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From: "Paul E. McKenney" <>
To: Steven Rostedt <>
Cc: LKML <>
Subject: Re: [BUG] stack tracing causes: kernel/module.c:271 module_assert_mutex_or_preempt
Date: Wed, 5 Apr 2017 12:08:10 -0700
Message-ID: <> (raw)
In-Reply-To: <20170405145425.357937b8@gandalf.local.home>

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" <> 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

							Thanx, Paul

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

  reply index

Thread overview: 16+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2017-04-05 13:32 Steven Rostedt
2017-04-05 16:25 ` Paul E. McKenney
2017-04-05 16:45   ` Steven Rostedt
2017-04-05 17:59     ` Paul E. McKenney
2017-04-05 18:54       ` Steven Rostedt
2017-04-05 19:08         ` Paul E. McKenney [this message]
2017-04-05 19:21           ` Steven Rostedt
2017-04-05 19:39             ` Paul E. McKenney
2017-04-05 19:52               ` Steven Rostedt
2017-04-05 20:42                 ` Paul E. McKenney
2017-04-06  1:31                   ` Steven Rostedt
2017-04-06  4:07                     ` Paul E. McKenney
2017-04-06  2:12       ` Steven Rostedt
2017-04-06  4:15         ` Paul E. McKenney
2017-04-06 14:14           ` Steven Rostedt
2017-04-06 14:59             ` Paul E. McKenney

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