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    SubjectRe: [PATCH v1] mm/memory_hotplug: Don't take the cpu_hotplug_lock
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    On Thu, 2019-09-26 at 13:52 +0200, Michal Hocko wrote:
    > On Thu 26-09-19 07:19:27, Qian Cai wrote:
    > >
    > >
    > > > On Sep 26, 2019, at 3:26 AM, Michal Hocko <mhocko@kernel.org> wrote:
    > > >
    > > > OK, this is using for_each_online_cpu but why is this a problem? Have
    > > > you checked what the code actually does? Let's say that online_pages is
    > > > racing with cpu hotplug. A new CPU appears/disappears from the online
    > > > mask while we are iterating it, right? Let's start with cpu offlining
    > > > case. We have two choices, either the cpu is still visible and we update
    > > > its local node configuration even though it will disappear shortly which
    > > > is ok because we are not touching any data that disappears (it's all
    > > > per-cpu). Case when the cpu is no longer there is not really
    > > > interesting. For the online case we might miss a cpu but that should be
    > > > tolerateable because that is not any different from triggering the
    > > > online independently of the memory hotplug. So there has to be a hook
    > > > from that code path as well. If there is none then this is buggy
    > > > irrespective of the locking.
    > > >
    > > > Makes sense?
    > >
    > > This sounds to me requires lots of audits and testing. Also, someone who is more
    > > familiar with CPU hotplug should review this patch.
    >
    > Thomas is on the CC list.
    >
    > > Personally, I am no fun of
    > > operating on an incorrect CPU mask to begin with, things could go wrong really
    > > quickly...
    >
    > Do you have any specific arguments? Just think of cpu and memory
    > hotplugs being independent operations. There is nothing really
    > inherently binding them together. If the cpu_online_mask really needs a
    > special treatment here then I would like to hear about that. Handwaving
    > doesn't really helps us.

    That is why I said it needs CPU hotplug experts to confirm that things including
    if CPU masks are tolerate to this kind of "abuse", or in-depth analysis of each
    calls sites that access CPU masks in both online_pages() and offline_pages() as
    well as ideally, more testing data in those areas.

    However, many kernel commits were merged with the expectations that people are
    going to deal with the aftermath, so I am not going to insist.

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