* 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.
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.
Ingo
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