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

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:

quoted

On 28/07/21 01:08, Frederic Weisbecker wrote:

quoted

On Wed, Jul 21, 2021 at 12:51:17PM +0100, Valentin Schneider wrote:

quoted

Signed-off-by: Valentin Schneider <redacted>
---
 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.

quoted

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