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    SubjectRe: [RFC patch 1/5] kthread: Implement park/unpark facility
    Hello,

    I've taken a stab at converting stop_machine to use kthread_park and
    kthread_unpark. I don't think stop_machine can use
    smpboot_park_threads (in their current implementation) because, among
    other reasons, _cpu_down calls __stop_machine after parking the
    stopper threads via smpboot_park_threads.

    Silas

    Signed-off-by: Silas Boyd-Wickizer <sbw@mit.edu>
    ---
    kernel/stop_machine.c | 41 ++++++++++++++++++-----------------------
    1 file changed, 18 insertions(+), 23 deletions(-)

    diff --git a/kernel/stop_machine.c b/kernel/stop_machine.c
    index 2f194e9..631f133 100644
    --- a/kernel/stop_machine.c
    +++ b/kernel/stop_machine.c
    @@ -244,7 +244,7 @@ int try_stop_cpus(const struct cpumask *cpumask, cpu_stop_fn_t fn, void *arg)
    return ret;
    }

    -static int cpu_stopper_thread(void *data)
    +static __noreturn int cpu_stopper_thread(void *data)
    {
    struct cpu_stopper *stopper = data;
    struct cpu_stop_work *work;
    @@ -253,10 +253,8 @@ static int cpu_stopper_thread(void *data)
    repeat:
    set_current_state(TASK_INTERRUPTIBLE); /* mb paired w/ kthread_stop */

    - if (kthread_should_stop()) {
    - __set_current_state(TASK_RUNNING);
    - return 0;
    - }
    + if (kthread_should_park())
    + kthread_parkme();

    work = NULL;
    spin_lock_irq(&stopper->lock);
    @@ -308,23 +306,23 @@ static int __cpuinit cpu_stop_cpu_callback(struct notifier_block *nfb,

    switch (action & ~CPU_TASKS_FROZEN) {
    case CPU_UP_PREPARE:
    - BUG_ON(stopper->thread || stopper->enabled ||
    - !list_empty(&stopper->works));
    - p = kthread_create_on_node(cpu_stopper_thread,
    - stopper,
    - cpu_to_node(cpu),
    - "migration/%d", cpu);
    - if (IS_ERR(p))
    - return notifier_from_errno(PTR_ERR(p));
    - get_task_struct(p);
    - kthread_bind(p, cpu);
    - sched_set_stop_task(cpu, p);
    - stopper->thread = p;
    + if (!stopper->thread) {
    + BUG_ON(stopper->enabled ||
    + !list_empty(&stopper->works));
    + p = kthread_create_on_cpu(cpu_stopper_thread,
    + stopper,
    + cpu,
    + "migration/%d");
    + if (IS_ERR(p))
    + return notifier_from_errno(PTR_ERR(p));
    + get_task_struct(p);
    + stopper->thread = p;
    + }
    + sched_set_stop_task(cpu, stopper->thread);
    break;

    case CPU_ONLINE:
    - /* strictly unnecessary, as first user will wake it */
    - wake_up_process(stopper->thread);
    + kthread_unpark(stopper->thread);
    /* mark enabled */
    spin_lock_irq(&stopper->lock);
    stopper->enabled = true;
    @@ -339,16 +337,13 @@ static int __cpuinit cpu_stop_cpu_callback(struct notifier_block *nfb,

    sched_set_stop_task(cpu, NULL);
    /* kill the stopper */
    - kthread_stop(stopper->thread);
    + kthread_park(stopper->thread);
    /* drain remaining works */
    spin_lock_irq(&stopper->lock);
    list_for_each_entry(work, &stopper->works, list)
    cpu_stop_signal_done(work->done, false);
    stopper->enabled = false;
    spin_unlock_irq(&stopper->lock);
    - /* release the stopper */
    - put_task_struct(stopper->thread);
    - stopper->thread = NULL;
    break;
    }
    #endif

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