Subject: Re: [PATCH 0/4] RCU: introduce noref debug

From: Paolo Abeni <[email-masked]>
Date: Fri, 06 Oct 2017 17:10:09 +0200

Hi,

On Fri, 2017-10-06 at 06:34 -0700, Paul E. McKenney wrote:
> On Fri, Oct 06, 2017 at 02:57:45PM +0200, Paolo Abeni wrote:
> > The networking subsystem is currently using some kind of long-lived
> > RCU-protected, references to avoid the overhead of full book-keeping.
> > 
> > Such references - skb_dst() noref - are stored inside the skbs and can be
> > moved across relevant slices of the network stack, with the users
> > being in charge of properly clearing the relevant skb - or properly refcount
> > the related dst references - before the skb escapes the RCU section.
> > 
> > We currently don't have any deterministic debug infrastructure to check
> > the dst noref usages - and the introduction of others noref artifact is
> > currently under discussion.
> > 
> > This series tries to tackle the above introducing an RCU debug infrastructure
> > aimed at spotting incorrect noref pointer usage, in patch one. The
> > infrastructure is small and must be explicitly enabled via a newly introduced
> > build option.
> > 
> > Patch two uses such infrastructure to track dst noref usage in the networking
> > stack.
> > 
> > Patch 3 and 4 are bugfixes for small buglet found running this infrastructure
> > on basic scenarios.

Thank you for the prompt reply!
> 
> This patchset does not look like it handles rcu_read_lock() nesting.
> For example, given code like this:
> 
> 	void foo(void)
> 	{
> 		rcu_read_lock();
> 		rcu_track_noref(&key2, &noref2, true);
> 		do_something();
> 		rcu_track_noref(&key2, &noref2, false);
> 		rcu_read_unlock();
> 	}
> 
> 	void bar(void)
> 	{
> 		rcu_read_lock();
> 		rcu_track_noref(&key1, &noref1, true);
> 		do_something_more();
> 		foo();
> 		do_something_else();
> 		rcu_track_noref(&key1, &noref1, false);
> 		rcu_read_unlock();
> 	}
> 
> 	void grill(void)
> 	{
> 		foo();
> 	}
> 
> It looks like foo()'s rcu_read_unlock() will complain about key1.
> You could remove foo()'s rcu_read_lock() and rcu_read_unlock(), but
> that will break the call from grill().

Actually the code should cope correctly with your example; when foo()'s
rcu_read_unlock() is called, 'cache' contains:

{ { &key1, &noref1, 1},  // ...

and when the related __rcu_check_noref() is invoked preempt_count() is
2 - because the check is called before decreasing the preempt counter.

In the main loop inside __rcu_check_noref() we will hit always the
'continue' statement because 'cache->store[i].nesting != nesting', so
no warn will be triggered.

> Or am I missing something subtle here?  Given patch 3/4, I suspect not...

The problem with the code in patch 3/4 is different; currently
ip_route_input_noref() is basically doing:

rcu_read_lock();

rcu_track_noref(&key1, &noref1, true);

rcu_read_unlock();

So the rcu lock there silence any RCU based check inside
ip_route_input_noref() but does not really protect the noref dst.

Please let me know if the above clarify the scenario.

Thanks,

Paolo