Re: call_rcu from trace_preempt

Paul E. McKenney
2015-06-16T16:05:58Z

On Tue, Jun 16, 2015 at 11:37:38AM -0400, Steven Rostedt wrote:
> On Tue, 16 Jun 2015 05:27:33 -0700
> "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> wrote:
>
> > On Mon, Jun 15, 2015 at 10:45:05PM -0700, Alexei Starovoitov wrote:
> > > On 6/15/15 7:14 PM, Paul E. McKenney wrote:
> > > >
> > > >Why do you believe that it is better to fix it within call_rcu()?
> > >
> > > found it:
> > > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> > > index 8cf7304b2867..a3be09d482ae 100644
> > > --- a/kernel/rcu/tree.c
> > > +++ b/kernel/rcu/tree.c
> > > @@ -935,9 +935,9 @@ bool notrace rcu_is_watching(void)
> > > {
> > > bool ret;
> > >
> > > - preempt_disable();
> > > + preempt_disable_notrace();
> > > ret = __rcu_is_watching();
> > > - preempt_enable();
> > > + preempt_enable_notrace();
> > > return ret;
> > > }
> > >
> > > the rcu_is_watching() and __rcu_is_watching() are already marked
> > > notrace, so imo it's a good 'fix'.
> > > What was happening is that the above preempt_enable was triggering
> > > recursive call_rcu that was indeed messing 'rdp' that was
> > > prepared by __call_rcu and before __call_rcu_core could use that.
> >
> > > btw, also noticed that local_irq_save done by note_gp_changes
> > > is partially redundant. In __call_rcu_core path the irqs are
> > > already disabled.
> >
>
> If rcu_is_watching() and __rcu_is_watching() are both marked as
> notrace, it makes sense to use preempt_disable/enable_notrace() as it
> otherwise defeats the purpose of the notrace markers on rcu_is_watching.
>
> That is regardless of what the rest of this thread is about.

Good enough! Alexei, are you OK with my adding your Signed-off-by
to the above patch? (Still not sold on reentrant call_rcu() and
kfree_rcu(), but getting notrace set up correctly is worthwhile.)

Thanx, Paul