What is the difference between busy-wait and polling?
From the Wikipedia article on Polling
Polling, or polled operation, in computer science, refers to actively sampling the status of an external device by a client program as a synchronous activity. Polling is most often used in terms of input/output (I/O), and is also referred to as polled I/O or software driven I/O.
Polling is sometimes used synonymously with busy-wait polling (busy waiting). In this situation, when an I/O operation is required the computer does nothing other than check the status of the I/O device until it is ready, at which point the device is accessed. In other words the computer waits until the device is ready.
Polling also refers to the situation where a device is repeatedly checked for readiness, and if it is not the computer returns to a different task. Although not as wasteful of CPU cycles as busy-wait, this is generally not as efficient as the alternative to polling, interrupt driven I/O.
So, when a thread doesn't use the "condition variables", will it be called "polling" for the data change or "busy waiting"?
Answer
The difference between the two is what the application does between polls.
If a program polls a device say every second, and does something else in the mean time if no data is available (including possibly just sleeping, leaving the CPU available for others), it's polling.
If the program continuously polls the device (or resource or whatever) without doing anything in between checks, it's called a busy-wait.
This isn't directly related to synchronization. A program that blocks on a condition variable (that should signal when a device or resource is available) is neither polling nor busy-waiting. That's more like event-driven/interrupt-driven I/O.
(But for example a thread that loops around a try_lock is a form of polling, and possibly busy-waiting if the loop is tight.)