Deadlocks

1
Deadlocks

• Chapter 3

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
• 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 Preventiona) 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.
• Bankers algorithm (by Dijkstra)

19
Deadlock AvoidanceResource 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)
unsafe
safe
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