On Mon, Aug 28, 2017 at 05:06:17PM +0200, Peter Zijlstra wrote:
> On Mon, Aug 28, 2017 at 04:58:08PM +0200, Peter Zijlstra wrote:
> > On Fri, Aug 25, 2017 at 12:03:04PM +0200, Borislav Petkov wrote:
> > > Hey,
> > >
> > > tglx says I have something for ya :-)
> > >
> > > ======================================================
> > > WARNING: possible circular locking dependency detected
> > > 4.13.0-rc6+ #1 Not tainted
> > > ------------------------------------------------------
> > > watchdog/3/27 is trying to acquire lock:
> > > (cpu_hotplug_lock.rw_sem){++++}, at: [<ffffffff8100c489>]
> > > release_ds_buffers+0x29/0xd0
> > >
> > > but now in release context of a crosslock acquired at the following:
> > > ((complete)&self->parked){+.+.}, at: [<ffffffff810895f6>]
> > > kthread_park+0x46/0x60
> >
> >
> > So I'm thinking this one is an actual deadlock.
> >
> > So, as far as I can tell this ends up being:
> >
> > CPU0 CPU1
> >
> > (smpboot_regiser_percpu_thread_cpumask)
> >
> > get_online_cpus()
> > __smpboot_create_thread()
> > kthread_park();
> > wait_for_completion(&X)
> >
> >
> > (smpboot_thread_fn)
> >
> > ->park() := watchdog_disable()
> > watchdog_nmi_disable()
> > perf_event_release_kernel();
> > put_event()
> > _free_event()
> > ->destroy() := hw_perf_event_destroy()
> > x86_release_hardware()
> > release_ds_buffers()
> > get_online_cpus()
> >
> >
> > kthread_parkme()
> > complete(&X)
> >
> >
> >
> > So CPU0 holds cpus_hotplug_lock while wait_for_completion() and CPU1
> > needs to acquire before complete().
> >
> > So if, in between, CPU2 does down_write(), things will get unstuck.
> >
> > What's worse, there's also:
> >
> > cpus_write_lock()
> > ...
> > takedown_cpu()
> > smpboot_park_threads()
> > smpboot_park_thread()
> > kthread_park()
> > ->park() := watchdog_disable()
> > watchdog_nmi_disable()
> > perf_event_release_kernel();
> > put_event()
> > _free_event()
> > ->destroy() := hw_perf_event_destroy()
> > x86_release_hardware()
> > release_ds_buffers()
> > get_online_cpus()
> >
> > which as far as I can tell, spells instant deadlock..
>
> Aah, but that latter will never happen.. because each CPU will have a
> &pmc_refcount and we can't unplug _all_ CPUs.
>
> So the first one will only ever happen on boot, where we park() the very
> first watchdog thread and is a potential deadlock, but won't happen
> because nobody is around to do down_write() just yet.
I suspect however it is possible to interleave:
sysctl.kernel.nmi_watchdog = 0
proc_watchdog_common()
with:
hot un-plug
watchdog_disable()
to tickle that exact problem. Just needs a bit of luck.
