Operating Systems

================ Start Lecture #12 ================

3.2.2: Deadlock Modeling

On the right is the Resource Allocation Graph, also called the Reusable Resource Graph.

Homework: 5.

Consider two concurrent processes P1 and P2 whose programs are.

P1: request R1       P2: request R2
    request R2           request R1
    release R2           release R1
    release R1           release R2

On the board draw the resource allocation graph for various possible executions of the processes, indicating when deadlock occurs and when deadlock is no longer avoidable.

There are four strategies used for dealing with deadlocks.

  1. Ignore the problem
  2. Detect deadlocks and recover from them
  3. Avoid deadlocks by carefully deciding when to allocate resources.
  4. Prevent deadlocks by violating one of the 4 necessary conditions.

3.3: Ignoring the problem--The Ostrich Algorithm

The ``put your head in the sand approach''.

3.4: Detecting Deadlocks and Recovering From Them

3.4.1: Detecting Deadlocks with Single Unit Resources

Consider the case in which there is only one instance of each resource.

To find a directed cycle in a directed graph is not hard. The algorithm is in the book. The idea is simple.

  1. For each node in the graph do a depth first traversal (hoping the graph is a DAG (directed acyclic graph), building a list as you go down the DAG.
  2. If you ever find the same node twice on your list, you have found a directed cycle and the graph is not a DAG and deadlock exists among the processes in your current list.
  3. If you never find the same node twice, the graph is a DAG and no deadlock occurs.
  4. The searches are finite since the list size is bounded by the number of nodes.

3.4.2: Detecting Deadlocks with Multiple Unit Resources

This is more difficult.

3.4.3: Recovery from deadlock


Perhaps you can temporarily preempt a resource from a process. Not likely.


Database (and other) systems take periodic checkpoints. If the system does take checkpoints, one can roll back to a checkpoint whenever a deadlock is detected. Somehow must guarantee forward progress.

Kill processes

Can always be done but might be painful. For example some processes have had effects that can't be simply undone. Print, launch a missile, etc.

Remark: We are doing 3.6 before 3.5 since 3.6 is easier.

3.6: Deadlock Prevention

Attack one of the coffman/havender conditions

3.6.1: Attacking Mutual Exclusion

Idea is to use spooling instead of mutual exclusion. Not possible for many kinds of resources

3.6.2: Attacking Hold and Wait

Require each processes to request all resources at the beginning of the run. This is often called One Shot.

3.6.3: Attacking No Preempt

Normally not possible.

3.6.4: Attacking Circular Wait

Establish a fixed ordering of the resources and require that they be requested in this order. So if a process holds resources #34 and #54, it can request only resources #55 and higher.

It is easy to see that a cycle is no longer possible.

Homework: 7.

3.5: Deadlock Avoidance

Let's see if we can tiptoe through the tulips and avoid deadlock states even though our system does permit all four of the necessary conditions for deadlock.

An optimistic resource manager is one that grants every request as soon as it can. To avoid deadlocks with all four conditions present, the manager must be smart not optimistic.

3.5.1 Resource Trajectories

We plot progress of each process along an axis. In the example we show, there are two processes, hence two axes, i.e., planar. This procedure assumes that we know the entire request and release pattern of the processes in advance so it is not a practical solution. I present it as it is some motivation for the practical solution that follows, the Banker's Algorithm.

Homework: 10, 11, 12.