Unsynchronized data races

memory.fill is currently specified as writing its bytes sequentially from lowest address to highest: https://webassembly.github.io/spec/core/exec/instructions.html#xref-syntax-instructions-syntax-instr-memory-mathsf-memory-fill. This behavior is unobservable to the thread executing the instruction, and I would presume most implementations don't write a byte at a time. But with multiple threads, I understand the goal is to allow races. What do we guarantee in that case?

1. Will a thread reading memory being concurrently filled via memory.fill see a linearly advancing fill?
2. Will a thread reading memory being concurrently filled via memory.fill see either the old data or the new data? Could it see something else?

I am particularly interested in the last question: is seeing something else allowed.

Start with the memory.fill execution semantics in the linked WebAssembly specification section. Investigate the guarantees for reads concurrent with memory.fill, including whether partial or non-sequential observations are allowed, then clarify the specification once the behavior is decided.