Atomic Operation

An atomic operation is one that cannot be interrupted. These operations are typically used for acquiring locks.

Consider the following code, intended to set a “lock” in shared memory so that only one thread or process performs a particular action at a time:

 if (lock == 0) {
   lock++;
   /* Perform exclusive action */
   lock--;
 } else {
   abort();
 }

If another process or thread writes to the lock variable between the lock comparison (lock == 0) and setting the lock (lock++), then the lock could fail to ensure exclusive action.

There are two ways to accomplish an atomic operation:

1. Use a single machine instruction.
2. Tell the operating system in advance not to interrupt the operation.

GCC provides intrinsics for some atomic operations.

(Note that in the above example, even the lock++ statement may not be atomic - it will typically compile into the three-operation sequence “load-increment-store”, and its possible that two processes or threads could each perform this operation in an interleaved fashion and increment lock by just one instead of two).