linux-kernel - Re: call_rcu from trace

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Message-ID: <20150616113738.3ab2667b@gandalf.local.home>
Date:	Tue, 16 Jun 2015 11:37:38 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	Alexei Starovoitov <ast@...mgrid.com>,
	Daniel Wagner <daniel.wagner@...-carit.de>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: call_rcu from trace_preempt

On Tue, 16 Jun 2015 05:27:33 -0700
"Paul E. McKenney" <paulmck@...ux.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.

-- Steve
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