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    SubjectRe: linux-next-20110923: warning kernel/rcutree.c:1833
    From
    2011/9/26 Frederic Weisbecker <fweisbec@gmail.com>:
    > On Sun, Sep 25, 2011 at 09:48:04AM -0700, Paul E. McKenney wrote:
    >> This is required for RCU_FAST_NO_HZ, which checks to see whether the
    >> current CPU can accelerate the current grace period so as to enter
    >> dyntick-idle mode sooner than it would otherwise.  This takes effect
    >> in the situation where rcu_needs_cpu() sees that there are callbacks.
    >> It then notes a quiescent state (which is illegal in an RCU read-side
    >> critical section), calls force_quiescent_state(), and so on.  For this
    >> to work, the current CPU must be in an RCU read-side critical section.
    >
    > You mean it must *not* be in an RCU read-side critical section (ie: in a
    > quiescent state)?
    >
    > That assumption at least fails anytime in idle for the RCU
    > sched flavour given that preemption is disabled in the idle loop.
    >
    >> If this cannot be made to work, another option is to call a new RCU
    >> function in the case where rcu_needs_cpu() returned false, but after
    >> the RCU read-side critical section has exited.
    >
    > You mean when rcu_needs_cpu() returns true (when we have callbacks
    > enqueued)?
    >
    >> This new RCU function
    >> could then attempt to rearrange RCU so as to allow the CPU to enter
    >> dyntick-idle mode more quickly.  It is more important for this to
    >> happen when the CPU is going idle than when it is executing a user
    >> process.
    >>
    >> So, is this doable?
    >
    > At least not when we have RCU sched callbacks enqueued, given preemption
    > is disabled. But that sounds plausible in order to accelerate the switch
    > to dyntick-idle mode when we only have rcu and/or rcu bh callbacks.

    But the RCU sched case could be dealt with if we embrace every use of
    it with rcu_read_lock_sched() and rcu_read_unlock_sched(), or some light
    version that just increases a local counter that rcu_needs_cpu() could check.

    It's an easy thing to add: we can ensure preempt is disabled when we call it
    and we can force rcu_dereference_sched() to depend on it.
    --
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