Exchange

The simplest atomic RMW operation is an exchange: the current value is read and replaced with a new one. To see where this might be useful, let’s tweak our example from chapter Atomicity: instead of displaying the total number of processed files, the UI might want to show how many were processed per second. We could implement this by having the UI thread read the counter then zero it each second. But we could get the following race condition if reading and zeroing are separate steps:

1. The UI thread reads the counter.
2. Before the UI thread has the chance to zero it, the worker thread increments it again.
3. The UI thread now zeroes the counter, and the previous increment is lost.

If the UI thread atomically exchanges the current value with zero, the race disappears.