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
