Chapter 6 Deadlocks.What Is Deadlock? How To Avoid Deadlock and Other Deadlock Related Content.

MODERN OPERATING SYSTEMS
Third Edition
ANDREW S. TANENBAUM
Chapter 6
Deadlocks
Tanenbaum, Modern Operating Systems 3 e, (c) 2008 Prentice-Hall, Inc. All rights reserved. 0-13-6006639

Preemptable and Nonpreemptable
Resources
Sequence of events required to use a
resource:
1. Request the resource.
2. Use the resource.
3. Release the resource.

Figure 6-1. Using a semaphore to protect resources.
(a) One resource. (b) Two resources.
Resource Acquisition (1)

Figure 6-2. (a)
Deadlock-free
code.

Figure 6-2. (b) Code with a
potential deadlock.

Introduction To Deadlocks
Deadlock can be defined formally as follows:
A set of processes is deadlocked if each
process in the set is waiting for an event
that only another process in the set can
cause.

Conditions for Resource Deadlocks
1. Mutual exclusion condition
2. Hold and wait condition.
3. No preemption condition.
4. Circular wait condition.

Figure 6-3. Resource allocation graphs. (a) Holding a resource.
(b) Requesting a resource. (c) Deadlock.
Deadlock Modeling (1)

Figure 6-4. An example of how deadlock occurs
and how it can be avoided.

Strategies for dealing with deadlocks:
1. Just ignore the problem.
2. Detection and recovery. Let deadlocks
occur, detect them, take action.
3. Dynamic avoidance by careful resource
allocation.
4. Prevention, by structurally negating one
of the four required conditions.

Figure 6-5. (a) A resource graph. (b) A cycle extracted from (a).
Deadlock Detection with
One Resource of Each Type (1)

Algorithm for detecting deadlock:
1. For each node, N in the graph, perform the
following five steps with N as the starting node.
2. Initialize L to the empty list, designate all arcs
as unmarked.
3. Add current node to end of L, check to see if
node now appears in L two times. If it does,
graph contains a cycle (listed in L), algorithm
terminates.
…

4. From given node, see if any unmarked
outgoing arcs. If so, go to step 5; if not, go to
step 6.
5. Pick an unmarked outgoing arc at random and
mark it. Then follow it to the new current node
and go to step 3.
6. If this is initial node, graph does not contain any
cycles, algorithm terminates. Otherwise, dead
end. Remove it, go back to previous node,
make that one current node, go to step 3.

Figure 6-6. The four data structures needed
by the deadlock detection algorithm.
Deadlock Detection with Multiple
Resources of Each Type (1)

Deadlock detection algorithm:
1. Look for an unmarked process, Pi , for which
the i-th row of R is less than or equal to A.
2. If such a process is found, add the i-th row of C
to A, mark the process, and go back to step 1.
3. If no such process exists, the algorithm
terminates.

Figure 6-7. An example for the deadlock detection algorithm.

Recovery from Deadlock
• Recovery through preemption
• Recovery through rollback
• Recovery through killing processes

Figure 6-8. Two process resource trajectories.
Deadlock Avoidance

Figure 6-9. Demonstration that the state in (a) is safe.
Safe and Unsafe States (1)

Figure 6-10. Demonstration that the state in (b) is not safe.
Safe and Unsafe States (2)

Figure 6-11. Three resource allocation states:
(a) Safe. (b) Safe. (c) Unsafe.
The Banker’s Algorithm
for a Single Resource

Figure 6-12. The banker’s algorithm with multiple resources.
for Multiple Resources (1)

for Multiple Resources (2)
Algorithm for checking to see if a state is safe:
1. Look for row, R, whose unmet resource needs all
≤ A. If no such row exists, system will eventually
deadlock since no process can run to completion
2. Assume process of row chosen requests all resources
it needs and finishes. Mark process as terminated, add
all its resources to the A vector.
3. Repeat steps 1 and 2 until either all processes marked
terminated (initial state was safe) or no process left
whose resource needs can be met (there is a
deadlock).

Deadlock Prevention
• Attacking the mutual exclusion condition
• Attacking the hold and wait condition
• Attacking the no preemption condition
• Attacking the circular wait condition

Figure 6-13. (a) Numerically ordered resources.
(b) A resource graph.
Attacking the
Circular Wait Condition

Figure 6-14. Summary of approaches to deadlock prevention.
Approaches to Deadlock Prevention

Other Issues
• Two-phase locking
• Communication deadlocks
• Livelock
• Starvation

Figure 6-15. A resource deadlock in a network.
Communication Deadlocks

Figure 6-16. Busy waiting that can lead to livelock.
Livelock

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