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From: Joel Fernandes <>
To: Paul McKenney <>
Cc: Steven Rostedt <>,
	Mathieu Desnoyers <>,
	LKML <>
Subject: rcu-bh design
Date: Fri, 04 May 2018 16:20:11 +0000
Message-ID: <> (raw)

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


- Joel

             reply index

Thread overview: 15+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-05-04 16:20 Joel Fernandes [this message]
2018-05-04 16:30 ` Steven Rostedt
2018-05-04 17:15   ` Joel Fernandes
2018-05-04 17:43     ` Paul E. McKenney
2018-05-04 18:34       ` Joel Fernandes
2018-05-04 18:49         ` Paul E. McKenney
2018-05-04 19:57           ` Joel Fernandes
2018-05-04 20:11             ` Paul E. McKenney
2018-05-04 20:33               ` Joel Fernandes
2018-05-04 22:49                 ` Paul E. McKenney
2018-05-04 23:20                   ` Joel Fernandes
2018-05-04 23:43                     ` Paul E. McKenney
2018-05-05  0:39                       ` Joel Fernandes
2018-05-04 17:32   ` Paul E. McKenney
2018-05-04 17:37     ` Steven Rostedt

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