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

From: Valentin Schneider
Date: Thu Jul 29 2021 - 06:51:43 EST

On 28/07/21 18:04, Paul E. McKenney wrote:
> 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:
>> > 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.

It does look like it.

> 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.

I'll go poke around, thank you both!