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

[Valentin Schneider]

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!