Re: bug 2400

[James Bottomley <James.Bottomley@steeleye.com>]

On Thu, 2004-04-01 at 16:15, Andrew Morton wrote:
> Apparently there is some controversy over whether this:
> http://bugme.osdl.org/show_bug.cgi?id=2400 is a usb-storage problem or a
> SCSI problem.

Well, I know why this happens, but I'm not entirely clear how to fix it.

The problem comes because the cdrom open and close take and release
references to the SCSI generic device (as they're supposed to).

However, Upper level Drivers like sr are implemented as generic device
drivers in the driverfs model.

When a USB unplug comes along, it calls scsi_remove_device, which
eventually calls device_del().  The problem is that device_del triggers
the ->remove methods of all the attached drivers and the sr_remove
method calls cdrom_unregister which throws away the cdrom device state,
even though the actual device has active references.

Some time later, the device is closed but there's now bogus state
because the sr_remove method has kfreed the struct scsi_cd which
contains the struct cdrom_device_info.

Now, the questions are, whose issue is this and how do we fix it?  I can
see that a driver needs early notification of unplugs so it can deny all
access to a gone device.  On the other hand, for a user land open where
we still have to hold resources in the driver, we'd like the driver to
have a notify when the device reference count drops to zero so we can
clean up.

This problem, by the way, exists in a lesser form for sd: the sd remove
method will free the device for reattachment even though it might have
active references.

James