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

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Message-ID: <20080710103036.GA32507@digi.com>
Date:	Thu, 10 Jul 2008 12:30:36 +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

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.

> The easiest fix is probably to use test_and_set and friends for each I/O
> operation.
Actually using spinlock + irq_disabled variable is new in V2 of this
patch.  Don't know why this changed, though.

>            You would then not need the lock to protect ->irq_disabled.
> Propogating that throughout means your user space has to handle the case
> of an IRQ arriving after disable returns but would be a fair bit saner I
> think ?
I think I didn't understand you right here, with the lock this can
happen, too, doesn't it?

Best regards
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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