Another interesting issue with hotplug which may need SCSI-layer changes:
If you unplug a device that some process already has open (for
example, md), the device is removed, but the internal scsi_device
struct is not actually freed until ALL references to this device
(including that still held by md) release it.
When you re-plugin the device at this time (scsi_device has been
marked as 'deleted' but not actually freed), the code properly detects
that while there is still a scsi_device for this host/channel/id/lun
combination, it is marked as "SDEV_DEL" and so should not be used.
Fine - a new scsi_device struct is allocated and added to the back of
the Scsi_Host->__devices list.
The problem: From now on, whenever someone wants to use this scsi
device, they will end up calling scsi_device_lookup, which calls
__scsi_device_lookup, which will always return the first scsi_device
struct, the one which is now SDEV_DEL, so scsi_device_lookup will
return NULL. This means that subsequent hot-unplugs will not remove
the most-recently-allocated scsi_device struct, so no hotplug events
are sent and no udev devices are removed. This is bad -> a memory
leak.
The workaround: This probably has ramifications I'm not considering,
but the following replacement for __scsi_device_lookup in scsi.c seems
to work for me. It returns the LAST matching scsi_device (and
therefore most-recently allocated), as opposed to the old code which
returned the FIRST matching scsi_device:
struct scsi_device *__scsi_device_lookup(struct Scsi_Host *shost,
uint channel, uint id, uint lun)
{
struct scsi_device *found_sdev = NULL;
struct scsi_device *sdev;
list_for_each_entry(sdev, &shost->__devices, siblings) {
printk( "device lookup checking %d:%d:%d:%d against
%d:%d:%d:%d\n",
shost->host_no, channel, id, lun,
shost->host_no, sdev->channel,
sdev->id, sdev->lun );
if (sdev->channel == channel && sdev->id == id &&
sdev->lun ==lun)
{
found_sdev = sdev;
}
}
return found_sdev;
}
How else could this be solved? I suppose adding newly allocated
scsi_device structs to the FRONT of the Scsi_Host->__devices list
would have the same effect as my alternate search routine, but I don't
know which would be better. I can't think of any solution that
wouldn't touch the existing SCSI layer, though.
--
Jim Ramsay
"Me fail English? That's unpossible!"
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