What are atomic operations for newbies?

Question

I am a newbie to operating systems and every answer I've found on Stackoverflow is so complicated that I am unable to understand. Can someone provide an explanation for what is an

atomic operation

For a newbie?

My understanding: My understanding is that atomic operation means it executes fully with no interruption? Ie, it is a blocking operation with no scope of interruption?

Imprimatur

Answer 1

Pretty much, yes. "Atom" comes from greek "atomos" = "uncuttable", and has been used in the sense "indivisible smallest unit" for a very long time (till physicists found that, in fact, there are smaller things than atoms). In concurrent programming, it means that there will be no context switch during it - nothing can affect the execution of atomic command.

An example: a web poll, open-ended questions, but we want to sum up how many people give the same answer. You have a database table where you insert answers and counts of that answer. The code is straightforward:

get the row for the given answer
if the row didn't exist:
  create the row with answer and count 1
else:
  increment count
  update the row with new count

Or is it? See what happens when multiple people do it at the same time:

user A answers "ham and eggs"       user B answers "ham and eggs"
get the row: count is 1             get the row: count is 1
okay, we're updating!               okay, we're updating!
count is now 2                      count is now 2
store 2 for "ham and eggs"          store 2 for "ham and eggs"

"Ham and eggs" only jumped by 1 even though 2 people voted for it! This is clearly not what we wanted. If only there was an atomic operation "increment if it exists or make a new record"... for brevity, let's call it "upsert" (for "update or insert")

user A answers "ham and eggs"       user B answers "ham and eggs"
upsert by incrementing count        upsert by incrementing count

Here, each upsert is atomic: the first one left count at 2, the second one left it at 3. Everything works.

Note that "atomic" is contextual: in this case, the upsert operation only needs to be atomic with respect to operations on the answers table in the database; the computer can be free to do other things as long as they don't affect (or are affected by) the result of what upsert is trying to do.

Coaming

Thanks for the answer, its very helpful! So in that sense, atomic operations must execute sequentially? Ie, if I have 2 atomic operations they cannot run concurrently and must run sequentially? – Imprimatur

Yup. (Also, it should be obvious that my example is on a macro level; but machine language level atomicity is the same concept.) – Coaming

Re, "it means that there will be no context switch during it". On a single-processor machine, that's a stronger guarantee than you need. On a multi-processor machine, it isn't strong enough. What "atomic" really means is, no other thread will be able to see the operation in a partially-completed state. – Recognizor

@besmirched: Thanks, that's a much better way of putting it than I managed to. – Coaming

@Ohm'sLawman Could you elaborate more? Do you mean, other threads in a different process can see the operation before it is done, even if the operation is atomic? – Grazing