A deadlock is a situation where in two transactions wait for each other to give up their respective locks.
Mutual Exclusion – not required for sharable resources; must hold for non-sharable resources.
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 sources before it begins execution, or allow process to request resources only when the process has none. Low resource utilization; starvation possible. Restrain the ways request can be made.
No Preemption – If a process that is holding some resources requests another resource that cannot be immediately allocated to it, and 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
Simplest and most useful model requires that each process declare the maximum number of resources of each type that it may need.
The deadlock-avoidance algorithm dynamically examines the resource-allocation state to ensure that there can never be a circular-wait condition.
Resource-allocation state is defined by the number of available and allocated resources, and the maximum demands of the processes.