On Fri, May 04, 2018 at 12:30:50PM -0400, Steven Rostedt wrote: > On Fri, 04 May 2018 16:20:11 +0000 > Joel Fernandes <joe...@google.com> wrote: > > > 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? > > I thought rcu-bh uses softirqs as a quiescent state. Thus, blocking > softirqs from happening makes sense. I don't think an > rcu_read_lock_bh() makes sense in a softirq.
Agreed, any place in the code where bottom halves are enabled is an RCU-bh quiescent state. > > 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. > > It doesn't? It does, but only sort of. A context switch really is always an RCU-preempt quiescent state from the perspective of the CPU. However, from the perspective of the task, context switch is a quiescent state only if the task is not in an RCU-preempt read-side critical section at the time. > > 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? > > Maybe I'm confused by what you are asking. The existence and use of RCU-bh in no way degrades the preemptibility of RCU-preempt read-side critical sections. The reason is that RCU-bh and RCU-preempt are completely separate, represented by different rcu_state structure instances. Thanx, Paul