Here's the backtrace:

#0  sem_wait () at ../nptl/sysdeps/unix/sysv/linux/x86_64/sem_wait.S:85
#1  0x00007fcd0234a6e8 in PyThread_acquire_lock (lock=0x183f3f0, waitflag=128) at Python/thread_pthread.h:349
#2  0x00007fcd02311fa4 in PyEval_RestoreThread (tstate=0x17a9b00) at Python/ceval.c:356
#3  0x00007fcd02336a28 in PyGILState_Ensure () at Python/pystate.c:592
#4  0x00007fcd013fcec1 in _CallPythonObject (cif=0x183f3f0, resp=0x80, args=0x7fccff153f40, userdata=<value optimized out>)
    at /home/marcin/Desktop/python/python2.x-hg/Modules/_ctypes/callbacks.c:220
#5  closure_fcn (cif=0x183f3f0, resp=0x80, args=0x7fccff153f40, userdata=<value optimized out>)
    at /home/marcin/Desktop/python/python2.x-hg/Modules/_ctypes/callbacks.c:374
#6  0x00007fcd014044f0 in ffi_closure_unix64_inner (closure=<value optimized out>, rvalue=<value optimized out>, reg_args=0x246, argp=0x0)
    at /home/marcin/Desktop/python/python2.x-hg/Modules/_ctypes/libffi/src/x86/ffi64.c:566
#7  0x00007fcd01404b68 in ffi_closure_unix64 () at /home/marcin/Desktop/python/python2.x-hg/Modules/_ctypes/libffi/src/x86/unix64.S:230
#8  0x00007fccff572429 in ?? () from /lib/libfuse.so.2
#9  0x00007fccff5704fa in fuse_session_loop () from /lib/libfuse.so.2
#10 0x00007fccff1576e4 in ?? ()
#11 0x00007fccff154188 in ?? ()
#12 0x00007fccff3580e0 in ?? ()
#13 0x000000000192e2f0 in ?? ()
#14 0x0000000400000000 in ?? ()
#15 0x0000000000000001 in ?? ()
#16 0x00000000016ff5e0 in ?? ()
#17 0x0000000000000001 in ?? ()
#18 0x0000000000000000 in ?? ()

All calls to native POSIX functions seem to be wrapped with Py_BEGIN_ALLOW_THREADS/Py_END_ALLOW_THREADS except of opendir() and closedir(). When I fix it (as in attached patch), call_hello.py doesn't block anymore.