Re: linux-next-20110923: warning kernel/rcutree.c:1833

Frederic Weisbecker
2011-10-03T13:04:00Z

On Sun, Oct 02, 2011 at 05:32:47PM -0700, Paul E. McKenney wrote:
> > > -void rcu_irq_enter(void)
> > > +int rcu_is_cpu_idle(void)
> > > {
> > > - rcu_exit_nohz();
> > > + return (atomic_read(&__get_cpu_var(rcu_dynticks).dynticks) & 0x1) == 0;
> > > }
> >
> > So that's not used in this patch but it's interesting for me
> > to backport "rcu: Detect illegal rcu dereference in extended quiescent state".
>
> Yep, that is why it is there.

Ok.

>
> > The above should be read from a preempt disabled section though
> > (remember "rcu: Fix preempt-unsafe debug check of rcu extended quiescent state")
>
> Yes, and that is why the last line of the header comment reads "The
> caller must have at least disabled preemption." Disabling preemption
> is not necessary in Tiny RCU because there is no other CPU for the task
> to go to. (Right?)

Right.

> > Those functions should probably lay in a separate patch. But I don't mind
> > much keeping the things as is and use these APIs in my next patches though.
> > I'll just fix the preempt enabled thing above.
>
> Or were you saying that you wish to make calls to rcu_is_cpu_idle()
> that have preemption enabled?

Yeah. That's going to be called from places like rcu_read_lock_held()
and things like this that don't need to disable preemption themselves.

Would be better to disable preemption from that function.

> And I can split the patch easily enough while keeping the diff the same,
> so you should be able to do your porting on top of the existing code.

No I'm actually pretty fine with the current state. Whether that's defined
in this patch or a following one is actually not important.

Thanks!