Will atomic operations block other threads?

Question

I am trying to make \"atomic vs non atomic\" concept settled in my mind. My first problem is I could not find \"real-life analogy\" on that. Like customer/restaurant relationshi

Answer 1

The atomic operations you describe are instructions within the processor and the hardware will make sure that a read cannot happen on a memory location until the atomic write is complete. This guarantees that a thread either reads the value before write or the value after the write operation, but nothing in-between - there's no chance of reading half of the bytes of the value from before the write and the other half from after the write.

Code running against the processor is not even aware of this block but it's really no different from using a lock statement to make sure that a more complex operation (made up of many low-level instructions) is atomic.

A single atomic operation is always thread-safe - the hardware guarantees that the effect of the operation is atomic - it'll never get interrupted in the middle.

A set of atomic operations is not atomic in the vast majority of cases (I'm not an expert so I don't want to make a definitive statement but I can't think of a case where this would be different) - this is why locking is needed for complex operations: the entire operation may be made up of multiple atomic instructions but the whole of the operation may still be interrupted between any of those two instructions, creating the possibility of another thread seeing half-baked results. Locking ensures that code operating on shared data cannot access that data until the other operation completes (possibly over several thread switches).

Some examples are shown in this question / answer but you find many more by searching.