Chapter 7 Deadlock

Outline Problem illustration Deadlock definition Resource types Resource allocation graph Deadlock conditions Deadlock avoidance Deadlock detection

Dining Philosophers Problem

Dining Philosophers Problem

First Solution

Second Solution

A Solution based on Monitor

A Solution based on Monitor

Deadlock Permanent blocking of a set of processes that either compete for system resources or communicate with each other Often involve conflicting needs for resources by two or more processes No efficient solution prevention, avoidance, detection

Two Resource Types Reusable resources Consumable resources

Reusable Resources Used by only one process at a time and not depleted by that use Processes obtain resources that they later release for reuse by other processes

Reusable Resources Processors, I/O channels, main and secondary memory, devices, and data structures such as files and databases Deadlock occurs if each process holds one resource and requests the other

Reusable Resources Space is available for allocation of 200Kbytes, and the following sequence of events occur Deadlock occurs if both processes progress to their second request P1 P2 . . . . . . Request 80 Kbytes; Request 70 Kbytes; . . . . . . Request 60 Kbytes; Request 80 Kbytes;

Consumable Resources Created (produced) and destroyed (consumed) Interrupts, signals, messages, and information in I/O buffers

Example of Deadlock Deadlock occurs if receive is blocking . . . . . . Receive(P2); Receive(P1); . . . . . . Send(P2, M1); Send(P1, M2);

Conditions for Deadlock Mutual exclusion Only one process may use a resource at a time Hold-and-wait A process may hold allocated resources while awaiting assignment of others

Conditions for Deadlock No preemption No resource can be forcibly removed from a process holding it Circular wait A closed chain of processes exists, such that each process holds at least one resource needed by the next process in the chain

Resource Allocation Graphs Directed graph that depicts a state of the system of resources and processes

Resource Allocation Graphs

Resource Allocation Graph for Figure (b)

Resource Allocation Graph for Figure (b)

Possibility of Deadlock Mutual Exclusion No preemption Hold and wait

Existence of Deadlock Mutual Exclusion No preemption Hold and wait Circular wait Self-read Chap 7.4, to know How to prevent deadlock? What’s the disadvantage of using deadlock prevention?

Deadlock Avoidance A decision is made dynamically on whether the current resource allocation request will, if granted, potentially lead to a deadlock Requires knowledge of future process requests

Two Approaches to Deadlock Avoidance Resource-allocation-graph algorithm (Chap 7.5.2) is only applicable to systems with single instance of each resource type Do not grant an incremental resource request to a process if this allocation might lead to deadlock (Banker’s Algorithm) Can be used to a banking system to allocate cash.

Resource Allocation Denial Referred to as the banker’s algorithm State of the system is the current allocation of resources to processes Safe state is where there is at least one sequence that does not result in deadlock Unsafe state is where any sequence will result in deadlock

Determination of a Safe State Is there a resource allocation sequence that does not result in deadlock

Determination of a Safe State

Determination of a Safe State

Determination of a Safe State

Safe to Grant P1’s Request? P1 requests one unit each of R1 and R3

Safe to Grant P1’s Request?

Determination of an Unsafe State

Deadlock Avoidance Logic

Deadlock Avoidance Logic

In-Class Exercise Consider a system consisting of four processes and a single resource. The current state of the claim and allocation matrices are Claim Matrix = [3, 2, 9, 7]T Allocation Matrix = [1, 1, 3, 2]T Resource Vector = [?] What is the minimum number of units of the resource needed to be available for this state to be safe?

C = [3, 2, 9, 7]T A = [1, 1, 3, 2]T RV = [?], AV[?-7] C = [3, 2, 9, 7]T A = [1, 1, 3, 2]T RV = [7] AV = [0] X Cannot make progress on any process!  need at least 1 extra instance of resource

C = [3, 2, 9, 7]T A = [1, 1, 3, 2]T RV = [8] AV = [1] Finish P2: C = [3, 0, 9, 7]T A = [1, 0, 3, 2]T AV = [2] Finish P1: C = [0, 0, 9, 7]T A = [0, 0, 3, 2]T AV = [3] X Cannot make progress on any process!  need at least 2 extra instances of resource

C = [0, 0, 9, 7]T A = [0, 0, 3, 2]T RV = [10] AV = [5] Finish P4: C = [0, 0, 9, 0]T A = [0, 0, 3, 0]T AV = [7] Finish P3: C = [0, 0, 0, 0]T A = [0, 0, 0, 0]T AV = [10]

Deadlock Avoidance Maximum resource requirement must be stated in advance Processes under consideration must be independent; their execution order must be unconstrained by any synchronization requirements There must be a fixed number of resources to allocate No process may exit while holding resources

Deadlock Detection Available vector

Deadlock Detection Algorithm Mark each process that has a row in the allocation matrix of all zeros Initialize a temporary vector W to equal to available vector

Deadlock Detection Algorithm Find an index i such that process i is currently unmarked and the ith row of request matrix is less than or equal to W. If no such row is found, terminate the algorithm and all unmarked processes are those involved in the deadlock. If such a row is found, mark process i and add the corresponding row of the allocation matrix to W.

Strategies Once Deadlock Detected Abort all deadlocked processes Back up each deadlocked process to some previously defined checkpoint, and restart all processes Original deadlock may occur

Strategies Once Deadlock Detected Successively abort deadlocked processes until deadlock no longer exists Successively preempt resources until deadlock no longer exists

Advantages and Disadvantages