rcu/tree: Protect rcu_rdp_is_offloaded() invocations on RT

[Valentin Schneider]

On 30/09/21 12:53, Frederic Weisbecker wrote:
> On Thu, Sep 30, 2021 at 10:00:39AM +0100, Valentin Schneider wrote:
>> My reasoning for adding protection in the outer functions was to prevent
>> impaired unlocks of rcu_nocb_{un}lock_irqsave(), as that too depends on the
>> offload state. Cf. Frederic's writeup:
>>
>>   http://lore.kernel.org/r/20210727230814.GC283787@lothringen
>
> I was wrong about that BTW!
> Because rcu_nocb_lock() always require IRQs to be disabled, which of course disables
> preemption, so the offloaded state can't change between
> rcu_nocb_lock[_irqsave]() and rcu_nocb_unlock[_irqrestore]() but anyway there
> were many other issues to fix :-)
>

Ooooh... Even with you pointing it out, it took me a while to see it that
way. It's tough to get out of holidays mode :-)