On Wed, Feb 14, 2007 at 11:09:04PM +0300, Oleg Nesterov wrote:
> 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() ?
What is ugly abt kthread_stop in workqueue.c?
I feel it is nice if the cleanup is synchronous i.e when cpu_down() is
complete, all the dead cpu's worker threads would have terminated.
Otherwise we expose races between CPU_UP_PREPARE/kthread_create and the
(old) thread exiting.
> 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 :)
wait_to_die is not a new "hack"! Its already used in several other
places ..
> > -static DEFINE_MUTEX(workqueue_mutex);
> > +static DEFINE_SPINLOCK(workqueue_lock);
>
> No. We can't do this. see below.
Ok ..
> > 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.
and try to kthread_stop() uninitialized cwq->thread?
How abt retaining the break above but setting cwq->thread = NULL in
create_workqueue_thread in failure case?
> > +static void take_over_work(struct workqueue_struct *wq, unsigned int cpu)
> > +{
<snip>
> > +}
>
> I think this is unneeded complication, but ok, should work.
This is required if we want to stop per-cpu threads synchronously.
> > + 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.
Ok yes ..thanks for pointing out!
--
Regards,
vatsa
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