Ah, yes, that makes sense now. Thanks for decoding this properly!
It does look like rt_sigreturn is being used:
--- SIGCHLD {si_signo=SIGCHLD, si_code=CLD_EXITED, si_pid=4909,
si_uid=33, si_status=0, si_utime=1, si_stime=2} ---
rt_sigreturn() = 0
Rather disconcerting this problem. I'll keep digging...
--david