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    SubjectRe: [PATCH RFC] v1 expedited "big hammer" RCU grace periods
    Paul E. McKenney wrote:
    > On Thu, Apr 23, 2009 at 02:11:58PM +0800, Lai Jiangshan wrote:
    >> Paul E. McKenney wrote:
    >
    > [ . . . ]
    >
    >> Hi, Paul
    >>
    >> I just typed codes in email, very like these two pathes:
    >>
    >> [PATCH 1/2] sched: Introduce APIs for waiting multi events
    >> http://lkml.org/lkml/2009/4/14/733
    >>
    >> [PATCH 2/2] rcupdate: use struct ref_completion
    >> http://lkml.org/lkml/2009/4/14/734
    >>
    >> Lai.
    >> --------------
    >
    > Interesting approach! This would get a second use for your multi-events
    > waiting code above. ;-)
    >
    > Looks like the idea is to have the task doing the
    > synchronize_rcu_bh_expedited() hold a reference across the process,
    > and have each rcu_bh_fast_qs() also acquire a reference, which would
    > be released in the softirq handler synchronize_rcu_bh_expedited_help().
    >
    > One question -- does this approach correctly handle all the CPU hotplug
    > scenarios? (I think that it might, but am not completely certain.)
    >
    > Thanx, Paul

    Ah, raise_softirq(RCU_EXPEDITED_SOFTIRQ) can not ensure the softirq
    will be called when hotplug.

    It needs get_online_cpus() and put_online_cpus().

    >
    >> #ifndef CONFIG_SMP
    >>
    >> static void __init synchronize_rcu_expedited_init(void)
    >> {
    >> }
    >>
    >> void synchronize_rcu_bh_expedited(void)
    >> {
    >> cond_resched();
    >> }
    >>
    >> #else /* #ifndef CONFIG_SMP */
    >>
    >> static DEFINE_MUTEX(synchronize_rcu_bh_mutex);
    >> static DEFINE_PER_CPU(int, call_only_once); /* is it need ? */
    >> static struct ref_completion rcu_bh_expedited_completion
    >>
    >> static void synchronize_rcu_bh_expedited_help(struct softirq_action *unused)
    >> {
    >> if (__get_cpu_var(call_only_once)) {
    >> smp_mb();
    >> ref_completion_put(&rcu_bh_expedited_completion);
    >> __get_cpu_var(call_only_once) = 0;
    >> }
    >> }
    >>
    >> static void rcu_bh_fast_qs(void *unused)
    >> {
    >> __get_cpu_var(call_only_once) = 1;
    >> ref_completion_get(&rcu_bh_expedited_completion);
    >> raise_softirq(RCU_EXPEDITED_SOFTIRQ);
    >> }
    >>
    >> static void __init synchronize_rcu_expedited_init(void)
    >> {
    >> open_softirq(RCU_EXPEDITED_SOFTIRQ, synchronize_rcu_bh_expedited_help);
    >> }
    >>
    >> void synchronize_rcu_bh_expedited(void)
    >> {
    >> mutex_lock(&synchronize_rcu_bh_mutex);
    >>
    >> ref_completion_get_init(&rcu_bh_expedited_completion);
    >>
    >> smp_call_function(rcu_bh_fast_qs, NULL, 1);
    >>
    >> ref_completion_put_init(&rcu_bh_expedited_completion);
    >> ref_completion_wait(&rcu_bh_expedited_completion);
    >>
    >> mutex_unlock(&synchronize_rcu_bh_mutex);
    >> }
    >>
    >> #endif /* #else #ifndef CONFIG_SMP */
    >>
    >
    >
    >




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