Deadlock

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Both processes need resources to continue execution. P1 requires additional resource R1 and is in possession of resource R2, P2 requires additional resource R2 and is in possession of R1; neither process can continue.
Four processes (blue lines) compete for one resource (grey circle), following a right-before-left policy. A deadlock occurs when all processes lock the resource simultaneously (black lines). The deadlock can be resolved by breaking the symmetry.

In concurrent computing, deadlock is any situation in which no member of some group of entities can proceed because each waits for another member, including itself, to take action, such as sending a message or, more commonly, releasing a lock.[1] Deadlocks are a common problem in multiprocessing systems, parallel computing, and distributed systems, because in these contexts systems often use software or hardware locks to arbitrate shared resources and implement process synchronization.[2]

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.[3] If a process remains indefinitely unable to change its state because resources requested by it are being used by another process that itself is waiting, then the system is said to be in a deadlock.[4]

In a communications system, deadlocks occur mainly due to loss or corruption of signals rather than contention for resources.[5]

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

Individually necessary and jointly sufficient conditions for deadlock

A deadlock situation on a resource can arise only if all of the following conditions occur simultaneously in a system:[6]

  1. Mutual exclusion: At least one resource must be held in a non-shareable mode (we are assuming that one resource could have multiple instances); that is, only one process at a time can use the resource.[7] 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.[8]
  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: each 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 = {P1, P2, ..., PN}, 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 P1.[4][9]

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

Edward G. Coffman, Jr.[9]

While these conditions are sufficient to produce a deadlock on single-instance resource systems, they only indicate the possibility of deadlock on systems having multiple instances of resources.[10]

Deadlock handling

Most current operating systems cannot prevent deadlocks.[11] When a deadlock occurs, different operating systems respond to them in different non-standard manners. Most approaches work by preventing one of the four Common conditions from occurring, especially the fourth one.[12] Major approaches are as follows.

Ignoring deadlock

In this approach, it is assumed that a deadlock will never occur. This is also an application of the

UNIX.[9]
This is used when the time intervals between occurrences of deadlocks are large and the data loss incurred each time is tolerable.

Ignoring deadlocks can be safely done if deadlocks are formally proven to never occur. An example is the RTIC framework.[14]

Detection

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 system.[13]

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 deadlock. This ensures that deadlock is resolved with certainty and speed.[citation needed] But the expense is high as partial computations will be lost. Or, one could choose to abort one process at a time until the deadlock is resolved. This approach has a high overhead because after each abort an algorithm must determine whether the system is still in deadlock.[citation needed] Several factors must be considered while choosing a candidate for termination, such as priority and age of the process.[citation needed]
  2. Resource preemption: resources allocated to various processes may be successively preempted and allocated to other processes until the deadlock is broken.[15][failed verification]

Prevention

(A) Two processes competing for one resource, following a first-come, first-served policy. (B) 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.

Deadlock avoidance

Similar to deadlock prevention, deadlock avoidance approach ensures that deadlock will not occur in a system. The term "deadlock avoidance" appears to be very close to "deadlock prevention" in a linguistic context, but they are very much different in the context of deadlock handling. Deadlock avoidance does not impose any conditions as seen in prevention but, here each resource request is carefully analyzed to see whether it could be safely fulfilled without causing deadlock.

Deadlock avoidance requires that the operating system be given in advance additional information concerning which resources a process will request and use during its lifetime. Deadlock avoidance algorithm analyzes each and every request by examining that there is no possibility of deadlock occurrence in the future if the requested resource is allocated. The drawback of this approach is its requirement of information in advance about how resources are to be requested in the future. One of the most used deadlock avoidance algorithms is Banker's algorithm.[17]

Livelock

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.

The term was coined by

resource starvation; the general definition only states that a specific process is not progressing.[20]

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 action.[21]

Distributed deadlock

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 do not 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 also

References

  1. .
  2. from the original on 18 April 2021. Retrieved 16 October 2020.
  3. . A deadlock is a condition that may happen in a system composed of multiple processes that can access shared resources. A deadlock is said to occur when two or more processes are waiting for each other to release a resource. None of the processes can make any progress.
  4. ^ from the original on 25 January 2022. Retrieved 16 October 2020.
  5. from the original on 18 April 2021. Retrieved 16 October 2020.
  6. from the original on 18 April 2021. Retrieved 16 October 2020.
  7. .
  8. ^ "ECS 150 Spring 1999: Four Necessary and Sufficient Conditions for Deadlock". nob.cs.ucdavis.edu. Archived from the original on 29 April 2018. Retrieved 29 April 2018.
  9. ^ from the original on 18 April 2021. Retrieved 16 October 2020.
  10. ^ "Operating Systems: Deadlocks". www.cs.uic.edu. Archived from the original on 28 May 2020. Retrieved 25 April 2020. If a resource category contains more than one instance then the presence of a cycle in the resource-allocation graph indicates the possibility of a deadlock, but does not guarantee one. Consider, for example, Figures 7.3 and 7.4 below:
  11. from the original on 18 April 2021. Retrieved 16 October 2020.
  12. ^ from the original on 18 April 2021. Retrieved 16 October 2020.
  13. ^ from the original on 18 April 2021. Retrieved 16 October 2020.
  14. ^ "Preface - Real-Time Interrupt-driven Concurrency". Archived from the original on 18 September 2020. Retrieved 1 October 2020.
  15. ^ "IBM Knowledge Center". www.ibm.com. Archived from the original on 19 March 2017. Retrieved 29 April 2018.
  16. from the original on 18 April 2021. Retrieved 16 October 2020.
  17. ^ "Deadlock Avoidance Algorithms in Operating System (OS)". Electronics Mind. 26 January 2022.
  18. .
  19. .
  20. ^ Anderson, James H.; Yong-jik Kim (2001). "Shared-memory mutual exclusion: Major research trends since 1986". Archived from the original on 25 May 2006.
  21. S2CID 38901737
    .

Further reading

External links