On 26.09.19 15:02, Qian Cai wrote: > 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@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. > > 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. > I am going to add documentation to build_all_zonelists() regarding locking and the details we discussed. Of course, I'll do more testing, and as Michal suggested, we should let this "mature" in linux-next for some time. -- Thanks, David / dhildenb
