linux-kernel - Re: + schedule_on_each_cpu-use-preempt

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Message-ID: <20061215162416.GB29191@elte.hu>
Date:	Fri, 15 Dec 2006 17:24:16 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Andrew Morton <akpm@...l.org>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: + schedule_on_each_cpu-use-preempt_disable.patch added to -mm tree


* Andrew Morton <akpm@...l.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.

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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