Und hier die Auflösung des Rätsels warum "Service_task" manchmal in sigsuspend() hängt und dann wieder man in der mutex/futex - abundzu wird ein Signalhandler aufgerufen während der thread an der mutex/futex wartet, und der signal-Handler wartet mittels sigsuspend auf SIGRTMIN1-SIGRTMIN8.

# strace -p 1075                  
Process 1075 attached
rt_sigsuspend(~[RTMIN RT_1 RT_8]


# gdb strack trace:
0xb6cfc5f8 in do_sigsuspend (set=0xb51be72c) at ../sysdeps/unix/sysv/linux/sigsuspend.c:32
32      ../sysdeps/unix/sysv/linux/sigsuspend.c: No such file or directory.
(gdb) where
#0  0xb6cfc5f8 in do_sigsuspend (set=0xb51be72c) at ../sysdeps/unix/sysv/linux/sigsuspend.c:32
#1  __GI___sigsuspend (set=0xb51be72c) at ../sysdeps/unix/sysv/linux/sigsuspend.c:46
#2  0x00160a40 in SysTaskWaitForResume ()
#3  0x0016058c in task_signalhandler ()
#4  <signal handler called>
#5  0xb6d954f0 in syscall () from /lib/libc.so.6
#6  0xb5fa27d4 in QMutexPrivate::wait(int) () from /usr/lib/libQtCore.so.4
#7  0xb5f9eb9c in QMutex::lockInternal() () from /usr/lib/libQtCore.so.4
#8  0xb6ac5b48 in data_in(char*, int) () from /usr/lib/libvt.so
#9  0xb6a8dbec in codesys_send () from /usr/lib/libvt.so
#10 0xb6b567a0 in IsoVtLib_NumericValue () from /usr/lib/libIsobus.so
#11 0xb6b55574 in isocmd_numericvalue () from /usr/lib/libIsobus.so
#12 0xb52799a0 in ?? ()
#13 0xb52799a0 in ?? ()
Backtrace stopped: previous frame identical to this frame (corrupt stack?)
(gdb) quit
A debugging session is active.