Strange switching [threadname] to secondary mode after exception #0 in kernel-space at 0x80272444

[François Legal via Xenomai]

Using RTNet with multiple interfaces, xenomai 3.1 with linux 4.4
Whenever a posix RT thread calls recvmsg (), I get this error.

I could track this down to line 422 in udp.c :     msg = rtnet_get_arg(fd, 
&_msg, u_msg, sizeof(_msg));

If I disassemble the kernel, the failing instruction (at 0x80272444) is a nop 
in arm_copy_from_user
Am I doing anything wrong ?

François