So, in principle, even one thread may have the problem of memory consistency. Think about write A and read A. The read should read from the write Buffer, right?

@Nian -- if there is a read from address X that follows a write to X in program order. If the pending write to X is in the write buffer, the subsequent read will be serviced from the write-buffer. As a result, it will reflect the value of the write, even if all the steps of the write (for storing it in the cache or in memory) have not been completed.

As a result, from the perspective of a thread on the processor, even if there is a write buffer, all operations appear as if they are carried out in program order.

This is very important! You need to have two parties running threads on different for consistency issues to arise -- ignoring other forms of parallelism i.e. network cards)