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    Subject[PATCH 4.14 040/237] cpu/hotplug, stop_machine: Fix stop_machine vs hotplug order
    Date
    From: Peter Zijlstra <peterz@infradead.org>

    [ Upstream commit 45178ac0cea853fe0e405bf11e101bdebea57b15 ]

    Paul reported a very sporadic, rcutorture induced, workqueue failure.
    When the planets align, the workqueue rescuer's self-migrate fails and
    then triggers a WARN for running a work on the wrong CPU.

    Tejun then figured that set_cpus_allowed_ptr()'s stop_one_cpu() call
    could be ignored! When stopper->enabled is false, stop_machine will
    insta complete the work, without actually doing the work. Worse, it
    will not WARN about this (we really should fix this).

    It turns out there is a small window where a freshly online'ed CPU is
    marked 'online' but doesn't yet have the stopper task running:

    BP AP

    bringup_cpu()
    __cpu_up(cpu, idle) --> start_secondary()
    ...
    cpu_startup_entry()
    bringup_wait_for_ap()
    wait_for_ap_thread() <-- cpuhp_online_idle()
    while (1)
    do_idle()

    ... available to run kthreads ...

    stop_machine_unpark()
    stopper->enable = true;

    Close this by moving the stop_machine_unpark() into
    cpuhp_online_idle(), such that the stopper thread is ready before we
    start the idle loop and schedule.

    Reported-by: "Paul E. McKenney" <paulmck@kernel.org>
    Debugged-by: Tejun Heo <tj@kernel.org>
    Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
    Tested-by: "Paul E. McKenney" <paulmck@kernel.org>
    Signed-off-by: Sasha Levin <sashal@kernel.org>
    ---
    kernel/cpu.c | 13 +++++++++----
    1 file changed, 9 insertions(+), 4 deletions(-)

    diff --git a/kernel/cpu.c b/kernel/cpu.c
    index 49273130e4f1e..96c0a868232ef 100644
    --- a/kernel/cpu.c
    +++ b/kernel/cpu.c
    @@ -494,8 +494,7 @@ static int bringup_wait_for_ap(unsigned int cpu)
    if (WARN_ON_ONCE((!cpu_online(cpu))))
    return -ECANCELED;

    - /* Unpark the stopper thread and the hotplug thread of the target cpu */
    - stop_machine_unpark(cpu);
    + /* Unpark the hotplug thread of the target cpu */
    kthread_unpark(st->thread);

    /*
    @@ -1064,8 +1063,8 @@ void notify_cpu_starting(unsigned int cpu)

    /*
    * Called from the idle task. Wake up the controlling task which brings the
    - * stopper and the hotplug thread of the upcoming CPU up and then delegates
    - * the rest of the online bringup to the hotplug thread.
    + * hotplug thread of the upcoming CPU up and then delegates the rest of the
    + * online bringup to the hotplug thread.
    */
    void cpuhp_online_idle(enum cpuhp_state state)
    {
    @@ -1075,6 +1074,12 @@ void cpuhp_online_idle(enum cpuhp_state state)
    if (state != CPUHP_AP_ONLINE_IDLE)
    return;

    + /*
    + * Unpart the stopper thread before we start the idle loop (and start
    + * scheduling); this ensures the stopper task is always available.
    + */
    + stop_machine_unpark(smp_processor_id());
    +
    st->state = CPUHP_AP_ONLINE_IDLE;
    complete_ap_thread(st, true);
    }
    --
    2.20.1


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