rcu-bh design

[Joel Fernandes]

Hi Paul, everyone,

I had some question(s) about rcu-bh design.
I am trying to understand the reasoning or need of it. I see that rcu-bh
will disable softirqs across read-side sections. But I am wondering why
this is needed. __do_softirq already disables softirq when a softirq
handler is running. The only reason I can see is, rcu-bh helps in
situations where - a softirq interrupts a preemptible RCU read-section and
prevents that read section from completing. But this problem would happen
if anyone where to use rcu-preempt - then does rcu-preempt even make sense
to use and shouldn't everyone be using rcu-bh?

The other usecase for rcu-bh seems to be if context-switch is used as a
quiescent state, then softirq flood can prevent that from happening and
cause rcu grace periods from completing. But preemptible RCU *does not* use
context-switch as a quiescent state. So in that case rcu-bh would make
sense only in a configuration where we're not using preemptible-rcu at all
and are getting flooded by softirqs. Is that the reason rcu-bh needs to
exist?

thanks!

- Joel