Deadlock

Read S&G, chapter 7
two or more processes are each waiting for a resource held by another process
if there are separate critical sections for two data structures, we have to be careful if we need both
example of deadlock in a resource allocation graph

Four Conditions for Deadlock
All four conditions must be true for a deadlock to occur.

mutual exclusion
- processes have exclusive control of resources
hold and wait
- processes have some resources and wait for more
  - example: Process 2 had mutex semaphores and waited for full condition
no preemption
- resource cannot be removed from a process
circular wait
- a circular chain of processes exists in which each process holds one or more resources requested by the next process in the chain
- example: first must acquire empty or full before mutex

Three Ways of Dealing with Deadlock

deadlock prevention
- one--shot algorithm where you must acquire all resources or none!
- hierarchical algorithm: acquire resources in a set order; acquire low priority resources before high priority ones
  ranked resources
  first acquire R1
  then acquire Rn
deadlock avoidance
- Dijkstra's Banker's Algorithm: assumes foreknowledge of all processes
- useless in practice
deadlock detection and recovery
- used in UNIX (most common in practice)
- deadlock occurs and then some processes are terminated and the mess is cleaned up

Sample Question:

(a) Draw the resource-allocation graph when the sets P, R, and E are:

P = {P1, P2, P3}

R = {R1, R2, R3, R4}

E = {P1 -->R1, P2 -->R1, P2 -->R2, P3 -->R4, R2 -->P3, R3 -->P1, and R4 -->P2}

Assume there exists one instance of each resource type. As well, assume resources R1, R2, R3, and R4 are nonpreemptive and cannot be shared.

(b) Could a deadlock exist in the system described in part (a)? Explain why or why not making reference to the necessary conditions for a deadlock.

Sample Answer:

(a) - not shown

(b) Yes, deadlock could exist. The four necessary conditions are all true.

mutual exclusion is true because the resource cannot be shared
hold and wait is true because process P2 holds resource R4 but also requests (waits for) resource R2
no preemption is stated in the question
circular wait exists: P2 wants R2 but R2 is held by P3, P3 wants R4 but R4 is held by P2