Re: [PATCH tip/core/rcu 1/3] ftrace: replace read_barrier_depends() with rcu_dereference_raw()

[Josh Triplett <josh@joshtriplett.org>]

On Fri, Mar 05, 2010 at 08:29:05PM -0500, Steven Rostedt wrote:
> On Fri, 2010-03-05 at 15:03 -0800, Paul E. McKenney wrote:
> > Replace the calls to read_barrier_depends() in ftrace_list_func() with
> > rcu_dereference_raw() to improve readability.  The reason that we use
> > rcu_dereference_raw() here is that removed entries are never freed,
> > instead they are simply leaked.  This is one of a very few cases where
> > use of rcu_dereference_raw() is the long-term right answer.  And I don't
> > yet know of any others.  ;-)
> > 
> > Cc: Steven Rostedt <rostedt@goodmis.org>
> 
> Acked-by: Steven Rostedt <rostedt@goodmis.org>
> 
> Thanks Paul!
> 
> > Cc: Frederic Weisbecker <fweisbec@gmail.com>
> > Cc: Ingo Molnar <mingo@redhat.com>
> > Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> > ---
> 
> >  
> > @@ -154,8 +159,7 @@ static int __register_ftrace_function(struct ftrace_ops *ops)
> >  	 * the ops->next pointer is valid before another CPU sees
> >  	 * the ops pointer included into the ftrace_list.
> >  	 */
> > -	smp_wmb();
> > -	ftrace_list = ops;
> > +	rcu_assign_pointer(ftrace_list, ops);
> 
> [ Off topic ]
> 
> I looked at rcu_assign_pointer() and it is:
> 
> #define rcu_assign_pointer(p, v) \
> 	({ \
> 		if (!__builtin_constant_p(v) || \
> 		    ((v) != NULL)) \
> 			smp_wmb(); \
> 		(p) = (v); \
> 	})
> 
> My question is, why that crazy if? The only time that will fail is if we
> are assigning the constant NULL to p. What makes NULL so important here?
> Can't there be a case when assigning NULL to p will require that wmb()?

The barrier ensures that the reader can't see the new p and the old
*p.  Since you can't look at *NULL, that concern doesn't apply.

- Josh Triplett