DeadlocksResourcesResources (1)Resources (2)Introduction to DeadlocksFour Conditions for DeadlockDeadlock Modeling (2)Deadlock Modeling (3)Deadlock Modeling (4)Deadlock Modeling (5)The Ostrich AlgorithmDetection with One Resource of Each Type (1)Detection with One Resource of Each Type (2)Detection with One Resource of Each Type (3)Recovery from Deadlock (1)Recovery from Deadlock (2)Deadlock AvoidanceResource TrajectoriesSafe and Unsafe States (1)Safe and Unsafe States (2)The Banker's Algorithm for a Single ResourceBanker's Algorithm for Multiple ResourcesDeadlock PreventionAttacking the Mutual Exclusion ConditionAttacking the Hold and Wait ConditionAttacking the No Preemption ConditionAttacking the Circular Wait Condition (1)Attacking the Circular Wait Condition (1)Other IssuesTwo-Phase LockingNonresource DeadlocksStarvation1Chapter 3Deadlocks3.1. Resource3.2. Introduction to deadlocks 3.3. The ostrich algorithm 3.4. Deadlock detection and recovery 3.5. Deadlock avoidance 3.6. Deadlock prevention 3.7. Other issues2Resources• Examples of computer resources– printers– tape drives– tables• Processes need access to resources in reasonable order• Suppose a process holds resource A and requests resource B– at same time another process holds B and requests A– both are blocked and remain so3Resources (1)• Deadlocks occur when …– processes are granted exclusive access to devices– we refer to these devices generally as resources• Preemptable resources– can be taken away from a process with no ill effects• Nonpreemptable resources– will cause the process to fail if taken away4Resources (2)• Sequence of events required to use a resource1. request the resource2. use the resource3. release the resource• Must wait if request is denied– requesting process may be blocked– may fail with error code5Introduction to Deadlocks• Formal definition :A set of processes is deadlocked if each process in the set is waiting for an event that only another process in the set can cause• Usually the event is release of a currently held resource• None of the processes can …– run– release resources– be awakened6Four Conditions for Deadlock1. Mutual exclusion condition• each resource assigned to 1 process or is available2. Hold and wait condition• process holding resources can request additional3. No preemption condition• previously granted resources cannot forcibly taken away4. Circular wait condition• must be a circular chain of 2 or more processes• each is waiting for resource held by next member of the chain7Deadlock Modeling (2)• Modeled with directed graphs– resource R assigned to process A– process B is requesting/waiting for resource S– process C and D are in deadlock over resources T and U8Deadlock Modeling (3)Strategies for dealing with Deadlocks1. just ignore the problem altogether2. detection and recovery3. dynamic avoidance • careful resource allocation4. prevention• negating one of the four necessary conditions9A B CDeadlock Modeling (4)How deadlock occurs10Deadlock Modeling (5)(o) (p) (q)How deadlock can be avoided11The Ostrich Algorithm• Pretend there is no problem• Reasonable if – deadlocks occur very rarely – cost of prevention is high• UNIX and Windows takes this approach• It is a trade off between – convenience– correctness12Detection with One Resource of Each Type (1)• Note the resource ownership and requests• A cycle can be found within the graph, denoting deadlock13Detection with One Resource of Each Type (2)Data structures needed by deadlock detection algorithm14Detection with One Resource of Each Type (3)An example for the deadlock detection algorithm15Recovery from Deadlock (1)• Recovery through preemption– take a resource from some other process– depends on nature of the resource• Recovery through rollback– checkpoint a process periodically– use this saved state – restart the process if it is found deadlocked16Recovery from Deadlock (2)• Recovery through killing processes– crudest but simplest way to break a deadlock– kill one of the processes in the deadlock cycle– the other processes get its resources – choose process that can be rerun from the beginning17Deadlock AvoidanceResource TrajectoriesTwo process resource trajectories18Safe and Unsafe States (1)(a) (b) (c) (d) (e)Demonstration that the state in (a) is safe19Safe and Unsafe States (2)(a) (b) (c) (d)Demonstration that the sate in b is not safe20The Banker's Algorithm for a Single Resource(a) (b) (c)• Three resource allocation states– safe– safe– unsafe21Banker's Algorithm for Multiple ResourcesExample of banker's algorithm with multiple resources22Deadlock PreventionAttacking the Mutual Exclusion Condition• Some devices (such as printer) can be spooled– only the printer daemon uses printer resource– thus deadlock for printer eliminated• Not all devices can be spooled• Principle:– avoid assigning resource when not absolutely necessary– as few processes as possible actually claim the resource23Attacking the Hold and Wait Condition• Require processes to request resources before starting– a process never has to wait for what it needs• Problems– may not know required resources at start of run– also ties up resources other processes could be using• Variation: – process must give up all resources– then request all immediately needed24Attacking the No Preemption Condition• This is not a viable option• Consider a process given the printer– halfway through its job– now forcibly take away printer– !!??25Attacking the Circular Wait Condition (1)(a) (b)• Normally ordered resources• A resource graph26Attacking the Circular Wait Condition (1)Summary of approaches to deadlock prevention27Other IssuesTwo-Phase Locking• Phase One– process tries to lock all records it needs, one at a time– if needed record found locked, start over– (no real work done in phase one)• If phase one succeeds, it starts second phase, – performing updates– releasing locks • Note similarity to requesting all resources at once• Algorithm works where programmer can arrange– program can be stopped, restarted28Nonresource Deadlocks• Possible for two