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    SubjectRe: [PATCH, RFC] v7 scalable classic RCU implementation
    Paul E. McKenney wrote:
    > +/*
    > + * If the specified CPU is offline, tell the caller that it is in
    > + * a quiescent state. Otherwise, whack it with a reschedule IPI.
    > + * Grace periods can end up waiting on an offline CPU when that
    > + * CPU is in the process of coming online -- it will be added to the
    > + * rcu_node bitmasks before it actually makes it online. Because this
    > + * race is quite rare, we check for it after detecting that the grace
    > + * period has been delayed rather than checking each and every CPU
    > + * each and every time we start a new grace period.
    > + */
    What about using CPU_DYING and CPU_STARTING?

    Then this race wouldn't exist anymore.
    > +static void force_quiescent_state(struct rcu_state *rsp, int relaxed)
    > +{
    > + [snip]
    > + case RCU_FORCE_QS:
    > +
    > + /* Check dyntick-idle state, send IPI to laggarts. */
    > + if (rcu_process_dyntick(rsp,
    > dyntick_recall_completed(rsp),
    > + rcu_implicit_dynticks_qs))
    > + goto unlock_ret;
    > +
    > + /* Leave state in case more forcing is required. */
    > +
    > + break;
    Hmm - your code must loop multiple times over the cpus.
    I've use a different approach: More forcing is only required for a nohz
    cpu when it was hit within a long-running interrupt.
    Thus I've added a '->kick_poller' flag, rcu_irq_exit() reports back when
    the long-running interrupt completes. Never more than one loop over the
    outstanding cpus is required.

    --
    Manfred


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