Re: call_rcu from trace_preempt

[Alexei Starovoitov]

On 6/16/15 9:05 AM, Paul E. McKenney wrote:
> 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@xxxxxxxxxxxxxxxxxx> 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?

sure.
Signed-off-by: Alexei Starovoitov <ast@xxxxxxxxxx>

> (Still not sold on reentrant call_rcu() and
> kfree_rcu(), but getting notrace set up correctly is worthwhile.)

I'm not sold on it either. So far trying to understand
all consequences.

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