Re: call_rcu from trace_preempt

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

On Tue, Jun 16, 2015 at 10:14:08AM -0700, Alexei Starovoitov wrote:
> 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@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.

And __rcu_is_watching() is marked notrace as well.

> >>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@kernel.org>
> 
> >(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.

Here is the updated patch.  Steven, I added your "Acked-by" based
on your positive comments above, please let me know if you would
like me to remove it.

							Thanx, Paul

------------------------------------------------------------------------

commit 9611f225d383a2edbdf74ca7f00c8d0b1e56dc45
Author: Alexei Starovoitov <ast@plumgrid.com>
Date:   Tue Jun 16 10:35:18 2015 -0700

    rcu: Make rcu_is_watching() really notrace
    
    Although rcu_is_watching() is marked notrace, it invokes preempt_disable()
    and preempt_enable(), both of which can be traced.  This defeats the
    purpose of the notrace on rcu_is_watching(), so this commit substitutes
    preempt_disable_notrace() and preempt_enable_notrace().
    
    Signed-off-by: Alexei Starovoitov <ast@plumgrid.com>
    Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
    Acked-by: Steven Rostedt <rostedt@goodmis.org>

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index fc0385380e97..c844ef3c2fae 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -973,9 +973,9 @@ bool notrace rcu_is_watching(void)
 {
 	bool ret;
 
-	preempt_disable();
+	preempt_disable_notrace();
 	ret = __rcu_is_watching();
-	preempt_enable();
+	preempt_enable_notrace();
 	return ret;
 }
 EXPORT_SYMBOL_GPL(rcu_is_watching);