Deadlock - Chapter 7

Threads can request resources such as IO or memory.

To get a resource:

• Request

• Use

• Release

Deadlock - when two or more threads both want resources that the other has a lock over

Livelock - when a thread keeps attempting a action that fails (i.e. two people trying to pass each other in a hallway)

What causes deadlock

Mutual exclusion - when only one (or limited) thread can hold a resource at a time

Hold and wait - when a thread is holding a resource and is waiting for another

No preemption - when a thread can only be released voluntarily

Circular wait - when each thread is waiting for a resource the next thread is holding in a circular fashion

Examples of deadlocks or not deadlocks with graphs

Not deadlock

Deadlock

Not deadlock (since p4 can be freed up)

How to stop deadlocks

1. Dont bother (common option)

2. Kill threads until the problem resolves

3. Dont let one of the 4 causing factors above happen

4. ... more in the book