linux-kernel - Re: [RFC PATCH 1/3] genirq: add support for per-cpu dev

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Date:	Fri, 16 Sep 2011 01:41:55 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Russell King - ARM Linux <linux@....linux.org.uk>
cc:	Marc Zyngier <marc.zyngier@....com>, linux-kernel@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org
Subject: Re: [RFC PATCH 1/3] genirq: add support for per-cpu dev_id
 interrupts

On Fri, 16 Sep 2011, Russell King - ARM Linux wrote:

> On Fri, Sep 16, 2011 at 12:49:10AM +0200, Thomas Gleixner wrote:
> > Marc,
> > 
> > On Thu, 15 Sep 2011, Marc Zyngier wrote:
> > > +
> > > +	raw_spin_unlock_irqrestore(&desc->lock, flags);
> > > +
> > > +	unregister_handler_proc(irq, action);
> > > +
> > > +	/* Make sure it's not being used on another CPU: */
> > > +	synchronize_irq(irq);
> > 
> > That's not helping w/o making synchronize_irq() aware of the percpu
> > stuff. Also there is the question whether we need the ability to
> > remove such interrupts in the first place. The target users are low
> > level arch interrupts not some random device drivers.
> 
> You do - think local timers which go away on hotunplug and come back
> on hotplug.  The alternative is requiring every local timer code to
> remember whether it registered its per-cpu handler on each CPU or not,
> and that just gets more messy than having them unregister on hotunplug.
> Not only would that be more prone to bugs but it will also mean extra
> complexity in arch code.

Yikes! That code is removing the GLOBAL action, so all users are going
to hell.

The point of the percpu_irq stuff is to have a single action with a
percpu dev_id and a per cpu enable/disable. So when you unplug your
cpu that very cpu calls the disable function and therefor removes
itself w/o causing the other cpus to die on action = NULL

Thanks,

	tglx


 
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