linux-kernel - Re: Unbinding drivers for resources that are in use

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Message-ID: <20110613154218.GA32124@kroah.com>
Date:	Mon, 13 Jun 2011 08:42:18 -0700
From:	Greg KH <greg@...ah.com>
To:	Alan Stern <stern@...land.harvard.edu>
Cc:	Kernel development list <linux-kernel@...r.kernel.org>,
	Hans de Goede <hdegoede@...hat.com>
Subject: Re: Unbinding drivers for resources that are in use

On Mon, Jun 13, 2011 at 11:10:57AM -0400, Alan Stern wrote:
> The kernel prevents modules from being unloaded if they are being used.  
> But it doesn't have any analogous mechanism for preventing a driver 
> being unbound from a device that's in use.
> 
> For example, suppose a SATA disk contains a mounted filesystem.  If the
> user writes the corresponding device name to
> /sys/bus/scsi/drivers/sd/unbind without unmounting the filesystem, the
> drive will become inaccessible and data may be lost.  The same problem
> arises with USB devices and programs using usbfs to unbind a device
> from its kernel driver.
> 
> It's true that the "unbind" attribute has mode 0200 and therefore can 
> be written only by the superuser.  Still, this puts the onus on 
> userspace to determine whether or not a device is being used.  The 
> kernel could easily keep track of this automatically and atomically 
> -- userspace can't do this without races.
> 
> Therefore I'm asking if the driver core should add a refcount to every
> struct device for keeping track of the number of open file references
> (or other types of resource) using this device.  If this number is
> nonzero, the kernel should prevent the device from being unbound from
> its driver -- except of course in cases where the device has been
> hot-unplugged; there's nothing we can do to prevent errors when this
> happens.
> 
> Changes to the refcount would have to propagate up the device tree: If 
> a device holds an important resource then we don't want any of the 
> device's ancestors to become inaccessible either.  This would be easy 
> to implement.
> 
> Should we do it?

No, people are starting to use 'unbind' as a poor-man's verison of
revoke(), by simulating the device removal from the driver, even if the
device is being used by someone at that point in time.

And that's a good thing, as that is what revoke() really wants to do,
you want to clean up whatever that device was doing and make the file
handles stale, and allow a different user to then connect to the device
if needed.

So I really would not want to disallow this type of functionality, which
adding reference counts and preventing unbind from working would cause.

thanks,

greg k-h
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