Re: [patch V2 24/24] cpu/hotplug: Convert hotplug locking to percpu rwsem

Steven Rostedt
2017-05-10T16:31:04Z

On Wed, 10 May 2017 10:49:09 +0200 (CEST)
Thomas Gleixner <tglx@linutronix.de> wrote:

> On Wed, 10 May 2017, Michael Ellerman wrote:
>
> > Thomas Gleixner <tglx@linutronix.de> writes:
> >
> > > @@ -130,6 +130,7 @@ void __static_key_slow_inc(struct static
> > > * the all CPUs, for that to be serialized against CPU hot-plug
> > > * we need to avoid CPUs coming online.
> > > */
> > > + lockdep_assert_hotplug_held();
> > > jump_label_lock();
> > > if (atomic_read(&key->enabled) == 0) {
> > > atomic_set(&key->enabled, -1);
> >
> > I seem to be hitting this assert from the ftrace event selftests,
> > enabled at boot with CONFIG_FTRACE_STARTUP_TEST=y, using next-20170509
> > (on powerpc).
> >
> > The stupidly obvious (or perhaps obviously stupid) patch below fixes it:
>
> Kinda. There is more horror in that area lurking and I'm still trying to
> figure out all the convoluted call pathes.

I finally got some time to look at this. I'm looking at your commit:

commit b53e5129c4c7ab47ec4f709fd8f5784ca45fb46d
Author: Thomas Gleixner <tglx@linutronix.de>
Date: Sun Apr 23 12:17:13 2017 +0200

trace/perf: Cure hotplug lock ordering issues

What were the circular locking dependencies that were uncovered.

event_mutex could possibly be broken up, if that helps. It sorta became
a catch all for various modifications to tracing.

-- Steve

>
> Thanks,
>
> tglx
>
> > diff --git a/kernel/trace/trace_events.c b/kernel/trace/trace_events.c
> > index daefdee9411a..5531f7ce8fa6 100644
> > --- a/kernel/trace/trace_events.c
> > +++ b/kernel/trace/trace_events.c
> > @@ -3241,9 +3241,19 @@ static __init void event_trace_self_tests(void)
> > continue;
> > }
> >
> > + get_online_cpus();
> > + mutex_lock(&event_mutex);
> > ftrace_event_enable_disable(file, 1);
> > + mutex_unlock(&event_mutex);
> > + put_online_cpus();
> > +
> > event_test_stuff();
> > +
> > + get_online_cpus();
> > + mutex_lock(&event_mutex);
> > ftrace_event_enable_disable(file, 0);
> > + mutex_unlock(&event_mutex);
> > + put_online_cpus();
> >
> > pr_cont("OK\n");
> > }
> >
> > cheers
> >