Re: call_rcu from trace_preempt

[Alexei Starovoitov <ast@plumgrid.com>]

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.
>>
>> 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.