linux-kernel - Re: [PATCH] uio: uio_pdrv

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Date:	Thu, 10 Jul 2008 11:02:25 +0100
From:	Alan Cox <alan@...rguk.ukuu.org.uk>
To:	Uwe Kleine-König <Uwe.Kleine-Koenig@...i.com>
Cc:	Magnus Damm <magnus.damm@...il.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"gregkh@...e.de" <gregkh@...e.de>,
	"akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
	"hjk@...utronix.de" <hjk@...utronix.de>,
	"lethal@...ux-sh.org" <lethal@...ux-sh.org>,
	"tglx@...utronix.de" <tglx@...utronix.de>
Subject: Re: [PATCH] uio: uio_pdrv_genirq V2

On Thu, 10 Jul 2008 12:30:36 +0200
Uwe Kleine-König <Uwe.Kleine-Koenig@...i.com> wrote:

> Alan Cox wrote:
> > 
> > > > +       else if (!irq_on && !priv->irq_disabled)
> > > > +               disable_irq(dev_info->irq);
> > > I'm not sure if this is a problem on SMP.  Should you use
> > > disable_irq_nosync here, too?  Probably it's OK.
> > 
> > That one will also deadlock.
> Can you explain why?  I think irqcontrol is only called in task context.
> I only see one possible deadlock and that's disable_irq being called
> while the irq is IRQ_INPROGRESS on the same cpu.  I'm always willing to
> learn.

	CPU0	 (UIO IRQ)		CPU1 (irqcontrol)
	take IRQ
					take spin lock
	spin on spinlock
					disable_irq (blocks)

> I think I didn't understand you right here, with the lock this can
> happen, too, doesn't it?

Actually yes - so it would simplify it without changing behaviour.

Alan
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