, xen-devel@lists.xenproject.org, devel@linuxdriverproject.org, David Hildenbrand , Jonathan Corbet , Michal Hocko , Andrew Morton Subject: [PATCH v1 6/6] memory-hotplug.txt: Add some details about locking internals Date: Tue, 18 Sep 2018 13:48:22 +0200 Message-Id: <20180918114822.21926-7-david@redhat.com> In-Reply-To: <20180918114822.21926-1-david@redhat.com> References: <20180918114822.21926-1-david@redhat.com> X-Scanned-By: MIMEDefang 2.79 on 10.5.11.16 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.25]); Tue, 18 Sep 2018 11:49:07 +0000 (UTC) Sender: linux-acpi-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-acpi@vger.kernel.org X-Virus-Scanned: ClamAV using ClamSMTP 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. Cc: Jonathan Corbet Cc: Michal Hocko Cc: Andrew Morton Reviewed-by: Pavel Tatashin Signed-off-by: David Hildenbrand --- Documentation/memory-hotplug.txt | 39 +++++++++++++++++++++++++++++++- 1 file changed, 38 insertions(+), 1 deletion(-) diff --git a/Documentation/memory-hotplug.txt b/Documentation/memory-hotplug.txt index 7f49ebf3ddb2..03aaad7d7373 100644 --- a/Documentation/memory-hotplug.txt +++ b/Documentation/memory-hotplug.txt @@ -3,7 +3,7 @@ Memory Hotplug ============== :Created: Jul 28 2007 -:Updated: Add description of notifier of memory hotplug: Oct 11 2007 +:Updated: Add some details about locking internals: Aug 20 2018 This document is about memory hotplug including how-to-use and current status. Because Memory Hotplug is still under development, contents of this text will @@ -495,6 +495,43 @@ 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 to +serialise memory hotplug (e.g. access to global/zone variables). + +In addition, mem_hotplug_lock (in contrast to device_hotplug_lock) allows +for a quite efficient get_online_mems/put_online_mems implementation. + + Future Work ===========