[PATCH] hotplug: make register and unregister notifier API symmetric

[Michal Hocko]

[Let's CC more people - the thread started
http://lkml.kernel.org/r/1480540516-6458-1-git-send-email-yuzhao@xxxxxxxxxx]

On Fri 02-12-16 09:56:26, Dan Streetman wrote:
> On Fri, Dec 2, 2016 at 9:44 AM, Michal Hocko <mhocko@xxxxxxxxxx> wrote:
> > On Fri 02-12-16 15:38:48, Michal Hocko wrote:
> >> On Fri 02-12-16 09:24:35, Dan Streetman wrote:
> >> > On Fri, Dec 2, 2016 at 8:46 AM, Michal Hocko <mhocko@xxxxxxxxxx> wrote:
> >> > > On Wed 30-11-16 13:15:16, Yu Zhao wrote:
> >> > >> __unregister_cpu_notifier() only removes registered notifier from its
> >> > >> linked list when CPU hotplug is configured. If we free registered CPU
> >> > >> notifier when HOTPLUG_CPU=n, we corrupt the linked list.
> >> > >>
> >> > >> To fix the problem, we can either use a static CPU notifier that walks
> >> > >> through each pool or just simply disable CPU notifier when CPU hotplug
> >> > >> is not configured (which is perfectly safe because the code in question
> >> > >> is called after all possible CPUs are online and will remain online
> >> > >> until power off).
> >> > >>
> >> > >> v2: #ifdef for cpu_notifier_register_done during cleanup.
> >> > >
> >> > > this ifedfery is just ugly as hell. I am also wondering whether it is
> >> > > really needed. __register_cpu_notifier and __unregister_cpu_notifier are
> >> > > noops for CONFIG_HOTPLUG_CPU=n. So what's exactly that is broken here?
> >> >
> >> > hmm, that's interesting, __unregister_cpu_notifier is always a noop if
> >> > HOTPLUG_CPU=n, but __register_cpu_notifier is only a noop if
> >> > HOTPLUG_CPU=n *and* MODULE.  If !MODULE, __register_cpu_notifier does
> >>
> >> OK, I've missed the MODULE part
> >>
> >> > actually register!  This was added by commit
> >> > 47e627bc8c9a70392d2049e6af5bd55fae61fe53 ('hotplug: Allow modules to
> >> > use the cpu hotplug notifiers even if !CONFIG_HOTPLUG_CPU') and looks
> >> > like it's to allow built-ins to register so they can notice during
> >> > boot when cpus are initialized.
> >>
> >> I cannot say I wound understand the motivation but that is not really
> >> all that important.
> >>
> >> > IMHO, that is the real problem - sure, without HOTPLUG_CPU, nobody
> >> > should ever get a notification that a cpu is dying, but that doesn't
> >> > mean builtins that register notifiers will never unregister their
> >> > notifiers and then free them.
> >>
> >> Yes that is true. That suggests that __unregister_cpu_notifier should
> >> the the symmetric thing to the __register_cpu_notifier for
> >> CONFIG_MODULE, right?
> >
> > I meant the following. Completely untested
> 
> agreed, but also needs the non-__ version, and kernel/cpu.c needs
> tweaking to move those functions out of the #ifdef CONFIG_HOTPLUG_CPU
> section.

OK, this is still only compile tested. Yu Zhao, assuming you were able
to trigger the original problem could you test with the below patch
please?
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