gdb

Getting a live backtrace

 pstack pid

or use gdb to attach to the running process
and then using "where" to print the backtrace
gdb prog pid

gdb apache2 26219
...
(gdb) where
#0 0xb7799424 in __kernel_vsyscall ()
#1 0xb7689b7b in semop () from /lib/i686/cmov/libc.so.6
#2 0xb7731a4f in ?? () from /usr/lib/libapr-1.so.0
#3 0x01158003 in ?? ()
#4 0xb7746548 in ?? () from /usr/lib/libapr-1.so.0
#5 0x00000001 in ?? ()
...

https://wiki.ubuntu.com/Backtrace