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

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Message-ID: <20080710104749.GA313@digi.com>
Date:	Thu, 10 Jul 2008 12:47:49 +0200
From:	Uwe Kleine-König <Uwe.Kleine-Koenig@...i.com>
To:	Alan Cox <alan@...rguk.ukuu.org.uk>
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

Hello Alan,

Alan Cox wrote:
> 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)
Ah, OK, that's because uio_pdrv_genirq_handler and
uio_pdrv_genirq_irqcontrol share the lock.

Is this something that lockdep can detect?
 
Best regards and thanks for clearifying
Uwe

-- 
Uwe Kleine-König, Software Engineer
Digi International GmbH Branch Breisach, Küferstrasse 8, 79206 Breisach, Germany
Tax: 315/5781/0242 / VAT: DE153662976 / Reg. Amtsgericht Dortmund HRB 13962
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