losetup -d --force for zombie loop devices?

[Thomas Orgis <thomas-forum@orgis.org>]

[-- Attachment #1: Type: text/plain, Size: 4391 bytes --]

Hi,

I am having an issue with a loop device that used to be connected to a file on NFS.
Not sure if this is actually a kernel bug, but I assume (perhaps ignorantly so)
that losetup could resolve the situation anyways.

0. have NFS share on /mnt/nfs  (rw,users,noexec,nosuid,nodev,hard,intr)
1. losetup --show -f /mnt/nfs/file.img
   /dev/loop0
# actually, it was using cryptsetup luksOpen/Close
2. mount /dev/loop0 && do_work && umount /dev/loop0
3. loose connection to NFS server (it went offline, client machine (laptop) switched networks ...)
4. umount -l /mnt/nfs
5. be reminded about stale loop device because of
  losetup --show -f /some/other.img
hanging (at accessing /dev/loop0, I presume), in need of kill -9
6. reconnect to NFS server, mount /mnt/nfs, hoping that that resolves the hang
7. does resolve hang, but now any operation on /dev/loop0 failing with "device or resource busy"

Well, at this point I just want to get rid of the stale loop device and I bump against this:

# strace losetup -d /dev/loop0
execve("/sbin/losetup", ["losetup", "-d", "/dev/loop0"], [/* 44 vars */]) = 0
brk(0)                                  = 0x1494000
mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x2ac8a5568000
access("/etc/ld.so.preload", R_OK)      = 0
open("/etc/ld.so.preload", O_RDONLY)    = 3
fstat(3, {st_mode=S_IFREG|0644, st_size=23, ...}) = 0
mmap(NULL, 23, PROT_READ|PROT_WRITE, MAP_PRIVATE, 3, 0) = 0x2ac8a5569000
close(3)                                = 0
munmap(0x2ac8a5569000, 23)              = 0
open("/etc/ld.so.cache", O_RDONLY)      = 3
fstat(3, {st_mode=S_IFREG|0644, st_size=194473, ...}) = 0
mmap(NULL, 194473, PROT_READ, MAP_PRIVATE, 3, 0) = 0x2ac8a5569000
close(3)                                = 0
open("/lib/libc.so.6", O_RDONLY)        = 3
read(3, "\177ELF\2\1\1\0\0\0\0\0\0\0\0\0\3\0>\0\1\0\0\0p\354\1\0\0\0\0\0@"..., 832) = 832
fstat(3, {st_mode=S_IFREG|0755, st_size=8820786, ...}) = 0
mmap(NULL, 3524616, PROT_READ|PROT_EXEC, MAP_PRIVATE|MAP_DENYWRITE, 3, 0) = 0x2ac8a576a000
mprotect(0x2ac8a58bd000, 2097152, PROT_NONE) = 0
mmap(0x2ac8a5abd000, 20480, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_DENYWRITE, 3, 0x153000) = 0x2ac8a5abd000
mmap(0x2ac8a5ac2000, 18440, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_ANONYMOUS, -1, 0) = 0x2ac8a5ac2000
close(3)                                = 0
mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x2ac8a5ac7000
mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x2ac8a5ac8000
arch_prctl(ARCH_SET_FS, 0x2ac8a5ac7b20) = 0
mprotect(0x2ac8a5abd000, 16384, PROT_READ) = 0
mprotect(0x2ac8a5767000, 4096, PROT_READ) = 0
munmap(0x2ac8a5569000, 194473)          = 0
brk(0)                                  = 0x1494000
brk(0x14b5000)                          = 0x14b5000
close(0)                                = 0
close(0)                                = -1 EBADF (Bad file descriptor)
uname({sys="Linux", node="dunkelstern", ...}) = 0
stat("/dev/loop-control", {st_mode=S_IFCHR|0600, st_rdev=makedev(10, 237), ...}) = 0
open("/dev/loop0", O_RDONLY)            = 0
ioctl(0, 0x4c01, 0)                     = -1 EBUSY (Device or resource busy)
write(2, "losetup: "..., 9losetup: )             = 9
write(2, "/dev/loop0: detach failed"..., 25/dev/loop0: detach failed) = 25
write(2, ": "..., 2: )                    = 2
write(2, "Device or resource busy\n"..., 24Device or resource busy
) = 24
close(0)                                = 0
exit_group(1)                           = ?

So I see that it might be tricky for losetup to decide on itself that the file
is not busy but actually a zombie. but is there a way to enforce deletion
of the loop device, ignoring the failing ioctl?

# I know what I'm doing, just get rid of the loop device.
losetup --force -d /dev/loop0

I did notice that this must be something peculiar with NFS ... 
well, for completeness, I am on vanilla linux 3.2.1 (plus ASPM patch for
that power management regression) on x86-64 and until-linux 2.21.1.
Distro is rather custom (it's an install of Source Mage).

In the case this is an obscure kernel/NFS bug/feature, I still would like to
know if such enforced delete could work from losetup ... sometimes we
have to live with bugs in other software.


Alrighty then,

Thomas

[-- Attachment #2: signature.asc --]
[-- Type: application/pgp-signature, Size: 197 bytes --]