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Quoting Serge E. Hallyn (serge.hallyn at canonical.com):
> I'm sure the patch should be tweaked (helpers moved elsewhere, whatever)
> but running euca-terminate-instances twice on an lxc container can cause
> oopses on the host without this.

Sorry, I guess when I re-typed it I erred on caps - s/true/True/ and
s/false/False/ to come up with the following, which is working rock-solid
for me.

diff -Nru nova-2011.3/debian/changelog nova-2011.3/debian/changelog
--- nova-2011.3/debian/changelog	2011-09-30 14:22:23.000000000 -0500
+++ nova-2011.3/debian/changelog	2011-10-03 20:14:58.000000000 -0500
@@ -1,3 +1,10 @@
+nova (2011.3-0ubuntu5) oneiric; urgency=low
+
+  * debian/patches/dont-detach-mounted-qemu-nbd.patch: work harder to prevent
+    host kernel oopses when a container is slow to shut down.  (LP: #861656)
+
+ -- Serge Hallyn <serge.hallyn at canonical.com>  Mon, 03 Oct 2011 20:14:05 -0500
+
 nova (2011.3-0ubuntu4) oneiric; urgency=low
 
   [James Page]
diff -Nru nova-2011.3/debian/patches/dont-detach-mounted-qemu-nbd.patch nova-2011.3/debian/patches/dont-detach-mounted-qemu-nbd.patch
--- nova-2011.3/debian/patches/dont-detach-mounted-qemu-nbd.patch	1969-12-31 18:00:00.000000000 -0600
+++ nova-2011.3/debian/patches/dont-detach-mounted-qemu-nbd.patch	2011-10-03 20:13:59.000000000 -0500
@@ -0,0 +1,40 @@
+Description: virt/disk.py: don't detach a mounted qemu-nbd drive
+ If a mounted qemu-nbd drive is detached, it can easily oops the kernel
+ and prevent the host from syncing or being cleanly shut down.
+Author: Serge Hallyn <serge.hallyn at canonical.com>
+Forwarded: yes
+Bug-Ubuntu: https://bugs.launchpad.net/ubuntu/+source/linux/+bug/861656
+
+Index: nova-2011.3/nova/virt/disk.py
+===================================================================
+--- nova-2011.3.orig/nova/virt/disk.py	2011-10-03 20:11:42.104067746 -0500
++++ nova-2011.3/nova/virt/disk.py	2011-10-03 20:11:51.920116412 -0500
+@@ -224,9 +224,28 @@
+         return out.strip()
+ 
+ 
++def is_mounted(device):
++    f=open("/proc/mounts", "r")
++    for l in f.readlines():
++        fields = l.split()
++        if fields[0] == device:
++            f.close()
++            return True
++    f.close()
++    return False
++
++def is_stuck_mounted(device):
++    if is_mounted(device):
++        utils.execute('umount', device, run_as_root=True)
++    if is_mounted(device):
++        return True
++    return False
++
+ def _unlink_device(device, nbd):
+     """Unlink image from device using loopback or nbd"""
+     if nbd:
++        if is_stuck_mounted(device):
++            raise exception.Error(_('Could not unmount device %s') % device)
+         utils.execute('qemu-nbd', '-d', device, run_as_root=True)
+         _free_device(device)
+     else:
diff -Nru nova-2011.3/debian/patches/series nova-2011.3/debian/patches/series
--- nova-2011.3/debian/patches/series	2011-09-30 14:22:23.000000000 -0500
+++ nova-2011.3/debian/patches/series	2011-10-03 20:11:37.000000000 -0500
@@ -10,3 +10,4 @@
 backport-lxc-container-console-fix.patch
 backport-snapshot-cleanup.patch
 fix-lp863305-images-permission.patch
+dont-detach-mounted-qemu-nbd.patch