Messages in this thread | | | Date | Fri, 5 Mar 2010 17:45:13 -0800 | From | Josh Triplett <> | Subject | Re: [PATCH tip/core/rcu 1/3] ftrace: replace read_barrier_depends() with rcu_dereference_raw() |
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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
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