DEADLOCK

DEADLOCK PREVENTION

The case of never enter.

Restrain the ways request can be made (at least one of the necessary conditions should not be true).

� Mutual Exclusion – not required for sharable resources; must hold for nonsharable resources.In general not possible to prevent deadlock by this, some resources are intrinsically non-sharable

� Hold and Wait – must guarantee that whenever a process requests a resource, it does not hold any other resources.

* Require process to request and be allocated all its resources before it begins execution, or allow process to request resources only when the process has none.

* Low resource utilization; starvation possible.

� No Preemption –

* If a process that is holding some resources requests another resource that cannot be immediately allocated to it,then all resources currently being held are released.

* Preempted resources are added to the list of resources for which the process is waiting.

* Process will be restarted only when it can regain its old resources, as well as the new ones that it is requesting.

� Circular Wait – impose a total ordering of all resource types, and require that each process requests resources in an increasing order of enumeration.