On Thu 26-09-19 07:19:27, Qian Cai wrote: > > > > On Sep 26, 2019, at 3:26 AM, Michal Hocko <mhocko@xxxxxxxxxx> 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. -- Michal Hocko SUSE Labs
