root@panda-maverick:~# ifconfig usb0 up
root@panda-maverick:~# strace ifconfig usb0 down
execve("/sbin/ifconfig", ["ifconfig", "usb0", "down"], [/* 15 vars */]) = 0
brk(0)                                  = 0x1e000
uname({sys="Linux", node="panda-maverick", ...}) = 0
access("/etc/ld.so.nohwcap", F_OK)      = -1 ENOENT (No such file or directory)
mmap2(NULL, 8192, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x40017000
access("/etc/ld.so.preload", R_OK)      = -1 ENOENT (No such file or directory)
open("/etc/ld.so.cache", O_RDONLY)      = 3
fstat64(3, {st_mode=S_IFREG|0644, st_size=16970, ...}) = 0
mmap2(NULL, 16970, PROT_READ, MAP_PRIVATE, 3, 0) = 0x40019000
close(3)                                = 0
access("/etc/ld.so.nohwcap", F_OK)      = -1 ENOENT (No such file or directory)
open("/lib/libgcc_s.so.1", O_RDONLY)    = 3
read(3, "\177ELF\1\1\1\0\0\0\0\0\0\0\0\0\3\0(\0\1\0\0\0\320%\0\0004\0\0\0"..., 512) = 512
fstat64(3, {st_mode=S_IFREG|0644, st_size=38488, ...}) = 0
mmap2(NULL, 37140, PROT_READ|PROT_EXEC, MAP_PRIVATE|MAP_DENYWRITE, 3, 0) = 0x40021000
mmap2(0x40029000, 8192, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_DENYWRITE, 3, 0x8) = 0x40029000
close(3)                                = 0
access("/etc/ld.so.nohwcap", F_OK)      = -1 ENOENT (No such file or directory)
open("/lib/libc.so.6", O_RDONLY)        = 3
read(3, "\177ELF\1\1\1\0\0\0\0\0\0\0\0\0\3\0(\0\1\0\0\0\301V\1\0004\0\0\0"..., 512) = 512
fstat64(3, {st_mode=S_IFREG|0755, st_size=988620, ...}) = 0
mmap2(NULL, 1025308, PROT_READ|PROT_EXEC, MAP_PRIVATE|MAP_DENYWRITE, 3, 0) = 0x4002b000
mprotect(0x40118000, 32768, PROT_NONE)  = 0
mmap2(0x40120000, 12288, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_DENYWRITE, 3, 0xed) = 0x40120000
mmap2(0x40123000, 9500, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_ANONYMOUS, -1, 0) = 0x40123000
close(3)                                = 0
mmap2(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x4001e000
mmap2(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x40126000
set_tls(0x401264c0, 0x4002091c, 0x40126b98, 0x401264c0, 0x40020000) = 0
mprotect(0x40120000, 8192, PROT_READ)   = 0
mprotect(0x40029000, 4096, PROT_READ)   = 0
mprotect(0x1b000, 4096, PROT_READ)      = 0
mprotect(0x4001f000, 4096, PROT_READ)   = 0
munmap(0x40019000, 16970)               = 0
brk(0)                                  = 0x1e000
brk(0x3f000)                            = 0x3f000
open("/usr/lib/locale/locale-archive", O_RDONLY|O_LARGEFILE) = 3
fstat64(3, {st_mode=S_IFREG|0644, st_size=1539648, ...}) = 0
mmap2(NULL, 1539648, PROT_READ, MAP_PRIVATE, 3, 0) = 0x40127000
close(3)                                = 0
uname({sys="Linux", node="panda-maverick", ...}) = 0
access("/proc/net", R_OK)               = 0
access("/proc/net/unix", R_OK)          = 0
socket(PF_FILE, SOCK_DGRAM, 0)          = 3
socket(PF_INET, SOCK_DGRAM, IPPROTO_IP) = 4
access("/proc/net/if_inet6", R_OK)      = 0
socket(PF_INET6, SOCK_DGRAM, IPPROTO_IP) = 5
access("/proc/net/ax25", R_OK)          = -1 ENOENT (No such file or directory)
access("/proc/net/nr", R_OK)            = -1 ENOENT (No such file or directory)
access("/proc/net/rose", R_OK)          = -1 ENOENT (No such file or directory)
access("/proc/net/ipx", R_OK)           = -1 ENOENT (No such file or directory)
access("/proc/net/appletalk", R_OK)     = -1 ENOENT (No such file or directory)
access("/proc/sys/net/econet", R_OK)    = -1 ENOENT (No such file or directory)
access("/proc/sys/net/ash", R_OK)       = -1 ENOENT (No such file or directory)
access("/proc/net/x25", R_OK)           = -1 ENOENT (No such file or directory)
open("/usr/share/locale/locale.alias", O_RDONLY) = 6
fstat64(6, {st_mode=S_IFREG|0644, st_size=2570, ...}) = 0
mmap2(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x40019000
read(6, "# Locale name alias data base.\n#"..., 4096) = 2570
read(6, "", 4096)                       = 0
close(6)                                = 0
munmap(0x40019000, 4096)                = 0
open("/usr/share/locale/en_US.UTF-8/LC_MESSAGES/net-tools.mo", O_RDONLY) = -1 ENOENT (No such file or directory)
open("/usr/share/locale/en_US.utf8/LC_MESSAGES/net-tools.mo", O_RDONLY) = -1 ENOENT (No such file or directory)
open("/usr/share/locale/en_US/LC_MESSAGES/net-tools.mo", O_RDONLY) = -1 ENOENT (No such file or directory)
open("/usr/share/locale/en.UTF-8/LC_MESSAGES/net-tools.mo", O_RDONLY) = -1 ENOENT (No such file or directory)
open("/usr/share/locale/en.utf8/LC_MESSAGES/net-tools.mo", O_RDONLY) = -1 ENOENT (No such file or directory)
open("/usr/share/locale/en/LC_MESSAGES/net-tools.mo", O_RDONLY) = -1 ENOENT (No such file or directory)
open("/usr/share/locale-langpack/en_US.UTF-8/LC_MESSAGES/net-tools.mo", O_RDONLY) = -1 ENOENT (No such file or directory)
open("/usr/share/locale-langpack/en_US.utf8/LC_MESSAGES/net-tools.mo", O_RDONLY) = -1 ENOENT (No such file or directory)
open("/usr/share/locale-langpack/en_US/LC_MESSAGES/net-tools.mo", O_RDONLY) = -1 ENOENT (No such file or directory)
open("/usr/share/locale-langpack/en.UTF-8/LC_MESSAGES/net-tools.mo", O_RDONLY) = -1 ENOENT (No such file or directory)
open("/usr/share/locale-langpack/en.utf8/LC_MESSAGES/net-tools.mo", O_RDONLY) = -1 ENOENT (No such file or directory)
open("/usr/share/locale-langpack/en/LC_MESSAGES/net-tools.mo", O_RDONLY) = -1 ENOENT (No such file or directory)
ioctl(4, SIOCGIFFLAGS, {ifr_name="usb0", ifr_flags=IFF_UP|IFF_BROADCAST|IFF_RUNNING|IFF_MULTICAST}) = 0
ioctl(4, SIOCSIFFLAGS

--> hangs

When it doesn't hangs it'll let your kernel very unstable, at the point you can't run any other process:
INFO: task ifconfig:557 blocked for more than 120 seconds.
"echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.