• Allow system to enter deadlock state
• Detection algorithm
• Recovery scheme

Recovery from Deadlock: Process Termination

• Abort all deadlocked processes.
• Abort one process at a time until the deadlock cycle is eliminated.
• In which order should we choose to abort?l Priority of the process.
• How long process has computed, and how much longer to completion.
• Resources the process has used.l Resources process needs to complete.l How many processes will need to be terminated.
• Is process interactive or batch?

Recovery from Deadlock: Resource Preemption

• Selecting a victim – minimize cost.
• Rollback – return to some safe state, restart process for that state.
• Starvation – same process may always be picked as victim, include number of rollback in cost factor.

• Mutual Exclusion – not required for sharable resources; must hold for nonsharable 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 resources 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, then all resources currently being held are released.
• Preempted resources are added to the list of resources for whichthe process is waiting.
• Process will be restarted only when it can regain its old resourcesas 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.

• Ensure that the system will never enter a deadlock state.
• Allow the system to enter a deadlock state and then recover.
• gnore the problem and pretend that deadlocks never occur in he system; used by most operating systems, including UNIX.

• Mutual exclusion: only one process at a time can use aresource.
• Hold and wait: a process holding at least one resource is waiting to acquire additional resources held by other processes.
• No preemption: a resource can be released only voluntarily by the process holding it, after that process has completed its task.
• Circular wait: there exists a set {P0, P1, …, P0} of waitingprocesses such that P0 is waiting for a resource that is held by P1, P1 is waiting for a resource that is held by P2, …, Pn–1 is waiting for a resource that is held by Pn, and P0 is waiting for a resource that is held by P0.Deadlock can arise if four conditions hold simultaneously.