linux-kernel - Re: [PATCH v2] drivers/uio/uio_pdrv_genirq.c: Fix memory freeing issues

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Date:	Fri, 7 Dec 2012 11:41:45 +0200
From:	Vitalii Demianets <vitas@...factor.kiev.ua>
To:	"Hans J. Koch" <hjk@...sjkoch.de>
Cc:	Cong Ding <dinggnu@...il.com>, linux-kernel@...r.kernel.org,
	"Greg Kroah-Hartman" <gregkh@...uxfoundation.org>
Subject: Re: [PATCH v2] drivers/uio/uio_pdrv_genirq.c: Fix memory freeing issues

On Friday 07 December 2012 00:15:59 Hans J. Koch wrote:
> On Thu, Dec 06, 2012 at 11:11:38AM +0200, Vitalii Demianets wrote:
> > On Thursday 06 December 2012 04:41:01 Hans J. Koch wrote:
> > > > @@ -63,7 +68,7 @@ static irqreturn_t uio_pdrv_genirq_handler(int irq,
> >
> > struct uio_info *dev_info)
> >
> > > >  	 * remember the state so we can allow user space to enable it
> > > > later. */
> > > >
> > > > -	if (!test_and_set_bit(0, &priv->flags))
> > > > +	if (!test_and_set_bit(UIO_IRQ_DISABLED, &priv->flags))
> > > >  		disable_irq_nosync(irq);
> > >
> > > That is not safe. That flag can only be changed by userspace, and if
> > > the userspace part is not running, the CPU on which the irq is pending
> > > will hang.
> > > The handler has to disable the interrupt in any case.
> > > (The original version had the same problem, I know...)
> >
> > Perhaps I'm missing something here, but I don't see your point. If
> > userspace is not (implicitly) calling uio_pdrv_genirq_irqcontrol() by
> > writing to the device file, then on the first invocation of interrupt
> > handler that IRQ is disabled by disable_irq_nosync() and that's the end
> > of story - no more irqs, no problems. Until userspace writes non-zero to
> > the device file, of course.
>
> If you run into the irq handler, the interrupt was obviously enabled. Since
> irq sharing is not supported by this driver, you ALWAYS have to disable it
> because it IS your interrupt. Storing the enabled/disabled status in a
> variable is not needed at all here. Or am I missing something?
>

Good point. I agree that having UIO_IRQ_DISABLED flag is redundant. Inside the 
irq handler we know for sure that irq is enabled. In 
uio_pdrv_genirq_irqcontrol() this knowledge is not mandatory, we could 
enable/disable irq unconditionally.

The only question: I feel uncomfortable mixing in one patch bug fixes (i.e. 
memory freeing issues) and optimization (removing UIO_IRQ_DISABLED flag). 
What do you think is the best course of action:
a) respin the current patch to the v3 with removing UIO_IRQ_DISABLED flag;
 or
b) do the removing of the flag in a separate patch, dependent on the "Fix 
memory freeing issues"?

> > > >  	priv->uioinfo = uioinfo;
> > > >  	spin_lock_init(&priv->lock);
> > > > -	priv->flags = 0; /* interrupt is enabled to begin with */
> > > > +	/* interrupt is enabled to begin with */
> > > > +	priv->flags = uioinfo_alloced ? (1 << UIO_INFO_ALLOCED) : 0;
> > >
> > > The comment doesn't describe the line it's supposed to comment.
> >
> > The comment draws attention to the fact that UIO_IRQ_DISABLED is not set
> > initially, and this is done on purpose.
>
> And that's right because the interrupt is initially enabled by
> uio_register_device().
>
> > Particularly it is done because of
> > the potential problem you noted earlier - if userspace is never setting
> > this flag, the interrupt handler will disable irq on the first invocation
> > thanks to the absence of the flag
> > (if(!test_and_set_bit(UIO_IRQ_DISABLED,...) will succeed).
> > So, I think we really need some comment explaining how the things are
> > arranged here; after all even you haven't got the whole picture on the
> > first glance. Maybe the current wording of the comment is not the best.
> > I've just left what was there before.
>
> OK, but we humans tend to read a text from top to bottom, so a comment
> should describe the code immediately following it. And that is a line that
> deals with memory allocation, not with interrupts.
>

That line deals with initialization of flags. The comment draws attention to 
the fact that UIO_IRQ_DISABLED flag is initialized to zero.
Anyways, if we agreed on the removing of that flag altogether, this would be 
no issue and the comment would be simply removed.
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