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Date: Fri, 15 Dec 2006 08:11:38 -0800
From: Andrew Morton <akpm@osdl.org>
To: Ingo Molnar <mingo@elte.hu>
Cc: linux-kernel@vger.kernel.org
Subject: Re: + schedule_on_each_cpu-use-preempt_disable.patch added to -mm
 tree
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On Fri, 15 Dec 2006 09:31:12 +0100
Ingo Molnar <mingo@elte.hu> wrote:

> 
> * akpm@osdl.org <akpm@osdl.org> wrote:
> 
> > -	mutex_lock(&workqueue_mutex);
> > +	preempt_disable();		/* CPU hotplug */
> >  	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.

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...