1. DPNM Lab. Dept. of CSE, POSTECH
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Chapter 4-2:
Deadlock
Mi-Jung Choi
DPNM Lab. Dept. of CSE, POSTECH

2. Contents Deadlock avoidance Deadlock prevention Other issues
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3. Deadlock Avoidance
Requires that the system has some additional a priori information available
Simplest and most useful model for DA requires that
each process declare the maximum number of resources of each type that it may need
The deadlock-avoidance algorithm dynamically examines the resource-allocation state to ensure that there can never be a circular-wait condition
Resource-allocation state is defined by the number of available and allocated resources, and the maximum demands of the processes
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4. Resource Allocation State: Safe, Unsafe , Deadlock
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5. Safe State When a process requests an available resource,
system must decide if immediate allocation leaves the system in a safe state
System is in safe state?
if there exists a safe sequence of all processes
Sequence <P1, P2, …, Pn> is safe if for each Pi, the resources that Pi can still request can be satisfied by currently available resources + resources held by all the Pj, with j<i
If Pi resource needs are not immediately available, then Pi can wait until all Pj have finished
When Pj is finished, Pi can obtain needed resources, execute, return allocated resources, and terminate.
When Pi terminates, Pi+1 can obtain its needed resources, and so on
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6. Basic Facts If a system is in safe state  no deadlocks
If a system is in unsafe state  possibility of deadlock
Avoidance  ensure that a system will never enter an unsafe state
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7. Deadlock Avoidance Resource Trajectories
Two process resource trajectories
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8. Safe and Unsafe States (1)
(a) (b) (c) (d) (e)
Demonstration that the state in (a) is safe
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9. Safe and Unsafe States (2)
(a) (b) (c) (d)
Demonstration that the sate in b is not safe
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10. Banker’s Algorithm Deadlock avoidance algorithm Assumption:
Resource-Allocation Graph is not applicable to a resource allocation system with multiple instance of each resource type
Supports multiple instances
Named because this algorithm could be used in a bank
Assumption:
Each process must make a priori claim for maximum use
When a process requests a resource it may have to wait
When a process gets all its resources it must return them in a finite amount of time
Two sub-algorithms
Safety Algorithm
Resource-Request Algorithm
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11. The Banker's Algorithm for a Single Resource
(a) (b) (c)
Three resource allocation states
safe
unsafe
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12. Banker's Algorithm for Multiple Resources
Example of banker's algorithm with multiple resources
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13. Deadlock Prevention (1)
By ensuring that at least one of the four conditions for deadlock cannot hold, we can prevent deadlock
Restrain the ways request can be made
Mutual Exclusion
Hold and Wait
No Preemption
Circular Wait
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14. Deadlock Prevention Attacking the Mutual Exclusion Condition
not required for sharable resources (read-only file)
must hold for nonsharable resources (printer)
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
Principle:
avoid assigning resource when not absolutely necessary
as few processes as possible actually claim the resource
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15. Attacking the Hold and Wait Condition
Hold and Wait – must guarantee that whenever a process requests a resource, it does not hold any other resources
require process to request and be allocated all its resources before it begins execution, or
allow process to request resources only when the process has none
Require processes to request resources before starting
a process never has to wait for what it needs
Problems
may not know required resources at start of run
also ties up resources other processes could be using
Low resource utilization; starvation possible
Variation
process must give up all resources
then request all immediately needed
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16. Attacking the No Preemption Condition
If a process that is holding some resources requests another resource that cannot be immediately allocated to it, then all resources currently being held are released
Preempted resources are added to the list of resources for which the process is waiting
Process will be restarted only when it can regain its old resources, as well as the new ones that it is requesting
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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17. Attacking the Circular Wait Condition
impose a total ordering of all resource types, and
require that each process requests resources in an increasing order of enumeration
(a) (b)
Normally ordered resources
A resource graph
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18. Summary to Deadlock Prevention (1)
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19. Summary to Deadlock Prevention (2)
Deadlock prevention algorithm
Prevent deadlocks by restraining how requests can be made
The restraints ensures that
at least one of the necessary conditions for deadlock cannot occur and, hence, that deadlocks cannot hold
Possible side effects deadlock prevention
Low device utilization
Reduced system throughput
Alternative method  Deadlock Avoidance
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20. Other Issues: Two-Phase Locking
Phase One
process tries to lock all records it needs, one at a time
if needed record found locked, start over
(no real work done in phase one)
If phase one succeeds, it starts second phase,
performing updates
releasing locks
Note similarity to requesting all resources at once
Algorithm works where programmer can arrange
program can be stopped, restarted
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21. Non-resource Deadlocks
Possible for two processes to deadlock
each is waiting for the other to do some task
Can happen with semaphores
each process required to do a down() on two semaphores (mutex and another)
if done in wrong order, deadlock results
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22. Starvation Algorithm to allocate a resource
may be to give to shortest job first
Works great for multiple short jobs in a system
May cause long job to be postponed indefinitely
even though not blocked
Solution:
First-come, first-serve policy
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23. Summary A deadlock requires that four conditions hold simultaneously
Mutual exclusion, hold and wait, no preemption, circular wait
Four principle methods for dealing with deadlocks
Ignore the deadlock problem
Deadlock detection-recovery scheme
Deadlock avoidance
Deadlock prevention
Deadlock avoidance requires a priori information such as the maximum number of each resource classes
Bankers algorithm
Deadlock prevention ensures that at least one of the necessary conditions never holds
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24. Review Deadlock avoidance Deadlock prevention Other issues
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