Re: [PATCH 2/2] ipvs: Use cond_resched_rcu_lock() helper when dumping connections

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

On Fri, Apr 26, 2013 at 07:19:49PM +0200, Peter Zijlstra wrote:
> On Fri, Apr 26, 2013 at 08:45:47AM -0700, Paul E. McKenney wrote:
> > On Fri, Apr 26, 2013 at 10:03:13AM +0200, Peter Zijlstra wrote:
> > > On Fri, Apr 26, 2013 at 10:45:08AM +0900, Simon Horman wrote:
> > > 
> > > > @@ -975,8 +975,7 @@ static void *ip_vs_conn_array(struct seq_file *seq, loff_t pos)
> > > >  				return cp;
> > > >  			}
> > > >  		}
> > > > -		rcu_read_unlock();
> > > > -		rcu_read_lock();
> > > > +		cond_resched_rcu_lock();
> > > >  	}
> > > 
> > > 
> > > While I agree with the sentiment I do find it a somewhat dangerous construct in
> > > that it might become far too easy to keep an RCU reference over this break and
> > > thus violate the RCU premise.
> > > 
> > > Is there anything that can detect this? Sparse / cocinelle / smatch? If so it
> > > would be great to add this to these checkers.
> > 
> > I have done some crude coccinelle patterns in the past, but they are
> > subject to false positives (from when you transfer the pointer from
> > RCU protection to reference-count protection) and also false negatives
> > (when you atomically increment some statistic unrelated to protection).
> > 
> > I could imagine maintaining a per-thread count of the number of outermost
> > RCU read-side critical sections at runtime, and then associating that
> > counter with a given pointer at rcu_dereference() time, but this would
> > require either compiler magic or an API for using a pointer returned
> > by rcu_dereference().  This API could in theory be enforced by sparse.
> 
> Luckily cond_resched_rcu_lock() will typically only occur within loops, and
> loops tend to be contained in a single sourcefile.
> 
> This would suggest a simple static checker should be able to tell without too
> much magic right? All it needs to do is track pointers returned from
> rcu_dereference*() and see if they're used after cond_resched_rcu_lock().
> 
> Also, cond_resched_rcu_lock() will only drop a single level of RCU refs; so
> that should be easier still.

Don't get me wrong, I am not opposing cond_resched_rcu_lock() because it
will be difficult to validate.  For one thing, until there are a lot of
them, manual inspection is quite possible.  So feel free to apply my
Acked-by to the patch.

But it is definitely not too early to start thinking about how best to
automatically validate this sort of thing!

							Thanx, Paul