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    SubjectRe: [PATCH v2] CPU hotplug: active_writer not woken up in some cases - deadlock
    > > Therefore we have to move the condition check inside the 
    > > __set_current_state(TASK_UNINTERRUPTIBLE) -> schedule();
    > > section to not miss any wake ups when the condition is satisfied.
    > >
    > > So wake_up_process() will either see TASK_RUNNING and do nothing or see
    > > TASK_UNINTERRUPTIBLE and set it to TASK_RUNNING, so schedule() will in
    > > fact be woken up again.
    >
    > Or the third alternative would be that 'active_writer' which was running
    > on CPU2 already terminated and wake_up_process() has a non-NULL pointer to
    > task_struct which is already dead.
    > Or is there anything that prevents this use-after-free race?

    Hmmm ... I think that is also a valid scenario.
    That would mean we need soemthing like this:

    void put_online_cpus(void)
    {
    + struct task_struct *awr;
    +
    if (cpu_hotplug.active_writer == current)
    return;
    if (!mutex_trylock(&cpu_hotplug.lock)) {
    + awr = ACCESS_ONCE(cpu_hotplug.active_writer);
    + if (unlikely(awr))
    + get_task_struct(awr);
    + /* inc after get_task_struct(), so the writer can't get NULL */
    atomic_inc(&cpu_hotplug.puts_pending);
    + /* we might be the last one */
    + if (unlikely(awr)) {
    + wake_up_process(awr);
    + put_task_struct(awr);
    + }
    cpuhp_lock_release();
    return;
    }


    Thanks!

    David



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