Discuss 4 conditions for deadlocks and 4 deadlock prevention techniques. Solutions like Ostrich Algorithm, and detection techniques with 1 resource type. Deadlock recovery and avoidance.

1. Deadlocks
CS4532 Concurrent Programming
Dilum Bandara
Dilum.Bandara@uom.lk
Slides extended from “Modern Operating Systems” by A. S. Tanenbaum

2. Resources
 Examples of computer resources
 Printers, tape drives, tables
 Processes need access to resources in
reasonable order
 Suppose a process holds resource T & requests
resource U, at same time another process holds
U & requests T
 Both are blocked & remain so
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3. Resources (Cont.)
 Deadlocks occur when …
 Processes are granted exclusive access to
devices/resources
 Sequence of events required to use a resource
 Request, use, & release
 Must wait, if request is denied
 Requesting process may be blocked or may fail
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4. 2 Types of Resources
 Preemptable resources
 Can be taken away from a process with no ill effects
 Nonpreemptable resources
 Will cause the process to fail, if taken away
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5. Deadlocks
 Formal definition
 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
 Usually the event is release of a currently held
resource
 None of the processes can …
 Run
 Release resources
 Be awakened
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6. 4 Conditions for Deadlock
1. Mutual exclusion condition
 Each resource assigned to 1 process or is available
2. Hold & wait condition
 Process holding resources can request additional
3. No preemption condition
 Previously granted resources cannot be forcibly taken
away
4. Circular wait condition
 Must be a circular chain of 2 or more processes
 Each is waiting for a resource held by next member of
the chain
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7. Deadlock Modeling
 Modeled with directed graphs
 Resource R assigned to process A
 Process B is requesting/waiting for resource S
 Process C & D are in deadlock over resources T & U
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8. Strategies for Dealing with Deadlocks
 Just ignore the problem altogether
 Detection & recovery
 Dynamic avoidance
 Careful resource allocation
 Prevention
 Negating one of the 4 necessary conditions
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9. 1. Ostrich Algorithm
 Pretend there is no problem
 Reasonable if
 Deadlocks occur very rarely
 Cost of prevention is high
 UNIX & Windows takes this approach
 It is a tradeoff between
 Convenience
 Correctness
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10. 2. Detection with One Resource of Each Type
 Note resource ownership & requests
 A cycle can be found within the graph, denoting
deadlock
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11. Detection with One Resource of Each
Type (Cont.)
 Data structures needed by deadlock detection
algorithm
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12. Example – Detection with One
Resource of Each Type
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13. Recovery From Deadlock
 Recovery through preemption
 Take a resource from some other process
 Depends on nature of resource
 Recovery through rollback
 Checkpoint a process periodically
 Use this saved state
 Restart process, if it is found deadlocked
 Recovery through killing processes
 Kill one of the processes in deadlock cycle
 Other processes get its resources
 Choose process that can be rerun from the beginning
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14. Example – How Deadlock Occurs?
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15. (o) (p) (q)
3. How Deadlock Can be Avoided?
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16. Deadlock Avoidance – Resource
Trajectories
 2 process resource trajectories
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17. Safe & Unsafe States – Banker’s
Algorithm
Demonstration that the state in (a) is safe
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(a) (b) (c) (d) (e)
(a) (b) (c) (d)
Demonstration that the sate in (b) is not safe

18. Banker’s Algorithm for Multiple
Resources
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19. 4. Deadlock Prevention
 Attacking Mutual Exclusion Condition
 Attacking Hold & Wait Condition
 Attacking No Preemption Condition
 Attacking Circular Wait Condition
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20. Attacking Mutual Exclusion Condition
 Some devices (such as printer) can be spooled
 Only printer daemon uses printer resource
 Thus, deadlock for printer eliminated
 Not all devices can be spooled
 Principle
 Avoid assigning resources when not absolutely
necessary
 As few processes as possible actually claim the
resource
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21. Attacking Hold & Wait Condition
 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
 Ties up resources other processes could be using
 Variation
 Process must give up all resources
 Then request all immediately needed
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22. Attacking 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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23. Attacking Circular Wait Condition
 Normally ordered resources
 Resource graph
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(a) (b)

24. Summary of Deadlock Prevention
Approaches
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25. Dealing with Deadlocks – Summary
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Dealing with Deadlocks
Just Ignore
Detection &
Recovery
Preemption
Rollback
Kill
Avoidance
Banker’s
Algorithm
Prevention
Spooling
Request All
Together
Take Away
Order
Numerically