ABA problem

The ABA problem occurs when multiple threads (or processes) accessing shared memory interleave. Below is the sequence of events that will result in the ABA problem:

• Process $P_{1}$ reads value A from shared memory,
• $P_{1}$ is preempted, allowing process $P_{2}$,
• $P_{2}$ modifies the shared memory value A to value B and back to A before preemption,
• $P_{1}$ begins execution again, sees that the shared memory value has not changed and continues.

Although $P_{1}$ can continue executing, it is possible that the behavior will not be correct due to the "hidden" modification in shared memory.

References

• Damian Dechev, Peter Pirkelbauer, and Bjarne Stroustrup. Lock-free Dynamically Resizable Arrays