From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753688AbdCBA1v (ORCPT ); Wed, 1 Mar 2017 19:27:51 -0500 Received: from mx0b-001b2d01.pphosted.com ([148.163.158.5]:42405 "EHLO mx0a-001b2d01.pphosted.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1753506AbdCBA1o (ORCPT ); Wed, 1 Mar 2017 19:27:44 -0500 Date: Wed, 1 Mar 2017 18:04:29 +0100 From: Heiko Carstens To: Dan Williams Cc: Michal Hocko , Sebastian Ott , Linux MM , "linux-kernel@vger.kernel.org" , Andrew Morton , "Rafael J. Wysocki" , Vladimir Davydov , Ben Hutchings Subject: Re: [PATCH] mm, add_memory_resource: hold device_hotplug lock over mem_hotplug_{begin, done} References: <20170227162031.GA27937@dhcp22.suse.cz> <20170228115729.GB13872@osiris> <20170301125105.GA5208@osiris> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.5.21 (2010-09-15) X-TM-AS-GCONF: 00 X-Content-Scanned: Fidelis XPS MAILER x-cbid: 17030117-0040-0000-0000-000003370369 X-IBM-AV-DETECTION: SAVI=unused REMOTE=unused XFE=unused x-cbparentid: 17030117-0041-0000-0000-000024581BF1 Message-Id: <20170301170429.GB5208@osiris> X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10432:,, definitions=2017-03-01_12:,, signatures=0 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 spamscore=0 suspectscore=0 malwarescore=0 phishscore=0 adultscore=0 bulkscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.0.1-1612050000 definitions=main-1703010156 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Mar 01, 2017 at 07:52:18AM -0800, Dan Williams wrote: > On Wed, Mar 1, 2017 at 4:51 AM, Heiko Carstens > wrote: > > Since it is anything but obvious why Dan wrote in changelog of b5d24fda9c3d > > ("mm, devm_memremap_pages: hold device_hotplug lock over > > mem_hotplug_{begin, done}") that write accesses to > > mem_hotplug.active_writer are coordinated via lock_device_hotplug() I'd > > rather propose a new private memory_add_remove_lock which has similar > > semantics like the cpu_add_remove_lock for cpu hotplug (see patch below). > > > > However instead of sprinkling locking/unlocking of that new lock around all > > calls of mem_hotplug_begin() and mem_hotplug_end() simply include locking > > and unlocking into these two functions. > > > > This still allows get_online_mems() and put_online_mems() to work, while at > > the same time preventing mem_hotplug.active_writer corruption. > > > > Any opinions? > > Sorry, yes, I didn't make it clear that I derived that locking > requirement from store_mem_state() and its usage of > lock_device_hotplug_sysfs(). > > That routine is trying very hard not trip the soft-lockup detector. It > seems like that wants to be an interruptible wait. If you look at commit 5e33bc4165f3 ("driver core / ACPI: Avoid device hot remove locking issues") then lock_device_hotplug_sysfs() was introduced to avoid a different subtle deadlock, but it also sleeps uninterruptible, but not for more than 5ms ;) However I'm not sure if the device hotplug lock should also be used to fix an unrelated bug that was introduced with the get_online_mems() / put_online_mems() interface. Should it? If so, we need to sprinkle around a couple of lock_device_hotplug() calls near mem_hotplug_begin() calls, like Sebastian already started, and give it additional semantics (protecting mem_hotplug.active_writer), and hope it doesn't lead to deadlocks anywhere.