1. Chapter 3 Deadlocks TOPICS Resource Deadlocks The ostrich algorithm
Deadlock detection and recovery
Deadlock prevention
Deadlock avoidance
Reference:
Operating Systems Design and Implementation (Second Edition)
by Andrew S. Tanenbaum, Albert S. Woodhull

2. Resources(1) Examples of computer resources printers tape drives
Tables

3. Resources (2) Deadlocks occur when … Preemptable resources
processes are granted exclusive access to devices
we refer to these devices generally as resources
Preemptable resources
can be taken away from a process with no ill effects
Example: process swapping form main memory
Nonpreemptable resources
will cause the process to fail if taken away
Example: print request by more than one proceses

4. Resources (3) Sequence of events required to use a resource
request the resource
use the resource
release the resource
Must wait if request is denied
requesting process may be blocked
may fail with error code

5. Deadlocks Suppose a process holds resource A and requests resource B
at same time another process holds B and requests A
both are blocked and remain so

6. Deadlock Modeling Modeled with directed graphs
resource R assigned to process A
process B is requesting/waiting for resource S
process C and D are in deadlock over resources T and U

7. Four Conditions for Deadlock
Four conditions must hold for there to be a deadlock:
Mutual exclusion condition
each resource assigned to 1 process or is available
Hold and wait condition
process holding resources can request additional
No preemption condition
previously granted resources cannot forcibly taken away
Circular wait condition
must be a circular chain of 2 or more processes
each is waiting for resource held by next member of the chain

8. How deadlock occurs
A B C

9. How deadlock can be avoided
(o) (p) (q)

10. Strategy to Deal with Deadlock
Strategies for dealing with Deadlocks
Just ignore the problem altogether
Detection and recovery
Prevention
Negating one of the four necessary conditions of deadlock
Dynamic avoidance
Careful resource allocation

11. Strategy 1: The Ostrich Algorithm
Just ignore the problem
Reasonable if
deadlocks occur very rarely
cost of prevention is high
UNIX and Windows takes this approach
It is a trade off between
convenience
correctness

12. Strategy 2: Detection and Recovery
Method 1:
Every time a resource is requested or released, the resource graph is updated, and a check is made to see if any cycle exist.
If a cycle exists, one of the process is the cycle is killed. If this does not break the deadlock, another process is killed and so on until the cycle is broken
Method2
Periodically check to see if there are any processes that have been continuously blocked for more than say 1 hour. Such processes are then killed

13. Strategy 3: Deadlock Prevention a) Attacking the Mutual Exclusion Condition
Some devices (such as printer) can be spooled
only the printer daemon uses printer resource
thus deadlock for printer eliminated
Not all devices can be spooled

14. b) Attacking the Hold and Wait Condition
Require processes to request resources before starting
A process is allowed to run if all resources it needed is available. Otherwise it will just wait.
Problems
May not know required resources at start of run
Resource will not be used optimally
Variation:
process must give up all resources and
then request all immediately needed

15. c) Attacking the No Preemption Condition
This is not a viable option
Consider a process given the printer
halfway through its job
now forcibly take away printer
!!??

16. d) Attacking the Circular Wait Condition
Numerically ordered resources
Resource Graph
A process may request 1st a printer, then tape dirve. But it may not request 1st a plotter, then a scanner.
Resource graph can never have cycle.

17. Deadlock Prevention Summary

18. Strategy 4: Deadlock Avoidance
Carefully analyze each resource request to see if it can be safely granted.
Need an algorithm that can always avoid deadlock by making right choice all the time.
Banker’s algorithm (by Dijkstra)

19. Deadlock Avoidance Resource Trajectories
Two process resource trajectories

20. Safe and Unsafe States (1)
(a) (b) (c) (d) (e)
Demonstration that the state in (a) is safe

21. Safe and Unsafe States (2)
(a) (b) (c) (d)
safe
unsafe
safe
safe

22. The Banker's Algorithm for a Single Resource
(a) (b) (c)
safe
safe
unsafe
Three resource allocation states

23. Banker's Algorithm for Multiple Resources
Example of banker's algorithm with multiple resources