linux-kernel - Re: call_rcu from trace

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Message-ID: <20150616160546.GL3913@linux.vnet.ibm.com>
Date:	Tue, 16 Jun 2015 09:05:46 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Steven Rostedt <rostedt@...dmis.org>
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, Jun 16, 2015 at 11:37:38AM -0400, Steven Rostedt wrote:
> 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.

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

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