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[14.202.194.140]) by smtp.gmail.com with ESMTPSA id n26-v6sm602291pgd.45.2018.09.18.18.22.09 (version=TLS1_2 cipher=ECDHE-RSA-CHACHA20-POLY1305 bits=256/256); Tue, 18 Sep 2018 18:22:10 -0700 (PDT) Date: Wed, 19 Sep 2018 11:22:07 +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: <20180919012207.GD8537@350D> References: <20180918114822.21926-1-david@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20180918114822.21926-1-david@redhat.com> User-Agent: Mutt/1.9.4 (2018-02-28) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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? Balbir Singh.