Re: call_rcu from trace_preempt

[Alexei Starovoitov <ast@plumgrid.com>]

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.

> Perhaps you are self-deadlocking within __call_rcu_core().  If you have
> not already done so, please try running with CONFIG_PROVE_LOCKING=y.

yes, I had CONFIG_PROVE_LOCKING on.

> I suspect that your problem may range quite a bit further than just
> call_rcu().  For example, in your stack trace, you have a recursive
> call to debug_object_activate(), which might not be such good thing.

nope :) recursive debug_object_activate() is fine.
with the above 'fix' the trace.patch is now passing.

Why I'm digging into all of these? Well, to find out when
it's safe to finally do call_rcu. If I will use deferred kfree
approach in bpf maps, I need to know when it's safe to finally
call_rcu and it's not an easy answer.
kprobes potentially can be placed in any part of call_rcu stack,
so things can go messy quickly. So it helps to understand the call_rcu
logic well enough to come up with good solution.