atomic operation definition and multiprocessor

Question

I'm learning about synchronization and now I'm confused about the definition of atomic operation. Through searching, I could only find out that atomic operation is uninterruptible operation.

Then, won't the atomic operation only be valid for uni processor system since for multiprocessor system, many operation can be run simultaneously?

Answer 1

This link explains it pretty much perfectly (emphasis mine):

On multiprocessor systems, ensuring atomicity exists is a little harder. It is still possible to use a lock (e.g. a spinlock) the same as on single processor systems, but merely using a single instruction or disabling interrupts will not guarantee atomic access. You must also ensure that no other processor or core in the system attempts to access the data you are working with. The easiest way to achieve this is to ensure that the instructions you are using assert the 'LOCK' signal on the bus, which prevents any other processor in the system from accessing the memory at the same time. On x86 processors, some instructions automatically lock the bus (e.g. 'XCHG') while others require you to specify a 'LOCK' prefix to the instruction to achieve this (e.g. 'CMPXCHG', which you should write as 'LOCK CMPXCHG op1, op2').