Re: timer: lockup in run_timer_softirq()

[Peter Zijlstra <peterz@infradead.org>]

On Wed, Jul 10, 2013 at 08:27:34AM -0400, Steven Rostedt wrote:
> On Wed, 2013-07-10 at 11:52 +0200, Peter Zijlstra wrote:
> 
> > Fun.. :-) we trace __local_bh_enable() and hit a ftrace callback between
> > telling lockdep we enabled softirqs and actually doing so.
> > 
> > I'm just a tad confused by the trace; it says we go:
> >   lock_is_held()
> >     check_flags()
> > 
> > Looking at perf_tp_event() this would most likely be from:
> > 
> >   ctx = rcu_dereference(task->perf_event_ctxp[perf_sw_context]);
> 
> Function tracing should not be treated as a normal trace point. It is
> much more invasive, and there's things that one should be weary about
> when using it.

Clearly.. yet someone forgot this when merging ced39002 :/

> > 
> > Where the lock_is_held() would be from rcu_dereference_check()'s
> > rcu_read_lock_sched_held(). However, by there we've already passed
> > rcu_read_lock() which includes rcu_lock_acquire() -> lock_acquire() ->
> > check_flags(). So it should've triggered there.
> > 
> > Ideally we'd not trace __local_bh_enable() at all, seeing as how any RCU usage
> > in there would be bound to trigger this.
> 
> I find it very useful to trace __local_bh_enable(). I also trace RCU
> calls.
> 
> When using function tracing, you need to use
> rcu_dereference_raw_notrace().
> 
> Also, function tracing callbacks should avoid rcu_read_lock(), as that's
> traced as well. You can use preempt_disable_notrace() for rcu usage.

but but but preempt_disable_notrace() isn't an rcu_read_lock().. You can only
do that for rcu_sched.

Anyway, I don't see a nice way out of this mess :/ the entire perf core uses
regular RCU and converting all that is going to me a nasty big patch.