The kernel notifies about leaving syscall before starting to deliver
signals. If you don't believe me, pls look at arch/x86/kernel/entry_64.S:

int_ret_from_sys_call
        syscall_trace_leave
        do_notify_resume
Signed-off-by: Andrey Vagin <avagin@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>