Re: rcu/tree: Protect rcu_rdp_is_offloaded() invocations on RT

Frederic Weisbecker
2021-09-22T11:31:28Z

On Tue, Sep 21, 2021 at 07:18:37PM -0700, Paul E. McKenney wrote:
> On Wed, Sep 22, 2021 at 01:36:27AM +0200, Frederic Weisbecker wrote:
> > Doing the local_irq_save() before checking that the segcblist is offloaded
> > protect that state from being changed (provided we lock the local rdp). Then we
> > can safely manipulate cblist, whether locked or unlocked.
> >
> > 2) The actual call to rcu_do_batch(). If we are preempted between
> > rcu_segcblist_completely_offloaded() and rcu_do_batch() with a deoffload in
> > the middle, we miss the callback invocation. Invoking rcu_core by the end of
> > deoffloading process should solve that.
>
> Maybe invoke rcu_core() at that point? My concern is that there might
> be an extended time between the missed rcu_do_batch() and the end of
> the deoffloading process.

Agreed!

>
> > > Reported-by: Valentin Schneider <valentin.schneider@arm.com>
> > > Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
> > > ---
> > > kernel/rcu/tree.c | 7 ++++---
> > > 1 file changed, 4 insertions(+), 3 deletions(-)
> > >
> > > --- a/kernel/rcu/tree.c
> > > +++ b/kernel/rcu/tree.c
> > > @@ -2278,13 +2278,13 @@ rcu_report_qs_rdp(struct rcu_data *rdp)
> > > {
> > > unsigned long flags;
> > > unsigned long mask;
> > > - bool needwake = false;
> > > - const bool offloaded = rcu_rdp_is_offloaded(rdp);
> > > + bool offloaded, needwake = false;
> > > struct rcu_node *rnp;
> > >
> > > WARN_ON_ONCE(rdp->cpu != smp_processor_id());
> > > rnp = rdp->mynode;
> > > raw_spin_lock_irqsave_rcu_node(rnp, flags);
> > > + offloaded = rcu_rdp_is_offloaded(rdp);
> > > if (rdp->cpu_no_qs.b.norm || rdp->gp_seq != rnp->gp_seq ||
> > > rdp->gpwrap) {
> >
> > BTW Paul, if we happen to switch to non-NOCB (deoffload) some time after
> > rcu_report_qs_rdp(), it's possible that the rcu_accelerate_cbs()
> > that was supposed to be handled by nocb kthreads on behalf of
> > rcu_core() -> rcu_report_qs_rdp() would not happen. At least not until
> > we invoke rcu_core() again. Not sure how much harm that could cause.
>
> Again, can we just invoke rcu_core() as soon as this is noticed?

Right. So I'm going to do things a bit differently. I'm going to add
a new segcblist state flag so that during the deoffloading process,
the first very step is an invoke_rcu_core() on the target after setting a
flag that requires handling all this things: accelerate/do_batch, etc...

Then will remain the "do we still have pending callbacks after do_batch?"
in which case we'll need to invoke the rcu_core again as long as we are in
the middle of deoffloading.

Ok, now to write the patches.