This is trace result of this "hanging" processes:
strace -p 9492
Process 9492 attached - interrupt to quit
futex(0x4a280c, FUTEX_WAIT, 2, NULL <unfinished ...>
Process 9492 detached
root[~]# strace -p 10231
Process 10231 attached - interrupt to quit
futex(0xaf8140, FUTEX_WAIT, 2, NULL...