linux-kernel - Re: [BUG RT] - rcupreempt.c:133 on 2.6.23-rc1-rt7

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Date:	Sun, 5 Aug 2007 08:04:49 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	Ingo Molnar <mingo@...e.hu>, Thomas Gleixner <tglx@...utronix.de>,
	RT <linux-rt-users@...r.kernel.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [BUG  RT] - rcupreempt.c:133 on 2.6.23-rc1-rt7

On Sun, Aug 05, 2007 at 10:24:15AM -0400, Steven Rostedt wrote:
> 
> --
> 
> On Sun, 5 Aug 2007, Ingo Molnar wrote:
> 
> >
> > * Steven Rostedt <rostedt@...dmis.org> wrote:
> >
> > > > I don't have time to look further now, and it's something that isn't
> > > > easily reproducible (Well, it happened once out of two boots). If
> > > > you need me to look further, or need a config or dmesg (I have
> > > > both), then just give me a holler.
> > >
> > > Silly me. FYI, I was running with !PREEMPT_RT, but with Hard and
> > > Softirqs as threads.  Must have copied the wrong config over :-/
> >
> > it's still not supposed to happen ... rcu read lock nesting that deep?
> >
> 
> The code on line 133 is:
> 
> 	WARN_ON_ONCE(current->rcu_read_lock_nesting > NR_CPUS);
> 
> I have NR_CPUS set to 2 since the box I'm running this on only has
> 2 cpus and I see no reason to waste more data structures.
> 
> Is rcu read lock nesting deeper than 2?

In networking, I would not be at all surprised, given things like fib_trie
and netfilter usage.  In addition, if rcu_read_lock() is called from
hardirq or NMI/SMI, it is necessary to add the nesting levels in these
environments as well.  In any case, rcu_read_lock() is freely nestable,
so there is no penalty for nesting pretty deeply.  I must have missed this
WARN_ON_ONCE() being added to rcu_read_lock() -- I did ack Daniel Walker's
check for negative values of rcu_read_lock_nesting in rcu_read_unlock(),
but saw no upper-limit checks.

So, are you running into a situation where rcu_read_lock_nesting is
growing unboundedly?

I would not expect the per-task nesting level to normally be a function
of the number of CPUs -- unless one was doing some sort of nested scan
of RCU-protected per-CPU data structures or some such.  So if you are
adding this to your local build as a debug check, I would suggest a fixed
limit -- but would -not- suggest putting such a check into a production
build, at least not for a small limit.

							Thanx, Paul
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