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    SubjectRe: [PATCH v3 tip/core/rcu 1/9] rcu: Add call_rcu_tasks()
    On Mon, Aug 04, 2014 at 02:25:15PM +0200, Peter Zijlstra wrote:
    > 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.

    Right, I detect userspace tasks using RCU's scheduling-clock handler.
    (And code to handle the NO_HZ_FULL case is under construction.)

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

    OK, got it.

    Thanx, Paul



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