On Fri, 15 Dec 2006 17:24:16 +0100
Ingo Molnar <[email protected]> wrote:
>
> * Andrew Morton <[email protected]> wrote:
>
> > > > for_each_online_cpu(cpu) {
> > > > INIT_WORK(per_cpu_ptr(works, cpu), func);
> > > > __queue_work(per_cpu_ptr(keventd_wq->cpu_wq, cpu),
> > > > per_cpu_ptr(works, cpu));
> > > > }
> > > > - mutex_unlock(&workqueue_mutex);
> > > > + preempt_enable();
> > >
> > > Why not cpu_hotplug_lock()?
> > >
> >
> > Because the workqueue code was explicitly switched over to
> > per-subsystem cpu-hotplug locking.
> >
> > Because lock_cpu_hotplug() is a complete turkey, source of deadlocks
> > and overall bad idea.
>
> not in the locking model i outlined earlier, which would turn it into a
> read-lock in essence.
>
> > This is actually a pretty simple problem. A subsystem has per-cpu
> > reosurces, and it needs to lock them while using them. duh. We know
> > how to do that sort of thing. But because the first implementation of
> > lock_cpu_hotplug() was conceived with magical properties, we seem to
> > think we need to retain magical properties. We don't...
>
> actually, we use two things here: cpu_online_map and the per-cpu keventd
> workqueues. cpu_online_map is pretty much attached to the CPU hotplug
> subsystem so it would be quite natural to use cpu_hotplug_read_lock()
> for that.
The two are connected, because cpu add/remove creates and kills keventd
threads.
> so i disagree that CPU hotplug locking should be per-subsystem. We
> should have one lightweight and scalable primitive that protects
> cpu_online_map use, and that same primitive can be used to protect other
> per-CPU resources too.
This problem can be (is being) solved using existing locking primitives.
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