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
1
Resources(1)
• Examples of computer resources
– printers
– tape drives
– Tables
2
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
3
Resources (3)
•
Sequence of events required to use a resource
1. request the resource
2. use the resource
3. release the resource
•
Must wait if request is denied
–
–
requesting process may be blocked
may fail with error code
4
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
5
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
6
Four Conditions for Deadlock
Four conditions must hold for there to be a deadlock:
1.
Mutual exclusion condition
•
each resource assigned to 1 process or is available
Hold and wait condition
2.
•
process holding resources can request additional
No preemption condition
3.
•
previously granted resources cannot forcibly taken away
Circular wait condition
4.
•
•
must be a circular chain of 2 or more processes
each is waiting for resource held by next member of the
chain
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How deadlock occurs
A
B
C
8
How deadlock can be avoided
(o)
(p)
(q)
9
Strategy to Deal with Deadlock
Strategies for dealing with Deadlocks
Just ignore the problem altogether
Detection and recovery
Prevention
1.
2.
3.
•
Negating one of the four necessary conditions of
deadlock
Dynamic avoidance
4.
•
Careful resource allocation
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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
11
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
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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
13
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
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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
– !!??
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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.
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Deadlock Prevention Summary
17
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)
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Deadlock Avoidance
Resource Trajectories
Two process resource trajectories
19
Safe and Unsafe States (1)
(a)
(b)
(c)
(d)
(e)
Demonstration that the state in (a) is safe
20
Safe and Unsafe States (2)
(a)
safe
(b)
unsafe
(c)
safe
(d)
safe
21
The Banker's Algorithm for a Single Resource
(a)
(b)
safe
safe
(c)
unsafe
Three resource allocation states
22
Banker's Algorithm for Multiple Resources
Example of banker's algorithm with multiple resources
23