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deadlock, deadlock in operating system
In concurrent computing, a deadlock is a state in which each member of a group of actions, is waiting for some other member to release a lock1 Deadlock is a common problem in multiprocessing systems, parallel computing, and distributed systems, where software and hardware locks are used to handle shared resources and implement process synchronization2

In an operating system, a deadlock occurs when a process or thread enters a waiting state because a requested system resource is held by another waiting process, which in turn is waiting for another resource held by another waiting process If a process is unable to change its state indefinitely because the resources requested by it are being used by another waiting process, then the system is said to be in a deadlock3

In a communications system, deadlocks occur mainly due to lost or corrupt signals rather than resource contention4

Two processes competing for two resources in opposite order
A A single process goes through
B The later process has to wait
C A deadlock occurs when the first process locks the first resource at the same time as the second process locks the second resource
D The deadlock can be resolved by cancelling and restarting the first process


  • 1 Necessary conditions
  • 2 Deadlock handling
    • 21 Ignoring deadlock
    • 22 Detection
    • 23 Prevention
  • 3 Livelock
  • 4 Distributed deadlock
  • 5 See also
  • 6 References
  • 7 Further reading
  • 8 External links

Necessary conditionsedit

A deadlock situation can arise if and only if all of the following conditions hold simultaneously in a system:5

  1. Mutual exclusion: The resources involved must be unshareable; otherwise, the processes would not be prevented from using the resource when necessary Only one process can use the resource at any given instant of time 6
  2. Hold and wait or resource holding: a process is currently holding at least one resource and requesting additional resources which are being held by other processes
  3. No preemption: a resource can be released only voluntarily by the process holding it
  4. Circular wait: a process must be waiting for a resource which is being held by another process, which in turn is waiting for the first process to release the resource In general, there is a set of waiting processes, P = , such that P1 is waiting for a resource held by P2, P2 is waiting for a resource held by P3 and so on until PN is waiting for a resource held by P137

These four conditions are known as the Coffman conditions from their first description in a 1971 article by Edward G Coffman, Jr7

Deadlock handlingedit

Most current operating systems cannot prevent deadlocks8 When a deadlock occurs, different operating systems respond to them in different non-standard manners Most approaches work by preventing one of the four Coffman conditions from occurring, especially the fourth one9 Major approaches are as follows

Ignoring deadlockedit

In this approach, it is assumed that a deadlock will never occur This is also an application of the Ostrich algorithm910 This approach was initially used by MINIX and UNIX7 This is used when the time intervals between occurrences of deadlocks are large and the data loss incurred each time is tolerable


Under the deadlock detection, deadlocks are allowed to occur Then the state of the system is examined to detect that a deadlock has occurred and subsequently it is corrected An algorithm is employed that tracks resource allocation and process states, it rolls back and restarts one or more of the processes in order to remove the detected deadlock Detecting a deadlock that has already occurred is easily possible since the resources that each process has locked and/or currently requested are known to the resource scheduler of the operating system10

After a deadlock is detected, it can be corrected by using one of the following methods:citation needed

  1. Process termination: one or more processes involved in the deadlock may be aborted One could choose to abort all competing processes involved in the deadlockcitation needed This ensures that deadlock is resolved with certainty and speedcitation needed But the expense is high as partial computations will be lostcitation needed Or, one could choose to abort one process at a time until the deadlock is resolvedcitation needed This approach has high overhead because after each abort an algorithm must determine whether the system is still in deadlockcitation needed Several factors must be considered while choosing a candidate for termination, such as priority and age of the processcitation needed
  2. Resource preemption: resources allocated to various processes may be successively preempted and allocated to other processes until the deadlock is brokencitation needed


Main article: Deadlock prevention algorithms A Two processes competing for one resource, following a first-come, first-served policy
B A deadlock occurs when both processes lock the resource simultaneously
C The deadlock can be resolved by breaking the symmetry of the locks
D The deadlock can be prevented by breaking the symmetry of the locking mechanism

Deadlock prevention works by preventing one of the four Coffman conditions from occurring

  • Removing the mutual exclusion condition means that no process will have exclusive access to a resource This proves impossible for resources that cannot be spooled But even with spooled resources, deadlock could still occur Algorithms that avoid mutual exclusion are called non-blocking synchronization algorithms
  • The hold and wait or resource holding conditions may be removed by requiring processes to request all the resources they will need before starting up or before embarking upon a particular set of operations This advance knowledge is frequently difficult to satisfy and, in any case, is an inefficient use of resources Another way is to require processes to request resources only when it has none Thus, first they must release all their currently held resources before requesting all the resources they will need from scratch This too is often impractical It is so because resources may be allocated and remain unused for long periods Also, a process requiring a popular resource may have to wait indefinitely, as such a resource may always be allocated to some process, resulting in resource starvation11 These algorithms, such as serializing tokens, are known as the all-or-none algorithms
  • The no preemption condition may also be difficult or impossible to avoid as a process has to be able to have a resource for a certain amount of time, or the processing outcome may be inconsistent or thrashing may occur However, inability to enforce preemption may interfere with a priority algorithm Preemption of a "locked out" resource generally implies a rollback, and is to be avoided, since it is very costly in overhead Algorithms that allow preemption include lock-free and wait-free algorithms and optimistic concurrency control If a process holding some resources and requests for some another resources that cannot be immediately allocated to it, the condition may be removed by releasing all the currently being held resources of that process
  • The final condition is the circular wait condition Approaches that avoid circular waits include disabling interrupts during critical sections and using a hierarchy to determine a partial ordering of resources If no obvious hierarchy exists, even the memory address of resources has been used to determine ordering and resources are requested in the increasing order of the enumeration3 Dijkstra's solution can also be used


A livelock is similar to a deadlock, except that the states of the processes involved in the livelock constantly change with regard to one another, none progressing This term was defined formally at some time during the 1970s—an early sighting in the published literature is in Babich's 1979 article on program correctness12 Livelock is a special case of resource starvation; the general definition only states that a specific process is not progressing13

Livelock is a risk with some algorithms that detect and recover from deadlock If more than one process takes action, the deadlock detection algorithm can be repeatedly triggered This can be avoided by ensuring that only one process chosen arbitrarily or by priority takes action14

Distributed deadlockedit

Distributed deadlocks can occur in distributed systems when distributed transactions or concurrency control is being used Distributed deadlocks can be detected either by constructing a global wait-for graph from local wait-for graphs at a deadlock detector or by a distributed algorithm like edge chasing

Phantom deadlocks are deadlocks that are falsely detected in a distributed system due to system internal delays but don't actually exist For example, if a process releases a resource R1 and issues a request for R2, and the first message is lost or delayed, a coordinator detector of deadlocks could falsely conclude a deadlock if the request for R2 while having R1 would cause a deadlock

See alsoedit


  1. ^ Coulouris, George 2012 Distributed Systems Concepts and Design Pearson p 716 ISBN 978-0-273-76059-7 
  2. ^ Padua, David 2011 Encyclopedia of Parallel Computing Springer p 524 ISBN 9780387097657 Retrieved 28 January 2012 
  3. ^ a b c Silberschatz, Abraham 2006 Operating System Principles 7th ed Wiley-India p 237 ISBN 9788126509621 Retrieved 29 January 2012 
  4. ^ Schneider, G Michael 2009 Invitation to Computer Science Cengage Learning p 271 ISBN 0324788592 Retrieved 28 January 2012 
  5. ^ Silberschatz, Abraham 2006 Operating System Principles 7 ed Wiley-India p 239 ISBN 9788126509621 Retrieved 29 January 2012 
  6. ^ http://nobcsucdavisedu/classes/ecs150-1999-02/dl-condhtml
  7. ^ a b c Shibu, K 2009 Intro To Embedded Systems 1st ed Tata McGraw-Hill Education p 446 ISBN 9780070145894 Retrieved 28 January 2012 
  8. ^ Silberschatz, Abraham 2006 Operating System Principles 7 ed Wiley-India p 237 ISBN 9788126509621 Retrieved 29 January 2012 
  9. ^ a b Stuart, Brian L 2008 Principles of operating systems 1st ed Cengage Learning p 446 Retrieved 28 January 2012 
  10. ^ a b Tanenbaum, Andrew S 1995 Distributed Operating Systems 1st ed Pearson Education p 117 Retrieved 28 January 2012 
  11. ^ Silberschatz, Abraham 2006 Operating System Principles 7 ed Wiley-India p 244 Retrieved 29 January 2012 
  12. ^ Babich, AF 1979 "Proving Total Correctness of Parallel Programs" IEEE Transactions on Software Engineering SE–5 6: 558–574 doi:101109/tse1979230192 
  13. ^ Anderson, James H; Yong-jik Kim 2001 "Shared-memory mutual exclusion: Major research trends since 1986" 
  14. ^ Zöbel, Dieter October 1983 "The Deadlock problem: a classifying bibliography" ACM SIGOPS Operating Systems Review 17 4: 6–15 doi:101145/850752850753 ISSN 0163-5980 

Further readingedit

  • Kaveh, Nima; Emmerich, Wolfgang "Deadlock Detection in Distributed Object Systems" PDF London: University College London 
  • Bensalem, Saddek; Fernandez, Jean-Claude; Havelund, Klaus; Mounier, Laurent 2006 "Confirmation of deadlock potentials detected by runtime analysis" Proceedings of the 2006 workshop on Parallel and distributed systems: Testing and debugging ACM: 41–50 doi:101145/11474031147412 ISBN 1595934146 
  • Coffman, Edward G, Jr; Elphick, Michael J; Shoshani, Arie 1971 "System Deadlocks" PDF ACM Computing Surveys 3 2: 67–78 doi:101145/356586356588 
  • Mogul, Jeffrey C; Ramakrishnan, K K 1997 "Eliminating receive livelock in an interrupt-driven kernel" ACM Transactions on Computer Systems 15 3: 217–252 doi:101145/263326263335 ISSN 0734-2071 
  • Havender, James W 1968 "Avoiding deadlock in multitasking systems" IBM Systems Journal 7 2: 74 doi:101147/sj720074 
  • Holliday, JoAnne L; El Abbadi, Amr "Distributed Deadlock Detection" Encyclopedia of Distributed Computing Kluwer Academic Publishers 
  • Knapp, Edgar 1987 "Deadlock detection in distributed databases" ACM Computing Surveys 19 4: 303–328 doi:101145/4507546163 ISSN 0360-0300 
  • Ling, Yibei; Chen, Shigang; Chiang, Jason 2006 "On Optimal Deadlock Detection Scheduling" IEEE Transactions on Computers 55 9: 1178–1187 doi:101109/tc2006151 

External linksedit

  • "Advanced Synchronization in Java Threads" by Scott Oaks and Henry Wong
  • Deadlock Detection Agents
  • DeadLock at the Portland Pattern Repository
  • Etymology of "Deadlock"

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