Messages in this thread | | | Date | Tue, 16 Jun 2015 10:14:08 -0700 | From | Alexei Starovoitov <> | Subject | Re: call_rcu from trace_preempt |
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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.
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