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Date:   Thu, 30 Aug 2018 19:36:43 +0000
From:   Pasha Tatashin <Pavel.Tatashin@...rosoft.com>
To:     David Hildenbrand <david@...hat.com>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>
CC:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-doc@...r.kernel.org" <linux-doc@...r.kernel.org>,
        "linuxppc-dev@...ts.ozlabs.org" <linuxppc-dev@...ts.ozlabs.org>,
        "linux-acpi@...r.kernel.org" <linux-acpi@...r.kernel.org>,
        "xen-devel@...ts.xenproject.org" <xen-devel@...ts.xenproject.org>,
        "devel@...uxdriverproject.org" <devel@...uxdriverproject.org>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Len Brown <lenb@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Juergen Gross <jgross@...e.com>,
        Nathan Fontenot <nfont@...ux.vnet.ibm.com>,
        John Allen <jallen@...ux.vnet.ibm.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Michal Hocko <mhocko@...e.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Oscar Salvador <osalvador@...e.de>,
        Mathieu Malaterre <malat@...ian.org>,
        YASUAKI ISHIMATSU <yasu.isimatu@...il.com>
Subject: Re: [PATCH RFCv2 2/6] mm/memory_hotplug: make add_memory() take the
 device_hotplug_lock

On 8/21/18 6:44 AM, David Hildenbrand wrote:
> add_memory() currently does not take the device_hotplug_lock, however
> is aleady called under the lock from
> 	arch/powerpc/platforms/pseries/hotplug-memory.c
> 	drivers/acpi/acpi_memhotplug.c
> to synchronize against CPU hot-remove and similar.
> 
> In general, we should hold the device_hotplug_lock when adding memory
> to synchronize against online/offline request (e.g. from user space) -
> which already resulted in lock inversions due to device_lock() and
> mem_hotplug_lock - see 30467e0b3be ("mm, hotplug: fix concurrent memory
> hot-add deadlock"). add_memory()/add_memory_resource() will create memory
> block devices, so this really feels like the right thing to do.
> 
> Holding the device_hotplug_lock makes sure that a memory block device
> can really only be accessed (e.g. via .online/.state) from user space,
> once the memory has been fully added to the system.
> 
> The lock is not held yet in
> 	drivers/xen/balloon.c
> 	arch/powerpc/platforms/powernv/memtrace.c
> 	drivers/s390/char/sclp_cmd.c
> 	drivers/hv/hv_balloon.c
> So, let's either use the locked variants or take the lock.
> 
> Don't export add_memory_resource(), as it once was exported to be used
> by XEN, which is never built as a module. If somebody requires it, we
> also have to export a locked variant (as device_hotplug_lock is never
> exported).

Reviewed-by: Pavel Tatashin <pavel.tatashin@...rosoft.com>

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