Date:	Wed, 17 Dec 2008 11:38:12 -0800
From:	Greg KH <greg@...ah.com>
To:	Hans Verkuil <hverkuil@...all.nl>
Cc:	linux-kernel@...r.kernel.org, v4l <video4linux-list@...hat.com>,
	Laurent Pinchart <laurent.pinchart@...net.be>
Subject: Re: [BUG] cdev_put() race condition

On Wed, Dec 17, 2008 at 08:30:32PM +0100, Hans Verkuil wrote:
> On Wednesday 17 December 2008 19:16:45 Greg KH wrote:
> > On Wed, Dec 17, 2008 at 02:37:33PM +0100, Hans Verkuil wrote:
> > > > Again, don't use cdev's reference counting for your own object
> > > > lifecycle, it is different and will cause problems, like you have
> > > > found out.
> > >
> > > Sigh. It has nothing to do with how v4l uses it. And to demonstrate
> > > this, here is how you reproduce it with the sg module (tested it with
> > > my USB harddisk).
> > >
> > > 1) apply this patch to char_dev.c:
> >
> > <snip>
> >
> > Ok, since I can't convince you that using a cdev for your reference
> > counting is incorrect, I'll have to go change the cdev code to prevent
> > you from doing this :(
> 
> Erm, you haven't told me yet why it's a bad idea. You just said "don't do 
> it", but I haven't seen the reason for it. There doesn't seem to be any 
> documentation on how to properly use cdev besides the Kernel Device Drivers 
> book, which (if memory serves) doesn't mention anything on this topic.

The reason is that it was not designed to do such a thing.  It was
designed to handle the character device lookup, and that's it.

> I really don't mind implementing refcounting in the v4l framework, I just 
> want to understand why it should be done like that!

Because the lifecycle of your device should be separate from the
lifecycle of the character device node, you should not tie the two
together.

> It seems to me that I will just be shadowing the refcounting of cdev if I 
> implement refcounting in v4l: init to 1 on creation, increase on open, 
> decrease on close, decrease on deletion. It's all terribly familiar...

You shouldn't, odds are there are other ways to grab the reference on
your device from outside of the character node, right?  Like the USB
probe/disconnect path?  So you should have an independant reference
count in order to handle all of the different ways the device can be
accessed.

> > Anyway, do you have a patch for the cdev code to propose how to fix this
> > issue you are having?
> 
> Sure, here it is:
> 
> --- fs/char_dev.c.orig	2008-12-17 20:28:40.000000000 +0100
> +++ fs/char_dev.c	2008-12-17 20:28:49.000000000 +0100
> @@ -345,7 +345,9 @@
>  {
>  	if (p) {
>  		struct module *owner = p->owner;
> +		spin_lock(&cdev_lock);
>  		kobject_put(&p->kobj);
> +		spin_unlock(&cdev_lock);
>  		module_put(owner);
>  	}
>  }
> @@ -415,14 +417,12 @@
>  
>  static void cdev_purge(struct cdev *cdev)
>  {
> -	spin_lock(&cdev_lock);
>  	while (!list_empty(&cdev->list)) {
>  		struct inode *inode;
>  		inode = container_of(cdev->list.next, struct inode, i_devices);
>  		list_del_init(&inode->i_devices);
>  		inode->i_cdev = NULL;
>  	}
> -	spin_unlock(&cdev_lock);
>  }
>  
>  /*
> @@ -478,7 +478,9 @@
>  void cdev_del(struct cdev *p)
>  {
>  	cdev_unmap(p->dev, p->count);
> +	spin_lock(&cdev_lock);
>  	kobject_put(&p->kobj);
> +	spin_unlock(&cdev_lock);
>  }
> 
> This solves this particular problem. But this will certainly break v4l as it 
> is right now, since the spin_lock means that the kref's release cannot do 
> any sleeps, which is possible in v4l. If we want to allow that in cdev, 
> then the spinlock has to be replaced by a mutex. But I have the strong 
> feeling that that's not going to happen :-)

True :)

I'll be glad to queue this change up for 2.6.29 if you are going to fix
up the v4l code to not break if I add this code.

thanks,

greg k-h
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