Re: + schedule_on_each_cpu-use-preempt_disable.patch added to -mm tree

[Andrew Morton <akpm@osdl.org>]

On Fri, 15 Dec 2006 17:24:16 +0100
Ingo Molnar <mingo@elte.hu> wrote:

> 
> * Andrew Morton <akpm@osdl.org> 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.