ABA problem

Article summary

In multithreaded computing, the ABA problem occurs during synchronization, when a location is read twice, has the same value for both reads, and the read value being the same twice is used to conclude that nothing has happened in the interim; however, another thread can execute between the two reads and change the value, do other work, then change the value back, thus fooling the first thread into thinking nothing has changed even though the second thread did work that violates that assumption. The ABA problem occurs when multiple threads (or processes) accessing shared data interleave. Below is a sequence of events that illustrates the ABA problem: # Process P_1 reads value A from some shared memory location, # P_1 is preempted, allowing process P_2 to run, # P_2 writes value B to the shared memory location # P_2 writes value A to the shared memory location # P_2 is preempted, allowing process P_1 to run, # P_1 reads value A from the shared memory location, # P_1 determines 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. A common case of the ABA problem is encountered when implementing a lock-free data structure.…

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