linux-kernel - Re: [PATCH v3 tip/core/rcu 1/9] rcu: Add call_rcu

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Date:	Mon, 4 Aug 2014 14:25:15 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	Lai Jiangshan <laijs@...fujitsu.com>, linux-kernel@...r.kernel.org,
	mingo@...nel.org, dipankar@...ibm.com, akpm@...ux-foundation.org,
	mathieu.desnoyers@...icios.com, josh@...htriplett.org,
	tglx@...utronix.de, rostedt@...dmis.org, dhowells@...hat.com,
	edumazet@...gle.com, dvhart@...ux.intel.com, fweisbec@...il.com,
	oleg@...hat.com, bobby.prani@...il.com
Subject: Re: [PATCH v3 tip/core/rcu 1/9] rcu: Add call_rcu_tasks()

On Mon, Aug 04, 2014 at 04:50:44AM -0700, Paul E. McKenney wrote:
> OK, I will bite...
> 
> What kinds of tasks are on a runqueue, but neither ->on_cpu nor
> PREEMPT_ACTIVE?

Userspace tasks, they don't necessarily get PREEMPT_ACTIVE when
preempted. Now obviously you're not _that_ interested in userspace tasks
for this, so that might be ok.

But the main point was, you cannot use ->on_cpu or PREEMPT_ACTIVE
without holding rq->lock.

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