What version of Go are you using (go version)?

master

Does this issue reproduce with the latest release?

Yes.

What operating system and processor architecture are you using (go env)?

Any.

What did you do?

Inspect runtime map for concurrent access checks. I am not talking about sync.map.

What did you expect to see?

hashWriting flag (un)set with atomic operations.

What did you see instead?

Non atomic operations like in those lines:
mapaccess flag set,
mapaccess flag reset.

These instructions compile to the following assembly instructions, without any prior lock:

1. mov from memory into register;
2. <binary_operation> (e.g. xor, and, ...);
3. mov from register back into memory.

These instructions, as well as the checks above them, are not guaranteed to be atomic.
As a consequence, we may have (rare but some) cases were a concurrent access is not detected.