Re: [PATCH 2/3] rcu/nocb: Check for migratability rather than pure preemptability

["Paul E. McKenney" <paulmck@kernel.org>]

On Thu, Jul 29, 2021 at 12:01:37AM +0200, Frederic Weisbecker wrote:
> On Wed, Jul 28, 2021 at 08:34:14PM +0100, Valentin Schneider wrote:
> > On 28/07/21 01:08, Frederic Weisbecker wrote:
> > > On Wed, Jul 21, 2021 at 12:51:17PM +0100, Valentin Schneider wrote:
> > >> Signed-off-by: Valentin Schneider <valentin.schneider@arm.com>
> > >> ---
> > >>  kernel/rcu/tree_plugin.h | 3 +--
> > >>  1 file changed, 1 insertion(+), 2 deletions(-)
> > >>
> > >> diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
> > >> index ad0156b86937..6c3c4100da83 100644
> > >> --- a/kernel/rcu/tree_plugin.h
> > >> +++ b/kernel/rcu/tree_plugin.h
> > >> @@ -70,8 +70,7 @@ static bool rcu_rdp_is_offloaded(struct rcu_data *rdp)
> > >>              !(lockdep_is_held(&rcu_state.barrier_mutex) ||
> > >>                (IS_ENABLED(CONFIG_HOTPLUG_CPU) && lockdep_is_cpus_held()) ||
> > >>                rcu_lockdep_is_held_nocb(rdp) ||
> > >> -		  (rdp == this_cpu_ptr(&rcu_data) &&
> > >> -		   !(IS_ENABLED(CONFIG_PREEMPT_COUNT) && preemptible())) ||
> > >> +		  (rdp == this_cpu_ptr(&rcu_data) && is_pcpu_safe()) ||
> > >
> > > I fear that won't work. We really need any caller of rcu_rdp_is_offloaded()
> > > on the local rdp to have preemption disabled and not just migration disabled,
> > > because we must protect against concurrent offloaded state changes.
> > >
> > > The offloaded state is changed by a workqueue that executes on the target rdp.
> > >
> > > Here is a practical example where it matters:
> > >
> > >            CPU 0
> > >            -----
> > >            // =======> task rcuc running
> > >            rcu_core {
> > >              rcu_nocb_lock_irqsave(rdp, flags) {
> > >                    if (!rcu_segcblist_is_offloaded(rdp->cblist)) {
> > >                      // is not offloaded right now, so it's going
> > >                        // to just disable IRQs. Oh no wait:
> > >            // preemption
> > >            // ========> workqueue running
> > >            rcu_nocb_rdp_offload();
> > >            // ========> task rcuc resume
> > >                      local_irq_disable();
> > >                    }
> > >                }
> > >              ....
> > >                      rcu_nocb_unlock_irqrestore(rdp, flags) {
> > >                    if (rcu_segcblist_is_offloaded(rdp->cblist)) {
> > >                        // is offloaded right now so:
> > >                        raw_spin_unlock_irqrestore(rdp, flags);
> > >
> > > And that will explode because that's an impaired unlock on nocb_lock.
> > 
> > Harumph, that doesn't look good, thanks for pointing this out.
> > 
> > AFAICT PREEMPT_RT doesn't actually require to disable softirqs here (since
> > it forces RCU callbacks on the RCU kthreads), but disabled softirqs seem to
> > be a requirement for much of the underlying functions and even some of the
> > callbacks (delayed_put_task_struct() ~> vfree() pays close attention to
> > in_interrupt() for instance).
> > 
> > Now, if the offloaded state was (properly) protected by a local_lock, do
> > you reckon we could then keep preemption enabled?
> 
> I guess we could take such a local lock on the update side
> (rcu_nocb_rdp_offload) and then take it on rcuc kthread/softirqs
> and maybe other places.
> 
> But we must make sure that rcu_core() is preempt-safe from a general perspective
> in the first place. From a quick glance I can't find obvious issues...yet.
> 
> Paul maybe you can see something?

Let's see...

o	Extra context switches in rcu_core() mean extra quiescent
	states.  It therefore might be necessary to wrap rcu_core()
	in an rcu_read_lock() / rcu_read_unlock() pair, because
	otherwise an RCU grace period won't wait for rcu_core().

	Actually, better have local_bh_disable() imply
	rcu_read_lock() and local_bh_enable() imply rcu_read_unlock().
	But I would hope that this already happened.

o	The rcu_preempt_deferred_qs() check should still be fine,
	unless there is a raw_bh_disable() in -rt. 

o	The set_tsk_need_resched() and set_preempt_need_resched()
	might preempt immediately.  I cannot think of a problem
	with that, but careful testing is clearly in order.

o	The values checked by rcu_check_quiescent_state() could now
	change while this function is running.	I don't immediately
	see a problematic sequence of events, but here be dragons.
	I therefore suggest disabling preemption across this function.
	Or if that is impossible, taking a very careful look at the
	proposed expansion of the state space of this function.

o	I don't see any new races in the grace-period/callback check.
	New callbacks can appear in interrupt handlers, after all.

o	The rcu_check_gp_start_stall() function looks similarly
	unproblematic.

o	Callback invocation can now be preempted, but then again it
	recently started being concurrent, so this should be no
	added risk over offloading/de-offloading.

o	I don't see any problem with do_nocb_deferred_wakeup().

o	The CONFIG_RCU_STRICT_GRACE_PERIOD check should not be
	impacted.

So some adjustments might be needed, but I don't see a need for
major surgery.

This of course might be a failure of imagination on my part, so it
wouldn't hurt to double-check my observations.

> > From a naive outsider PoV, rdp->nocb_lock looks like a decent candidate,
> > but it's a *raw* spinlock (I can't tell right now whether changing this is
> > a horrible idea or not), and then there's
> 
> Yeah that's not possible, nocb_lock is too low level and has to be called with
> IRQs disabled. So if we take that local_lock solution, we need a new lock.

No argument here!

							Thanx, Paul