Deadlocks

Question 1

What is a deadlock?

Answer 1

Processes unable to make progress due to both waiting on the same resource

Question 2

What are common causes of deadlocks?

Answer 2

Programming errors, wait and signal the wrong way round.

Question 3

Conditions for a deadlock

Answer 3

Mutual exclusion
Hold and wait
No resource Preemption
Circular wait

Question 4

Deadlock strategies

Answer 4

Prevention
Recovery
Ignore the problem

Question 5

How do we prevent deadlocks?

Answer 5

Avoid mutual exclusion

No hold-and-wait, limit to one resource per thread.

Allow resource preemption

Circular wait, impose lock ordering.

Question 6

What is mutual exclusion?

Answer 6

Only one process can use the resource at a given time.

Question 7

How do we avoid deadlocks?

Answer 7

Processes declare maximum resources needed

Ensure the system is in a safe state. This occurs when there is a safe sequence of processes such that deadlock cannot occur.

Question 8

How do we detect deadlocks?

Answer 8

Using a wait-for graph. If there are cycles, then deadlock is possible.

Question 9

How do we recover from a deadlock?

Answer 9

Process termination, abort all deadlocks processes one at a time.

Resource pre-emption, select a victim to rollback