Messages in this thread | | | Date | Wed, 4 Sep 2013 07:50:27 -0400 | From | Steven Rostedt <> | Subject | Re: [RFC][PATCH 01/18 v2] ftrace: Add hash list to save RCU unsafe functions |
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On Tue, 3 Sep 2013 21:18:28 -0700 "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> wrote:
> On Tue, Sep 03, 2013 at 10:03:25PM -0400, Steven Rostedt wrote: > > On Tue, 3 Sep 2013 22:01:15 -0400 > > Steven Rostedt <rostedt@goodmis.org> wrote: > > > > > On Tue, 3 Sep 2013 18:24:04 -0700 > > > "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> wrote: > > > > > > > > > > > static DEFINE_PER_CPU(unsigned long, ftrace_rcu_func); > > > > > @@ -588,15 +593,14 @@ static void > > > > > ftrace_unsafe_callback(unsigned long ip, unsigned long parent_ip, > > > > > struct ftrace_ops *op, struct pt_regs *pt_regs) > > > > > { > > > > > - int bit; > > > > > - > > > > > + /* Make sure we see disabled or not first */ > > > > > + smp_rmb(); > > > > > > > > smp_mb__before_atomic_inc()? > > > > > > > > > > Ah, but this is before an atomic_read(), and not an atomic_inc(), thus > > > the normal smp_rmb() is still required. > > > > > > > Here's the changes against this one: > > > > diff --git a/kernel/trace/trace_functions.c b/kernel/trace/trace_functions.c > > index cdcf187..9e6902a 100644 > > --- a/kernel/trace/trace_functions.c > > +++ b/kernel/trace/trace_functions.c > > @@ -569,14 +569,14 @@ void ftrace_unsafe_rcu_checker_disable(void) > > { > > atomic_inc(&ftrace_unsafe_rcu_disabled); > > /* Make sure the update is seen immediately */ > > - smp_wmb(); > > + smp_mb__after_atomic_inc(); > > } > > > > void ftrace_unsafe_rcu_checker_enable(void) > > { > > atomic_dec(&ftrace_unsafe_rcu_disabled); > > /* Make sure the update is seen immediately */ > > - smp_wmb(); > > + smp_mb__after_atomic_dec(); > > } > > > > static void > > > > > > > > Which is nice, because the smp_mb() are now in the really slow path. > > Looks good! > > But now that I look at it more carefully, including the comments... > The smp_mb__after_atomic_dec() isn't going to make the update be seen > faster -- instead, it will guarantee that if some other CPU sees this > CPU's later write, then that CPU will also see the results of the > atomic_dec().
I don't need to have it seen "faster", just before anything that comes next after the call to ftrace_unsafe_rcu_checker_disable(). That's what I meant by the comment. In other words, don't delay this write, it needs to go first.
-- Steve
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