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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

A CAS sees value A, another thread changes it to B and back to A, so CAS wrongly succeeds though the structure changed (e.g. a node was freed and reused). Fix with version tags or hazard pointers.

Where it is used

Lock-free stacks.

Common pitfall

Reusing memory addresses triggers it.

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.
What is the most common mistake with ABA Problem?
Reusing memory addresses triggers it.
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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