I create some loop devices within a docker container.
The container starts by the following command:

docker run --rm --privileged -it ubuntu bash

Then I execute the script inside the container.

#!/bin/bash

for idx in $(seq 3); do
    losetup --raw | grep $idx.img | awk '{print $1}' | xargs -I {} losetup -d {}
    rm -f $idx.img
    fallocate -l 1g $idx.img # create an empty file with 5GB 
    ls -l $idx.img
    loopDev=$(losetup -f) # figure out the loop device
    echo "Setup loop device $loopDev on file $idx.img"
    strace losetup $loopDev $idx.img 
    losetup $loopDev
    echo "#######################################################"
done

    -rw-r--r-- 1 root root 1073741824 Jun 15 14:57 1.img
    Setup loop device /dev/loop25 on file 1.img
    /dev/loop25: [66306]:17177356 (/workspace/infrastructure/test-env/rook/1.img)
    -rw-r--r-- 1 root root 1073741824 Jun 15 14:57 2.img
    Setup loop device /dev/loop26 on file 2.img
    /dev/loop26: [66306]:17177357 (/workspace/infrastructure/test-env/rook/2.img)
    -rw-r--r-- 1 root root 1073741824 Jun 15 14:57 3.img
    Setup loop device /dev/loop28 on file 3.img
    losetup: /dev/loop28: failed to set up loop device: No such file or directory
    losetup: /dev/loop28: failed to use device: No such device

It works for the first two loop devices but after a restart it maybe creates all or just one. I couldn’t figured out the reason why this behavior is so unpredictable.

The strace out put of the command which failed is:

    execve("/usr/sbin/losetup", ["losetup", "/dev/loop28", "3.img"], 0x7ffe844f02d0 /* 47 vars */) = 0
    arch_prctl(0x3001 /* ARCH_??? */, 0x7fffffd94760) = -1 EINVAL (Invalid argument)
    mmap(NULL, 8192, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7e2044058000
    access("/etc/ld.so.preload", R_OK)      = -1 ENOENT (No such file or directory)
    openat(AT_FDCWD, "/etc/ld.so.cache", O_RDONLY|O_CLOEXEC) = 3
    newfstatat(3, "", {st_mode=S_IFREG|0644, st_size=13419, ...}, AT_EMPTY_PATH) = 0
    mmap(NULL, 13419, PROT_READ, MAP_PRIVATE, 3, 0) = 0x7e2044054000
    read(3, "177ELF2113>1"..., 832) = 832
    newfstatat(3, "", {st_mode=S_IFREG|0755, st_size=2220400, ...}, AT_EMPTY_PATH) = 0
    pread64(3, "64@@@"..., 784, 64) = 784
    mmap(NULL, 2264656, PROT_READ, MAP_PRIVATE|MAP_DENYWRITE, 3, 0) = 0x7e2043e11000
    prlimit64(0, RLIMIT_STACK, NULL, {rlim_cur=8192*1024, rlim_max=RLIM64_INFINITY}) = 0
    newfstatat(3, "", {st_mode=S_IFREG|0644, st_size=3048928, ...}, AT_EMPTY_PATH) = 0
    openat(AT_FDCWD, "/dev/loop28", O_RDWR|O_CLOEXEC) = -1 ENOENT (No such file or directory)
    openat(AT_FDCWD, "/usr/share/locale/locale.alias", O_RDONLY|O_CLOEXEC) = 3
    newfstatat(3, "", {st_mode=S_IFREG|0644, st_size=2996, ...}, AT_EMPTY_PATH) = 0
    openat(AT_FDCWD, "/usr/share/locale/en_US.UTF-8/LC_MESSAGES/util-linux.mo", O_RDONLY) = -1 ENOENT (No such file or directory)
    write(2, "losetup: ", 9losetup: )                = 9
    write(2, "/dev/loop28: failed to set up lo"..., 41/dev/loop28: failed to set up loop device) = 41
    openat(AT_FDCWD, "/usr/share/locale/en_US.UTF-8/LC_MESSAGES/libc.mo", O_RDONLY) = -1 ENOENT (No such file or directory)
    write(2, ": No such file or directoryn", 28: No such file or directory