Deadlock conditions

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(New page: == Mutual exclusion == The resources involved must be unshareable; otherwise, the processes would not be prevented from using the resource when necessary. == Hold and wait or partial al...)
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== Mutual exclusion ==
== Mutual exclusion ==
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The resources involved must be unshareable; otherwise, the processes would not be prevented from using the resource when necessary.
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The resources involved must be exclusive. Once a thread gets on hold of a resource, nobody else can use it. Otherwise the concurrent threads would not be prevented from using the resource.
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== Hold and wait ==
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== Hold and wait or partial allocation ==
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A thread can hold and wait indefinitely. If it could not, and for example would time out, the deadlock can be resolved by letting the timed out thread to give up and release all resources.
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The processes must hold the resources they have already been allocated while waiting for other (requested) resources. If the process had to release its resources when a new resource or resources were requested, deadlock could not occur because the process would not prevent others from using resources that it controlled.
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== Cannot Take Away a Resource ==
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== No pre-emption ==
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Nobody can take away a resource from a thread. If there was a supervisor, that could take the resource away, it could effectively solve all deadlocks by detecting them (which is easy) and then choosing a thread to loose its resource while letting others proceed.
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The processes must not have resources taken away while that resource is being used. Otherwise, deadlock could not occur since the operating system could simply take enough resources from running processes to enable any process to finish.
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== Incremental Requests ==
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== Resource waiting or circular wait ==
 
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A circular chain of processes, with each process holding resources which are currently being requested by the next process in the chain, cannot exist. If it does, the cycle theorem (which states that "a cycle in the resource graph is necessary for deadlock to occur") indicated that deadlock could occur.
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There must be a way to request another resource while already holding some. If a thread can only request resource(s) while holding none (or there is some order of resources and those can be requested only in such order), a [[deadlock]] cannot be created.
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[http://www.google.com/search?q=four+necessary+condition+for+a+deadlock+to+appear&start=0&start=0&ie=utf-8&oe=utf-8 four necessary and also sufficient conditions]
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Revision as of 08:45, 1 August 2018

Contents

Mutual exclusion

The resources involved must be exclusive. Once a thread gets on hold of a resource, nobody else can use it. Otherwise the concurrent threads would not be prevented from using the resource.

Hold and wait

A thread can hold and wait indefinitely. If it could not, and for example would time out, the deadlock can be resolved by letting the timed out thread to give up and release all resources.

Cannot Take Away a Resource

Nobody can take away a resource from a thread. If there was a supervisor, that could take the resource away, it could effectively solve all deadlocks by detecting them (which is easy) and then choosing a thread to loose its resource while letting others proceed.

Incremental Requests

There must be a way to request another resource while already holding some. If a thread can only request resource(s) while holding none (or there is some order of resources and those can be requested only in such order), a deadlock cannot be created.

Retrieved from "http://wiki.apidesign.org/wiki/Deadlock_conditions"
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