[systemd-devel] removing dm-crypt mapping hangs on shutdown

[Colin Guthrie]

'Twas brillig, and Dave Reisner at 13/06/12 01:50 did gyre and gimble:
> Hi,
> 
> I'm having some problems with a few of my VMs which all share in common
> an encrypted root. They pivot into a shutdown ramfs and walk through the
> blockdevs in sysfs, breaking down and unmounting each device.
> Universally, these VMs all hang when calling 'cryptsetup remove', and
> only under systemd. The relevant bits of stack trace look something like
> this:
> 
> ioctl(3, DM_DEV_REMOVE, 0xe1b8b0)       = 0
> semget(0xd4d255f, 1, 0)                 = 229376
> semctl(229376, 0, GETVAL, 0xffffffffffffffff) = 2
> semop(229376, {{0, -1, IPC_NOWAIT}}, 1) = 0
> semop(229376, {{0, 0, 0}}, 1

I've seen similar symptoms in bug reports to your experience. Basically
hanging after detaching dm devices.

> I've found 2 solutions so far to avoiding this hang:
> 
> 1) Not letting systemd get its paws on my block devices to umount them.
> Specifically, avoiding the dm_detach_all() call seems to alleviate this.

Yeah, it wouldn't be right to avoid it completely, but I agree that it
does need to skip things a bit more robustly.

> So this, to me, raises 2 questions:
> 
> 1) Can systemd just leave block devices which it didn't assemble alone?
> I'd expect that if the initramfs assembled and mounted a device, it
> should be responsible for tearing it down. In theory, this could just be
> something as simple as just skipping over / and /usr.

I don't think a simply solution would work here as there are other cases
that may trigger the initramfs to enable certain devices (e.g. resume
from an encrypted swap partition for example).

I guess the initramfs should really write something in /run/initramfs
folder that indicates which devices it's "managing" such that systemd
can only deal with the remainder.


I'm not sure what the current status is, but I wonder what would happen
when systemd tries to unmount /usr just now... :s


> 2) What's the goal of calling mlockall() here? The original patch that
> added this (b1b2a107d15a) gives no indication of why it's wanted/needed.

No idea on this one.

Col


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Colin Guthrie
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