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 06:28:15PM -0800, Josh Triplett wrote:
> On Mon, Jan 04, 2010 at 06:19:19PM -0800, Paul E. McKenney wrote:
> > 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.
> 
> No, I think you could simply read the predecremented value into a local
> variable, test it once with == 0, then have the WARN_ON_ONCE, and hope
> that the compiler figures out it can just test the register once and
> then do multiple jumps on the same flags.
> 
> You could try it and see what code it generates.

I agree that a smart compiler could share condition-code state, but
there still will be the extra branch.  (Keep in mind that this is a
.h file, so #ifdef is permitted -- though I might nevertheless make
a one-line function/macro.)

						Thanx, Paul