Intel TSX hardware transactional memory what do non-transactional threads see?

Question

Suppose you have two threads, one creates a TSX transaction, and modifies some data structure. The other thread does no synchronization of any kind and reads the same data structure. Is the transaction atomic to it? I can't actually imagine that it can be true, since there is no way afaik to block or restart it if it tries reading a cache line modified by the transaction.

If the transaction is not atomic, then are the write ordering rules on x86 still respected? If it sees write #2, then it is guaranteed that it must be able to see the previous write #1. Does this still hold for writes that happen as part of a transaction?

I could not find answers to these questions anywhere, and I kind of doubt anyone on SO would know either, but at least when somebody finds out this is a Google friendly place to put an answer.

Answer 1

(My answer is based on Intel® 64 and IA-32 Architectures Optimization Reference Manual, Chapter 12)

The transaction is atomic to the read, in that the read will cause the transaction to abort, and thus appear that it never took place. In the transactional region, cache lines (tracked in the L1) read are considered the read-set and lines written to from the write-set. If another processor reads from the write-set (which is your example) or writes to either the read- or write-set, then there is a data conflict.

Data conflicts are detected through the cache coherence protocol. Data conflicts cause transactional aborts. In the initial implementation, the thread that detects the data conflict will transactionally abort.

Thus the thread attempting the transaction is tracking the line and will detect the conflict when the other thread makes its read request. It aborts and "the hardware will restart at the instruction address provided by the operation of the XBEGIN instruction". In this chapter, there are no distinctions as to what the second processor is doing. It does not matter whether it is attempting a transaction or performing a simple read.

To summarize, all threads (whether transactional or not) see either the full transaction or nothing. Only the thread in a TSX transaction can see the intermediate state of memory.