Messages in this thread | | | Date | Tue, 3 Sep 2013 21:18:28 -0700 | From | "Paul E. McKenney" <> | Subject | Re: [RFC][PATCH 01/18 v2] ftrace: Add hash list to save RCU unsafe functions |
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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().
Thanx, Paul
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