Re: [BUG] stack tracing causes: kernel/module.c:271 module_assert_mutex_or_preempt

[Paul E. McKenney]

On Wed, Apr 05, 2017 at 02:54:25PM -0400, Steven Rostedt wrote:
> On Wed, 5 Apr 2017 10:59:25 -0700
> "Paul E. McKenney" <paulmck@xxxxxxxxxxxxxxxxxx> wrote:
> 
> > > Note, this has nothing to do with trace_rcu_dyntick(). It's the
> > > function tracer tracing inside RCU, calling the stack tracer to record
> > > a new stack if it sees its larger than any stack before. All I need is
> > > a way to tell the stack tracer to not record a stack if it is in this
> > > RCU critical section.
> > > 
> > > If you can add a "in_rcu_critical_section()" function, that the stack
> > > tracer can test, and simply exit out like it does if in_nmi() is set,
> > > that would work too. Below is my current work around.  
> > 
> > Except that the rcu_irq_enter() would already have triggered the bug
> > that was (allegedly) fixed by my earlier patch.  So, yes, the check for
> > rcu_is_watching() would work around this bug, but the hope is that
> > with my earlier fix, this workaround would not be needed.
> 
> Note, if I had a "in_rcu_critical_section()" I wouldn't need to call
> rcu_irq_enter(). I could fall out before that. My current workaround
> does the check, even though it breaks things, it would hopefully fix
> things as it calls rcu_irq_exit() immediately.

OK, color me confused.  What would "in_rcu_critical_section()" do?

The rcu_is_watching() function tells you that RCU is not in an extended
quiescent state, though its return value can be iffy in the middle of
rcu_eqs_enter_common() -- which is why interrupts are disabled there.
In preemptible RCU, you can (but shouldn't) use rcu_preempt_depth()
to determine whether you are within an RCU read-side critical section,
which is what in_rcu_critical_section() sounds like to me, but I don't
see how this information would help in this situation.

What am I missing here?

> Would I would have is:
> 
> 	if (in_rcu_critical_section())
> 		goto out;
> 
> 	rcu_irq_enter();
> 
> which would probably be the easiest fix.
> 
> 
> > 
> > So could you please test my earlier patch?
> 
> I could, but it wouldn't tell me anything immediately. It's a hard race
> to hit. Which I never could hit it when I tried, but it would appear to
> hit immediately when testing other things :-p
> 
> Remember, it only triggers when a new max stack size is hit. The bug
> happens when that new max stack size is in the rcu critical section.
> 
> I guess I could force it to trigger by inserting a call in your code
> that clears the max stack size.

I know it is a bit of a hassle, but I would really appreciate the
testing.

							Thanx, Paul

> -- Steve
> 
> > 
> > This patch does not conflict with anything on -rcu, so you could
> > carry it if that helps.
> > 
>