Messages in this thread | | | Date | Tue, 16 Jun 2015 09:05:46 -0700 | From | "Paul E. McKenney" <> | Subject | Re: call_rcu from trace_preempt |
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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? (Still not sold on reentrant call_rcu() and kfree_rcu(), but getting notrace set up correctly is worthwhile.)
Thanx, Paul
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