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Intermediate Topic 28 of 51 Concurrency & Parallelism

ABA Problem

Hazard when a value changes and changes back unnoticed.

How it works

The ABA problem occurs in lock-free algorithms using compare-and-swap: a thread reads a value A from a location, is delayed, meanwhile other threads change the value to B and then back to A, and when the first thread's CAS compares and sees A, it succeeds, wrongly assuming nothing has changed, though the underlying state may be different. In a lock-free stack, a popped node may be freed and reallocated at the same address with different contents, corrupting the structure.

Where it is used

Lock-free stacks, queues and memory allocators encounter the ABA problem, which also appears in database optimistic concurrency checks based on values.

Common pitfall

Solutions include version counters or tagged pointers combined with the pointer in a double-width CAS, hazard pointers and epoch-based reclamation that prevent reuse of memory while it may be referenced, load-linked and store-conditional instructions that detect any write, and garbage-collected languages where addresses are not reused while referenced. Remember that equal values are not equal histories.

Where it fits in the course

ABA Problem is topic 28 of 51 in Concurrency & Parallelism, rated Intermediate. It follows Wait-Free Algorithms. The next topic is Software Transactional Memory.

Practice and explore

How to study this topic

  1. Read the explanation above and restate the definition in your own words.
  2. Open the interactive lesson and answer its mini quiz.
  3. Read the common pitfall, then write a short example that avoids it.
  4. Check your understanding in the Concurrency & Parallelism quiz, and revisit the topic if you miss a question.

Frequently asked questions

What is ABA Problem?
Hazard when a value changes and changes back unnoticed. It belongs to Concurrency & Parallelism and is an intermediate-level topic.
Where do people use ABA Problem?
Lock-free stacks, queues and memory allocators encounter the ABA problem, which also appears in database optimistic concurrency checks based on values.
What is the most common mistake with ABA Problem?
Solutions include version counters or tagged pointers combined with the pointer in a double-width CAS, hazard pointers and epoch-based reclamation that prevent reuse of memory while it may be referenced, load-linked and store-conditional instructions that detect any write, and garbage-collected languages where addresses are not reused while referenced.
What should I learn before this topic?
Start with Wait-Free Algorithms, the topic just before it in Concurrency & Parallelism.
How can I practice it?
Answer the mini quiz in the interactive lesson, then take the Concurrency & Parallelism quiz.
Is ABA Problem hard to learn?
It is an Intermediate topic. Be comfortable with the Beginner topics in this area first, then read the common pitfall carefully, since it is where most mistakes happen.
How long does it take to learn ABA Problem?
Most learners can read the explanation and finish the interactive lesson in under an hour. Fluency comes from applying the idea, so plan a few short practice sessions with the quiz.
Which topics are related to ABA Problem?
Wait-Free Algorithms, Software Transactional Memory, Lock-Free Data Structures are closely connected. The full list is in the related topics section of this page.
What should I study after ABA Problem?
Continue with Software Transactional Memory, the next topic in Concurrency & Parallelism.

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