The presence of interrupts is basically equivalent to not being single threaded, though. There are true single-threaded environments, it's just limited to userspace processes that don't handle signals.
It is even worse: re-entrancy safe is a different property than thread-safe. For example you can't use mutexes to guarantee mutual exclusions with interrupts.
Some algorithms are both thread safe and reentrancy safe, but generally this is not the case.
