What are the advantages of lock-free programming over spin lock？

Question

I am wondering which are the advantages of lock-free programming over spin lock？ I think that when we do lock free programming using CAS mechanism in a thread(called A), if other thread change the value in CAS, A thread still need to loop again. And I think it just like we use spin lock!

I am so confused about this. Although I know that CAS and spin-lock are suitable to use when the lock contention is not fierce, can someone explain in which scenarios lock free should be used and spin lock should be used?

Answer 1

Lock-freedom provides what is called progress guarantee . You are right that in your example thread A has do perform a retry (ie, loop again), but only if some other thread changed the value , which implies that that thread was able to make progress.

In contrast, a thread (let's call it X ) that holds a spin-lock prevents all other threads from making progress until the lock is released. So if thread X is preempted, execution of all threads waiting for the lock is effectively stalled until X can resume execution and finally release the lock. If X were to be stalled indefinitely, then all other threads would also be blocked indefinitely.

Such a situation is not possible with lock-free algorithms, since it is guaranteed that at any time at least one thread can make progress.

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Which should be used depends on the situation. Lock-free algorithms are inherently difficult to design, especially for more complex data structures like trees. And even if you have a lock-free algorithm, it is almost always slower than a serial one, so a serial version protected by a lock might perform better. Then again, if the data structure is heavily contended, a lock-free version will scale better than one protected by a lock. However, if your workload is mostly read-only, a read-write-lock will also provide good scalability. Unfortunately, there is no general rule here...

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If you want to learn more about lock-freedom (and more) I recommend the book The Art of Multiprocessor Programming .
If you prefer free alternatives I recommend Is Parallel Programming Hard, And, If So, What Can You Do About It? by Paul McKenney or Practicallock-freedom by Keir Fraser.