From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-pg1-x542.google.com (mail-pg1-x542.google.com [IPv6:2607:f8b0:4864:20::542]) (using TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)) (No client certificate requested) by lists.ozlabs.org (Postfix) with ESMTPS id 42Hryq4Rc4zF3LH for ; Sun, 23 Sep 2018 12:34:58 +1000 (AEST) Received: by mail-pg1-x542.google.com with SMTP id b129-v6so7678252pga.13 for ; Sat, 22 Sep 2018 19:34:58 -0700 (PDT) Date: Sun, 23 Sep 2018 12:34:52 +1000 From: Balbir Singh To: David Hildenbrand Cc: linux-mm@kvack.org, 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, Andrew Morton , Benjamin Herrenschmidt , Boris Ostrovsky , Dan Williams , Greg Kroah-Hartman , Haiyang Zhang , Heiko Carstens , John Allen , Jonathan Corbet , Joonsoo Kim , Juergen Gross , Kate Stewart , "K. Y. Srinivasan" , Len Brown , Martin Schwidefsky , Mathieu Malaterre , Michael Ellerman , Michael Neuling , Michal Hocko , Nathan Fontenot , Oscar Salvador , Paul Mackerras , Pavel Tatashin , Pavel Tatashin , Philippe Ombredanne , "Rafael J. Wysocki" , Rashmica Gupta , Stephen Hemminger , Thomas Gleixner , Vlastimil Babka , YASUAKI ISHIMATSU Subject: Re: [PATCH v1 0/6] mm: online/offline_pages called w.o. mem_hotplug_lock Message-ID: <20180923023452.GG8537@350D> References: <20180918114822.21926-1-david@redhat.com> <20180919012207.GD8537@350D> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii In-Reply-To: List-Id: Linux on PowerPC Developers Mail List List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , On Wed, Sep 19, 2018 at 09:35:07AM +0200, David Hildenbrand wrote: > Am 19.09.18 um 03:22 schrieb Balbir Singh: > > On Tue, Sep 18, 2018 at 01:48:16PM +0200, David Hildenbrand wrote: > >> Reading through the code and studying how mem_hotplug_lock is to be used, > >> I noticed that there are two places where we can end up calling > >> device_online()/device_offline() - online_pages()/offline_pages() without > >> the mem_hotplug_lock. And there are other places where we call > >> device_online()/device_offline() without the device_hotplug_lock. > >> > >> While e.g. > >> echo "online" > /sys/devices/system/memory/memory9/state > >> is fine, e.g. > >> echo 1 > /sys/devices/system/memory/memory9/online > >> Will not take the mem_hotplug_lock. However the device_lock() and > >> device_hotplug_lock. > >> > >> E.g. via memory_probe_store(), we can end up calling > >> add_memory()->online_pages() without the device_hotplug_lock. So we can > >> have concurrent callers in online_pages(). We e.g. touch in online_pages() > >> basically unprotected zone->present_pages then. > >> > >> Looks like there is a longer history to that (see Patch #2 for details), > >> and fixing it to work the way it was intended is not really possible. We > >> would e.g. have to take the mem_hotplug_lock in device/base/core.c, which > >> sounds wrong. > >> > >> Summary: We had a lock inversion on mem_hotplug_lock and device_lock(). > >> More details can be found in patch 3 and patch 6. > >> > >> I propose the general rules (documentation added in patch 6): > >> > >> 1. add_memory/add_memory_resource() must only be called with > >> device_hotplug_lock. > >> 2. remove_memory() must only be called with device_hotplug_lock. This is > >> already documented and holds for all callers. > >> 3. device_online()/device_offline() must only be called with > >> device_hotplug_lock. This is already documented and true for now in core > >> code. Other callers (related to memory hotplug) have to be fixed up. > >> 4. mem_hotplug_lock is taken inside of add_memory/remove_memory/ > >> online_pages/offline_pages. > >> > >> To me, this looks way cleaner than what we have right now (and easier to > >> verify). And looking at the documentation of remove_memory, using > >> lock_device_hotplug also for add_memory() feels natural. > >> > > > > That seems reasonable, but also implies that device_online() would hold > > back add/remove memory, could you please also document what mode > > read/write the locks need to be held? For example can the device_hotplug_lock > > be held in read mode while add/remove memory via (mem_hotplug_lock) is held > > in write mode? > > device_hotplug_lock is an ordinary mutex. So no option there. > > Only mem_hotplug_lock is a per CPU RW mutex. And as of now it only > exists to not require get_online_mems()/put_online_mems() to take the > device_hotplug_lock. Which is perfectly valid, because these users only > care about memory (not any other devices) not suddenly vanish. And that > RW lock makes things fast. > > Any modifications (online/offline/add/remove) require the > mem_hotplug_lock in write. > > I can add some more details to documentation in patch #6. > > "... we should always hold the mem_hotplug_lock (via > mem_hotplug_begin/mem_hotplug_done) in write mode to serialize memory > hotplug" ..." > > "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." > > Would that work for you? Yes, Thanks Balbir Singh.