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

Paul E. McKenney
2011-10-03T16:30:54Z

On Mon, Oct 03, 2011 at 03:03:48PM +0200, Frederic Weisbecker wrote:
> 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.

Hmmm... This might be a good use for the "drive-by" per-CPU access
functions.

No, that doesn't work. We could pick up the pointer, switch to another
CPU, the original CPU could run a task that blocks before we start running,
and then we could incorrectly decide that we were running in idle context,
issuing a spurious warning. This approach would only work in environments
that (unlike the Linux kernel) mapped all the per-CPU variables to the
same virtual address on all CPUs. (DYNIX/ptx did this, but this leads
to other problems, like being unable to reasonably access other CPUs'
variables. Double mapping has other issues on some architectures.)

OK, agreed. I will make this function disable preemption.

> > 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.

Fair enough!

Thanx, Paul