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ion no other process thread is allowed to access modify the same object until the first process has finished writing upon the data object critical section and released the object for other processes to read and write upon the requirement of mutual exclusion was first identified and solved by edsger w dijkstra in his seminal 1965 paper solution of a problem in concurrent programming control 1 2 which is credited as the first topic in the study of concurrent algorithms 3 a simple example of why mutual exclusion is important in practice can be visualized using a singly linked list of four items where the second and third are to be removed the removal of a node that sits between two other nodes is performed by changing the next pointer of the previous node to point to the next node in other words if node i is being removed then the next pointer of node i 1 is changed to point to node i 1 thereby removing from the linked list any reference to node i when such a linked list is being shared between multiple threads of execution two threads of execution may attempt to remove two different nodes simultaneously one thread of execution changing the next pointer of node i 1 to point to node i 1 while another thread of execution changes the next pointer of node i to point to node i 2 although both removal operations complete successfully the desired state of the linked list is not achieved node i 1 remains in the list because the next pointer of node i 1 points to node i 1 this problem called a race condition can be avoided by using the requirement of mutual exclusion to ensure that simultaneous updates to the same part of the list cannot occur the term mutual exclusion is also used in reference to the simultaneous writing of a memory address by one thread while the aforementioned memory address is being manipulated or read by one or more other threads problem description edit the problem which mutual exclusion addresses is a problem of resource sharing how can a software system control multiple processes access to a shared resource when each process needs exclusive control of that resource while doing its work the mutual exclusion solution to this makes the shared resource available only while the process is in a specific code segment called the critical section it controls access to the shared resource by controlling each mutual execution of that part of its program where the resource would be used a successful solution to this problem must have at least these two properties it must implement mutual exclusion only one process can be in the critical section at a time it must be free of deadlocks if processes are trying to enter the critical section one of them must eventually be able to do so successfully provided no process stays in the critical section permanently deadlock freedom can be expanded to implement one or both of these properties lockout freedom guarantees that any process wishing to enter the critical section will be able to do so eventually this is distinct from deadlock avoidance which requires that some waiting process be able to get access to the critical section but does not require that every process gets a turn if two processes continually trade a resource between them a third process could be locked out and experience resource starvation even though the system is not in deadlock if a system is free of lockouts it ensures that every process can get a turn at some point in the future a k bounded waiting property gives a more precise commitment than lockout freedom lockout freedom ensures every process can access the critical section eventually it gives no guarantee about how long the wait will be in practice a process could be overtaken an arbitrary or unbounded number of times by other higher priority processes before it gets its turn under a k bounded waiting property each process has a finite maximum wait time this works by setting a limit to the number of times other processes can cut in line so that no process can enter the critical section more than k times while another is waiting 4 every process s program can be partitioned into four sections resulting in four states program execution cycles through these four states in order 5 the cycle of sections of a single process non critical section operation is outside the critical section the process is not using or requesting the shared resource trying the process attempts to enter the critical section critical section the process is allowed to access the shared resource in this section exit the process leaves the critical section and makes the shared resource available to other processes if a process wishes to enter the critical section it must first execute the trying section and wait until it acquires access to the critical section after the process has executed its critical section and is finished with the shared resources it needs to execute the exit section to release them for other processes use the process then returns to its non critical section enforcing mutual exclusion edit hardware solutions edit on uni processor systems the simplest solution to achieve mutual exclusion is to disable interrupts during a process s critical section this will prevent any interrupt service routines from running effectively preventing a process from being preempted although this solution is effective it leads to many problems if a critical section is long then the system clock will drift every time a critical section is executed because the timer interrupt is no longer serviced so tracking time is impossible during the critical section also if a process halts during its critical section control will never be returned to another process effectively halting the entire system a more elegant method for achieving mutual exclusion is the busy wait busy waiting is effective for both uniprocessor and multiprocessor systems the use of shared memory and an atomic test and set instruction provide the mutual exclusion a process can test and set on a location in shared memory and since the operation is atomic only one process can set the flag at a time any process that is unsuccessful in setting the flag can either go on to do other tasks and try again later release the processor to another process and try again later or continue to loop while checking the flag until it is successful in acquiring it preemption is still possible so this method allows the system to continue to function even if a process halts while holding the lock several other atomic operations can be used to provide mutual exclusion of data structures most notable of these is compare and swap cas cas can be used to achieve wait free mutual exclusion for any shared data structure by creating a linked list where each node represents the desired operation to be performed cas is then used to change the pointers in the linked list 6 during the insertion of a new node only one process can be successful in its cas all other processes attempting to add a node at the same time will have to try again each process can then keep a local copy of the data structure and upon traversing the linked list can perform each operation from the list on its local copy software solutions edit in addition to hardware supported solutions some software solutions exist that use busy waiting to achieve mutual exclusion examples include dekker s algorithm peterson s algorithm lamport s bakery algorithm 7 szymański s algorithm taubenfeld s black white bakery algorithm 2 maekawa s algorithm these algorithms do not work if out of order execution is used on the platform that executes them programmers have to specify strict ordering on the memory operations within a thread 8 it is often preferable to use synchronization facilities provided by an operating system s multithreading library which can take advantage of hardware support when available but fall back on software mechanisms when necessary for example when an operating system s lock facility is used and a thread attempts to acquire a lock that is already held the operating system may suspend the thread via a context switch and schedule another runnable thread or place the processor into a low power state if no other thread is available to run as a result most modern mutual exclusion techniques aim to reduce latency and busy waiting by relying on queuing and context switching however if the overhead of suspending and later restoring a thread is demonstrably greater than the time the thread would spend waiting for the lock to become available in a specific scenario then spinlocks can be an acceptable solution in that context 9 10 bound on the mutual exclusion problem edit one binary test set register is sufficient to provide the deadlock free solution to the mutual exclusion problem but a solution built with a test set register can possibly lead to the starvation of some processes which become caught in the trying section 4 in fact ω n displaystyle omega sqrt n distinct memory states are required to avoid lockout to avoid unbounded waiting n distinct memory states are required 11 recoverable mutual exclusion edit most algorithms for mutual exclusion are designed with the assumption that no failure occurs while a process is running inside the critical section however in reality such failures may be commonplace for example a sudden loss of power or faulty interconnect might cause a process in a critical section to experience an unrecoverable error or otherwise be unable to continue if such a failure occurs conventional non failure tolerant mutual exclusion algorithms may deadlock or otherwise fail key liveness properties to deal with this problem several solutions using crash recovery mechanisms have been proposed 12 types of mutual exclusion devices edit the solutions explained above can be used to build the synchronization primitives below locks mutexes readers writer locks recursive locks semaphores monitors message passing tuple space many forms of mutual exclusion have side effects for example classic semaphores permit deadlocks in which one process gets a semaphore another process gets a second semaphore and then both wait till the other semaphore to be released other common side effects include starvation in which a process never gets sufficient resources to run to completion priority inversion in which a higher priority thread waits for a lower priority thread and high latency in which response to interrupts is not prompt much research is aimed at eliminating the above effects often with the goal of guaranteeing non blocking progress no perfect scheme is known blocking system calls used to sleep an entire process until such calls became threadsafe there was no proper mechanism for sleeping a single thread within a process see polling 13 see also edit atomicity programming concurrency control dining philosophers problem exclusive or mutually exclusive events reentrant mutex semaphore spinlock load link store conditional references edit dijkstra e w 1965 solution of a problem in concurrent programming control communications of the acm 8 9 569 doi 10 1145 365559 365617 s2cid 19357737 1 2 taubenfeld the black white bakery algorithm in proc distributed computing 18th international conference disc 2004 vol 18 56 70 2004 podc influential paper award 2002 acm symposium on principles of distributed computing retrieved 24 august 2009 1 2 attiya hagit welch jennifer 25 march 2004 distributed computing fundamentals simulations and advanced topics john wiley sons inc isbn 978 0 471 45324 6 lamport leslie 26 june 2000 the mutual exclusion problem part ii statement and solutions pdf journal of the association for computing machinery 33 2 313 348 doi 10 1145 5383 5384 s2cid 12012739 harris timothy l 2001 a pragmatic implementation of non blocking linked lists pdf distributed computing lecture notes in computer science 2180 300 314 doi 10 1007 3 540 45414 4_21 isbn 978 3 540 42605 9 retrieved 1 december 2022 cite journal cs1 maint periodical has isbn link lamport leslie august 1974 a new solution of dijkstra s concurrent programming problem communications of the acm 17 8 453 455 doi 10 1145 361082 361093 s2cid 8736023 holzmann gerard j bosnacki dragan 1 october 2007 the design of a multicore extension of the spin model checker pdf ieee transactions on software engineering 33 10 659 674 doi 10 1109 tse 2007 70724 s2cid 9080331 archived pdf from the original on 9 october 2022 silberschatz abraham galvin peter b gagne greg 2018 operating system concepts 10th ed wiley pp 233 239 isbn 978 1119320913 herlihy maurice shavit nir 2012 the art of multiprocessor programming 2nd ed morgan kaufmann pp 11 15 isbn 978 0123973375 burns james e paul jackson nancy a lynch january 1982 data requirements for implementation of n process mutual exclusion using a single shared variable pdf journal of the association for computing machinery 33 2 313 348 golab wojciech ramaraju aditya july 2016 recoverable mutual exclusion proceedings of the 2016 acm symposium on principles of distributed computing pp 65 74 doi 10 1145 2933057 2933087 isbn 9781450339643 s2cid 8621532 blieberger johann burgstaller bernd 2018 safe non blocking synchronization in ada2x in casimiro antónio ferreira pedro m eds reliable software technologies ada europe 2018 vol 10873 cham springer international publishing pp 53 69 arxiv 1803 10067 doi 10 1007 978 3 319 92432 8_4 isbn 978 3 319 92431 1 retrieved 23 june 2026 further reading edit michel raynal algorithms for mutual exclusion mit press isbn 0 262 18119 3 sunil r das pradip k srimani distributed mutual exclusion algorithms ieee computer society isbn 0 8186 3380 8 thomas w christopher george k thiruvathukal high performance java platform computing prentice hall isbn 0 13 016164 0 gadi taubenfeld synchronization algorithms and concurrent programming pearson prentice hall isbn 0 13 197259 6 external links edit common threads posix threads explained the little things called mutexes by daniel robbins mutual exclusion petri net at the wayback machine archived 2016 06 02 mutual exclusion with locks an introduction mutual exclusion variants in openmp the black white bakery algorithm retrieved from https en wikipedia org w index php title mutual_exclusion oldid 1366280185 category concurrency control hidden categories articles with short description short description is different from wikidata use dmy dates from december 2019 cs1 maint periodical has isbn webarchive template wayback links this page was last edited on 27 july 2026 at 03 56 utc page was rendered with parsoid text is available under the creative commons attribution sharealike 4 0 license additional terms may apply by using this site you agree to the terms of use and privacy policy wikipedia is a registered trademark of the wikimedia foundation inc a non profit organization privacy policy about wikipedia disclaimers contact wikipedia legal safety contacts code of conduct developers statistics cookie statement 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