It's worse that that, actually. With hard mounts (the default), the only way to interrupt is with SIGKILL, which if my memory serves me correctly, is a process-wide signal. So you'd have to do all NFS I/O in a separate child process.

With soft mounts (does EFS support this?) you can exchange SIGKILL for religiously checking return values for all I/O syscalls, retrying partial writes and whatnot.