Chapter 6: Deadlocks

A deadlock occurs when a every member of a set of processes is waiting for an event that can only be caused by a member of the set.

Often the event waited for is the release of a resource.

In the automotive world deadlocks are called gridlocks.

• The processes are the cars.
• The resources are the spaces occupied by the cars

Reward: One point extra credit on the final exam for anyone who brings a real (e.g., newspaper) picture of an automotive deadlock. You must bring the clipping to the final and it must be in good condition. Hand it in with your exam paper.

For a computer science example consider two processes A and B that each want to print a file currently on tape.

1. A has obtained ownership of the printer and will release it after printing one file.
2. B has obtained ownership of the tape drive and will release it after reading one file.
3. A tries to get ownership of the tape drive, but is told to wait for B to release it.
4. B tries to get ownership of the printer, but is told to wait for A to release the printer.

Bingo: deadlock!

6.1: Resources:

The resource is the object granted to a process.

• Resources come in two types
  1. Preemptable, meaning that the resource can be taken away from its current owner (and given back later). An example is memory.
  2. Non-preemptable, meaning that the resource cannot be taken away. An example is a printer.
• The interesting issues arise with non-preemptable resources so those are the ones we study.
• Life history of a resource is a sequence of
  1. Request
  2. Allocate
  3. Use
  4. Release
• Processes make requests, use the resourse, and release the resourse. The allocate decisions are made by the system and we will study policies used to make these decisions.

6.2: Deadlocks

To repeat: A deadlock occurs when a every member of a set of processes is waiting for an event that can only be caused by a member of the set.

Often the event waited for is the release of a resource.

6.3: Necessary conditions for deadlock

The following four conditions (Coffman; Havender) are necessary but not sufficient for deadlock. Repeat: They are not sufficient.

1. Mutual exclusion: A resource can be assigned to at most one process at a time (no sharing).
2. Hold and wait: A processing holding a resource is permitted to request another.
3. No preemption: A process must release its resources; they cannot be taken away.
4. Circular wait: There must be a chain of processes such that each member of the chain is waiting for a resource held by the next member of the chain.

6.2.2: Deadlock Modeling

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

• The processes are circles.
• The resources are squares.
• An arc (directed line) from a process P to a resource R signifies that process P has requested (but not yet been allocated) resource R.
• An arc from a resource R to a process P indicates that process P has been allocated resource R.

Homework: 1.

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.

6.3: Ignoring the problem--The Ostrich Algorithm

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

• If the likelihood of a deadlock is sufficiently small and the cost of avoiding a deadlock is sufficiently high it might be better to ignore the problem. For example if each PC deadlocks once per 100 years, the one reboot may be less painful that the restrictions needed to prevent it.
• Clearly not a good philosophy for nuclear missile launchers.
• For embedded systems (e.g., missile launchers) the programs run are fixed in advance so many of the questions Tanenbaum raises (such as many processes wanting to fork at the same time) don't occur.

6.4: Detecting Deadlocks and Recovering from them

6.4.1: Detecting Deadlocks with single unit resources

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

• So a request can be satisfied by only one specific resource.
• In this case the 4 necessary conditions for deadlock are also sufficient.
• Remember we are making an assumption (single unit resources) that is often invalid. For example, many systems have several printers and a request is given for ``a printer'' not a specific printer. Similarly, one can have many tape drives.
• So the problem comes down to finding a directed cycle in the resource allocation graph. Why?
  Answer: Because the other three conditions are either satisfied by the system we are studying or are not in which case deadlock is not a question. That is, conditions 1,2,3 are conditions on the system in general not on what is happening right now.

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.

6.4.2: Detecting Deadlocks with multiple unit resources

This is more difficult.

• The figure on the right shows a resource allocation graph with multiple unit resources.
• Each unit is represented by a dot in the box.
• Request edges are drawn to the box since they represent a request for any dot in the box.
• Allocation edges are drawn from the dot to represent that this unit of the resource has been assigned (but all units of a resource are equivalent and the choice of which one to assign is arbitrary).
• Note that there is a directed cycle in black, but there is no deadlock. Indeed the middle process might finish, erasing the magenta arc and satisfying the process on the right.
• The book gives an algorithm for detecting deadlocks in this more general setting. The idea is as follows.
  1. look for process that might be able to terminate (i.e. all its request arcs can be satisfied).
  2. If one is found pretend that it does terminate (erase all its arcs) and repeat step 1.
  3. If any processes remain, they are deadlocked.
• We will really do an algorithm later (the Banker's algorithm) that has some of this flavor.

6.4.3: Recovery from deadlock

Preemption

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

Rollback

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 6.6 before 6.5 since 6.6 is easier.

6.6: Deadlock Prevention

Attack one of the coffman/havender conditions

6.6.1: Attacking Mutual Exclusion

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

6.6.2: Attacking Hold and Wait

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

6.6.3: Attacking No Preempt

Normally not possible.

6.6.4: 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.

6.5: 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.

6.5.1 Resource Trajectories

• We have two processes H (horizontal) and V.
• The origin represents them both starting.
• Their combined state is a point on the graph.
• The parts where the printer and plotter are needed by each process are indicated.
• The dark green is where both processes have the plotter and hence execution cannot reach this point.
• Light green represents both having the printer; also impossible.
• Pink is both having both printer and plotter; impossible.
• Gold is possible (H has plotter, V has printer), but you can't get there.
• The upper right corner is the goal both processes finished.
• The red dot is ... (cymbals) deadlock. We don't want to go there.
• The cyan is safe. From anywhere in the cyan we have horizontal and vertical moves to the finish line without hitting any impossible area.
• The magenta interior is very interesting. It is
  • Possible: each processor has a different resource
  • Not deadlocked: each processor can move within the magenta
  • Deadly: deadlock is unavoidable. You will hit a magenta-green boundary and then will no choice but to turn and go to the red dot.
• The cyan-magenta border is the danger zone
• The dashed line represents a possible execution pattern.
• With a uniprocessor no diagonals are possible. We either move to the right meaning H is executing or move up indicating V.
• The trajectory shown represents.
  1. H excuting a little
  2. V excuting a little
  3. H executes; requests the printer; gets it; executes some more
  4. V executes; requests the plotter
• The crisis is at hand!
• If the resource manager gives V the plotter, the magenta has been entered and all is lost. ``Abandon all hope yee who enter here'' --Dante.
• The right thing to do is to deny the request, let H execute moving horizontally under the magenta and dark green. At the end of the dark green, no danger remains, both processes will complete successfully. Victory!