First of all, you should know what atomic operation is. My understanding is that operations that conform to multi-thread atomic operations are called atomic operations.
The atom is the smallest unit in the world and inseparable. For example, a = 0；; (A is not of type long and double) This operation
Is inseparable, then we say this operation is atomic operation. Another example is: a++；+; +; This operation is actually a = a+1;
Is separable, so it is not an atomic operation.
For example, there is no problem when you perform the operation a = 1, but there is a big problem when you perform A = B.
If other threads change the value of b at this time, then a = b will have different results, because you are not sure about the value of b,
For example, if your first expectation is a = b, and you think that the value of B is 3, then A is also 3, but in the case of multithreading, the program makes B become.
4, then A becomes 4, then this is not an atomic operation.