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    SubjectRe: [RFC PATCH(Experimental) 2/4] Revert changes to workqueue.c
    On 02/14, Gautham R Shenoy wrote:
    >
    > This patch reverts all the recent workqueue hacks added to make it
    > hotplug safe.

    In my opinion these hacks are cleanups :)

    Ok. If we use freezer then yes, we can remove cpu_populated_map and just
    use for_each_online_cpu(). This is easy and good.

    What else you don't like? Why do you want to remove cwq_should_stop() and
    restore an ugly (ugly for workqueue.c) kthread_stop/kthread_should_stop() ?

    We can restore take_over_works(), although I don't see why this is needed.
    But cwq_should_stop() will just work regardless, why do you want to add
    this "wait_to_die" ... well, hack :)

    > -static DEFINE_MUTEX(workqueue_mutex);
    > +static DEFINE_SPINLOCK(workqueue_lock);

    No. We can't do this. see below.

    > struct workqueue_struct *__create_workqueue(const char *name,
    > int singlethread, int freezeable)
    > {
    > @@ -798,17 +756,20 @@ struct workqueue_struct *__create_workqu
    > INIT_LIST_HEAD(&wq->list);
    > cwq = init_cpu_workqueue(wq, singlethread_cpu);
    > err = create_workqueue_thread(cwq, singlethread_cpu);
    > + if (!err)
    > + wake_up_process(cwq->thread);
    > } else {
    > - mutex_lock(&workqueue_mutex);
    > + spin_lock(&workqueue_lock);
    > list_add(&wq->list, &workqueues);
    > -
    > - for_each_possible_cpu(cpu) {
    > + spin_unlock(&workqueue_lock);
    > + for_each_online_cpu(cpu) {
    > cwq = init_cpu_workqueue(wq, cpu);
    > - if (err || !(cpu_online(cpu) || cpu == embryonic_cpu))
    > - continue;
    > err = create_workqueue_thread(cwq, cpu);
    > + if (err)
    > + break;

    No, we can't break. We are going to execute destroy_workqueue(), it will
    iterate over all cwqs.

    > +static void take_over_work(struct workqueue_struct *wq, unsigned int cpu)
    > +{
    > + struct cpu_workqueue_struct *cwq = per_cpu_ptr(wq->cpu_wq, cpu);
    > + struct list_head list;
    > + struct work_struct *work;
    > +
    > + spin_lock_irq(&cwq->lock);
    > + list_replace_init(&cwq->worklist, &list);
    > +
    > + while (!list_empty(&list)) {
    > + work = list_entry(list.next,struct work_struct,entry);
    > + list_del(&work->entry);
    > + __queue_work(per_cpu_ptr(wq->cpu_wq, smp_processor_id()), work);
    > + }
    > +
    > + spin_unlock_irq(&cwq->lock);
    > +}

    I think this is unneeded complication, but ok, should work.

    > static int __devinit workqueue_cpu_callback(struct notifier_block *nfb,
    > unsigned long action,
    > void *hcpu)
    > + case CPU_UP_CANCELED:
    > + list_for_each_entry(wq, &workqueues, list) {
    > + if (!per_cpu_ptr(wq->cpu_wq, hotcpu)->thread)
    > + continue;
    > + /* Unbind so it can run. */
    > + kthread_bind(per_cpu_ptr(wq->cpu_wq, hotcpu)->thread,
    > + any_online_cpu(cpu_online_map));
    > + cleanup_workqueue_thread(wq, hotcpu);
    > }
    > + break;
    > +
    > + case CPU_DEAD:
    > + list_for_each_entry(wq, &workqueues, list)
    > + take_over_work(wq, hotcpu);
    > + break;
    > +
    > + case CPU_DEAD_KILL_THREADS:
    > + list_for_each_entry(wq, &workqueues, list)
    > + cleanup_workqueue_thread(wq, hotcpu);
    > }

    Both CPU_UP_CANCELED and CPU_DEAD_KILL_THREADS runs after thaw_processes(),
    this means that workqueue_cpu_callback() is racy wrt create/destroy workqueue,
    we should take the mutex, and it can't be spinlock_t.

    Oleg.

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