Invoking writeln() from a lot of threads running concurrently --> crash

[IM]

For purposes of debugging, I'm using writeln() to print stuff out 
from tasks running concurrently on many threads. At some point it 
crashes with the following stack trace:

Thread 4 received signal SIGUSR1, User defined signal 1.
[Switching to Thread 0x7ffff5ec2700 (LWP 19267)]
__lll_lock_wait_private () at 
../sysdeps/unix/sysv/linux/x86_64/lowlevellock.S:95
95	../sysdeps/unix/sysv/linux/x86_64/lowlevellock.S: No such file 
or directory.
(gdb) bt
#0  __lll_lock_wait_private () at 
../sysdeps/unix/sysv/linux/x86_64/lowlevellock.S:95
#1  0x00007ffff7bcb2d5 in __flockfile (stream=0x7ffff728b780 
<_IO_stdfile_1_lock>) at ../sysdeps/pthread/flockfile.c:28
#2  0x000000000045f5b0 in 
_D3std5stdio4File17LockingTextWriter6__ctorMFNcNeKSQBxQBwQBtZSQCiQChQCeQCc ()
#3  0x000000000045f670 in 
_D3std5stdio4File17lockingTextWriterMFNfZSQBoQBnQBk17LockingTextWriter ()
#4  0x000000000044a1e0 in 
_D3std5stdio4File__T5writeTAyaTyiTaZQqMFNfQpyiaZv (this=..., 
_param_2=10 '\n', _param_1=12, _param_0=...)
     at /usr/include/dmd/phobos/std/stdio.d:1399

Note that I didn't add any synchronizations around the writeln() 
calls, should I? I assume the implementation *should* synchronize 
access to std_out, no?

Bug in phobos?