linux-kernel - Re: rcu-bh design

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Message-ID: <20180504133726.3f04d7bf@gandalf.local.home>
Date:   Fri, 4 May 2018 13:37:26 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:     Joel Fernandes <joelaf@...gle.com>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: rcu-bh design

On Fri, 4 May 2018 10:32:06 -0700
"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com> wrote:

> > > But preemptible RCU *does not* use context-switch as a quiescent state.  
> > 
> > It doesn't?  
> 
> It does, but only sort of.

Ah, my confusion. I was thinking of rcu_read_lock_sched() as
"preemptible RCU", not the "we can preempt in rcu_read_lock() context".

-- Steve


> 
> 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.