Re: [PATCH tip/core/rcu 2/3] rcu: add debug check for too many rcu_read_unlock()

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

On Mon, Jan 04, 2010 at 06:03:08PM -0800, Josh Triplett wrote:
> On Mon, Jan 04, 2010 at 04:04:01PM -0800, Paul E. McKenney wrote:
> > From: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> > 
> > TREE_PREEMPT_RCU maintains an rcu_read_lock_nesting counter in the
> > task structure, which happens to be a signed int.  So this patch adds a
> > check for this counter being negative at the end of __rcu_read_unlock().
> > This check is under CONFIG_PROVE_LOCKING, so can be thought of as being
> > part of lockdep.
> > 
> > Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> > ---
> >  kernel/rcutree_plugin.h |    3 +++
> >  1 files changed, 3 insertions(+), 0 deletions(-)
> > 
> > diff --git a/kernel/rcutree_plugin.h b/kernel/rcutree_plugin.h
> > index f11ebd4..e77cdf3 100644
> > --- a/kernel/rcutree_plugin.h
> > +++ b/kernel/rcutree_plugin.h
> > @@ -304,6 +304,9 @@ void __rcu_read_unlock(void)
> >  	if (--ACCESS_ONCE(t->rcu_read_lock_nesting) == 0 &&
> >  	    unlikely(ACCESS_ONCE(t->rcu_read_unlock_special)))
> >  		rcu_read_unlock_special(t);
> > +#ifdef CONFIG_PROVE_LOCKING
> > +	WARN_ON_ONCE(ACCESS_ONCE(t->rcu_read_lock_nesting) < 0);
> > +#endif /* #ifdef CONFIG_PROVE_LOCKING */
> >  }
> >  EXPORT_SYMBOL_GPL(__rcu_read_unlock);
> 
> Given that you *already* need to access t->rcu_read_lock_nesting here,
> why not just do the test all the time?  Ideally you could access
> t->rcu_read_lock_nesting once, decrement it, and test for both 0 and
> negative.

Because I was paranoid about the extra branch.  Perhaps needlessly
paranoid, but this is rcu_read_unlock() we are talking about here.  ;-)

You seem to be suggesting making the first test be "<=", then
sorting things out later, but given that both the equals-zero and the
greater-than-zero cases are quite common, I couldn't figure out how to
avoid the extra test and branch in the common case.  Hence the #ifdef.

							Thanx, Paul