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{{Distinguish|non-blocking I/O}}
In [[computer science]], an [[algorithm]] is called '''non-blocking''' if failure or [[Scheduling (computing)|suspension]] of any [[thread (computing)|thread]] cannot cause failure or suspension of another thread;<ref>{{cite book|last1=Göetz|first1=Brian|last2=Peierls|first2=Tim|last3=Bloch|first3=Joshua|last4=Bowbeer|first4=Joseph|last5=Holmes|first5=David|last6=Lea|first6=Doug|title=Java concurrency in practice|date=2006|publisher=Addison-Wesley|___location=Upper Saddle River, NJ|isbn=9780321349606|page=[https://archive.org/details/javaconcurrencyi00goet/page/41 41]|url-access=registration|url=https://archive.org/details/javaconcurrencyi00goet/page/41}}</ref> for some operations, these algorithms provide a useful alternative to traditional [[lock (computer science)|blocking implementations]]. A non-blocking algorithm is '''lock-free''' if there is guaranteed system-wide [[Resource starvation|progress]], and '''wait-free''' if there is also guaranteed per-thread progress. "Non-blocking" was used as a synonym for "lock-free" in the literature until the introduction of obstruction-freedom in 2003.<ref name=obs-free>{{cite conference|last1=Herlihy|first1=M.|last2=Luchangco|first2=V.|last3=Moir|first3=M.|title=Obstruction-Free Synchronization: Double-Ended Queues as an Example|conference=23rd [[International Conference on Distributed Computing Systems]]|year=2003|pages=522|url=
The word "non-blocking" was traditionally used to describe [[telecommunications network]]s that could route a connection through a set of relays "without having to re-arrange existing calls"{{Quote without source|date=November 2024}} (see [[Clos network]]). Also, if the telephone exchange "is not defective, it can always make the connection"{{Quote without source|date=November 2024}} (see [[nonblocking minimal spanning switch]]).
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Other problems are less obvious. For example, certain interactions between locks can lead to error conditions such as [[Deadlock (computer science)|deadlock]], [[livelock]], and [[priority inversion]]. Using locks also involves a trade-off between coarse-grained locking, which can significantly reduce opportunities for [[parallel computing|parallelism]], and fine-grained locking, which requires more careful design, increases locking overhead and is more prone to bugs.
Unlike blocking algorithms, non-blocking algorithms do not suffer from these downsides, and in addition are safe for use in [[interrupt handler]]s: even though the [[Pre-emptive multitasking|preempted]] thread cannot be resumed, progress is still possible without it. In contrast, global data structures protected by mutual exclusion cannot safely be accessed in an interrupt handler, as the preempted thread may be the one holding the lock. While this can be rectified by masking interrupt requests during the critical section, this requires the code in the critical section to have bounded (and preferably short) running time, or excessive [[interrupt latency]] may be observed.<ref name="monit">{{cite journal | doi = 10.1145/358818.358824 | url = http://research.microsoft.com/lampson/23-ProcessesInMesa/Abstract.html | title = Experience with Processes and Monitors in Mesa | author = Butler W. Lampson | author-link = Butler W. Lampson |author2=David D. Redell |author2-link=David D. Redell | journal = Communications of the ACM | volume = 23 | issue = 2 | pages = 105–117 |date=February 1980| citeseerx = 10.1.1.142.5765 | s2cid = 1594544 }}</ref>
A lock-free data structure can be used to improve performance.
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Several libraries internally use lock-free techniques,<ref>
[
</ref><ref>
[https://www.liblfds.org/ liblfds] - A library of lock-free data structures, written in C
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[[C++11]] programmers can use <code>std::atomic</code> in <code><atomic></code>,
and [[C11 (C standard revision)|C11]] programmers can use <code><stdatomic.h></code>,
both of which supply types and functions that tell the [[compiler]] not to re-arrange such instructions, and to insert the appropriate memory barriers.<ref>
Bruce Dawson.
[https://randomascii.wordpress.com/2020/11/29/arm-and-lock-free-programming/ "ARM and Lock-Free Programming"].
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All wait-free algorithms are lock-free.
In particular, if one thread is suspended, then a lock-free algorithm guarantees that the remaining threads can still make progress. Hence, if two threads can contend for the same mutex lock or [[spinlock]], then the algorithm is ''not'' lock-free. (If we suspend one thread that holds the lock, then the second thread will block.)
An algorithm is lock-free if infinitely often operation by some processors will succeed in a finite number of steps. For instance, if {{var|N}} processors are trying to execute an operation, some of the {{var|N}} processes will succeed in finishing the operation in a finite number of steps and others might fail and retry on failure. The difference between wait-free and lock-free is that wait-free operation by each process is guaranteed to succeed in a finite number of steps, regardless of the other processors.
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* [[Priority inversion]]
* [[Resource starvation]]
* [[Non-lock concurrency control]]
* [[Optimistic concurrency control]]
== References ==
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