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

2. Contents Resource Introduction to deadlocks Deadlock modeling
The ostrich algorithm
Deadlock detection and recovery
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3. Resources (1) Examples of computer resources
printers
tape drives
tables
Processes need access to resources in reasonable order
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
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4. 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
Nonpreemptable resources
will cause the process to fail if taken away
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5. 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
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6. Introduction to Deadlocks
Formal definition:
A 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 or
A set of blocked processes each holding a resource and waiting to acquire a resource held by another process in the set
Usually the event is release of a currently held resource
None of the processes can …
run
release resources
be awakened
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7. Deadlock Example Example P0 P1 System has 2 tape drives
P1 and P2 each hold one tape drive and each needs another one
semaphores A and B, initialized to 1
P0 P1
wait (A); wait (B);
wait (B); wait (A);
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8. Four Conditions for 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
Deadlock can arise if four conditions hold simultaneously
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9. Resource-Allocation Graph (1)
Deadlocks can be described more precisely in terms of a directed graph called a system resource-allocation graph: A set of vertices V and a set of edges E
V is partitioned into two types:
P = {P1, P2, …, Pn}, the set consisting of all the processes in the system
R = {R1, R2, …, Rm}, the set consisting of all resource types in the system
E is partitioned into two types:
request edge – directed edge Pi  Rj
assignment edge – directed edge Rj  Pi
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10. Resource-Allocation Graph (2)
Process
Resource Type with 4 instances
Pi requests a instance of Rj
Pi is holding an instance of Rj Pi Rj Pi Rj
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11. Resource-Allocation Graph (3)
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
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12. Example of a Resource Allocation Graph
Deadlock or not?
Mutual Exclusion
R1, R2, R3, R4
Hold and Wait
P1, P2
No Preemption
YES
Circular Wait NO
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13. Resource Allocation Graph With a Deadlock
Deadlock or not?
Mutual Exclusion
R1, R2, R3, R4
Hold and Wait
P1, P2 , P3
No Preemption
YES
Circular Wait
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14. Resource Allocation Graph With A Cycle But No Deadlock
Deadlock or not?
Mutual Exclusion
R1, R2
Hold and Wait
P1, P3
No Preemption
YES
Circular Wait
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15. Deadlock Modeling (1)
A B C
How deadlock occurs
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16. How deadlock can be avoided
Deadlock Modeling (2)
(o) (p) (q)
How deadlock can be avoided
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17. Strategies for dealing with Deadlocks
just ignore the problem altogether
Ignore the problem and pretend that deadlocks never occur in the system;
used by most operating systems, including UNIX and Windows
It is then up to the application developer to write programs that handle deadlocks
detection and recovery
Allow the system to enter a deadlock state and then recover
deadlock detection, deadlock recovery
dynamic avoidance
careful resource allocation
prevention
negating one of the four necessary conditions
ensure that the system will never enter a deadlock state
Use a protocol to prevent or avoid deadlocks
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18. The Ostrich Algorithm Pretend there is no 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
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19. Deadlock Detection Allow system to enter deadlock state
Detection algorithm
Recovery scheme
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20. Detection with One Resource of Each Type (1)
Note the resource ownership and requests
A cycle can be found within the graph, denoting deadlock
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21. Detection with One Resource of Each Type (2)
Data structures needed by deadlock detection algorithm
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22. Detection with One Resource of Each Type (3)
An example for the deadlock detection algorithm
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23. Recovery from Deadlock (1)
When a detection algorithm determines that a deadlock exists, several alternatives are available:
To inform the operator that a deadlock has occurred and to let the operator deal with the deadlock manually
Recovery through preemption
To preempt some resources from one or more of the deadlocked processes
take a resource from some other process
depends on nature of the resource
Recovery through rollback
checkpoint a process periodically
use this saved state
restart the process if it is found deadlocked
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24. Recovery from Deadlock (2)
Three issues in resource preemption
Selecting a victim – minimize cost
Rollback – return to some safe state, restart process from that state
Starvation – same process may always be picked as victim
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25. Recovery from Deadlock (3)
Recovery through killing processes = process termination
to abort one or more processes to break the circular wait
crudest but simplest way to break a deadlock
kill one of the processes in the deadlock cycle
the other processes get its resources
choose process that can be rerun from the beginning
In which order should we choose to abort?
Priority of the process
How long process has computed, and how much longer to completion
Resources the process has used
Resources process needs to complete
How many processes will need to be terminated
Is process interactive or batch?
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26. Summary A deadlock state occurs
when two or more processes are waiting indefinitely for an event that can be caused only by one of the waiting processes
A deadlock requires that four conditions hold simultaneously
Mutual exclusion, hold and wait, no preemption, circular wait
Four principle methods for dealing with deadlocks
Deadlock prevention
Deadlock avoidance
Deadlock detection-recovery scheme
Ignore the deadlock problem
Deadlock prevention ensures that at least one of the necessary conditions never holds
When deadlock is detected, the system must recover either by terminating some of the deadlocked processes or by preempting resources from some of the deadlocked processes
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27. Review Resource Introduction to deadlocks Deadlock modeling
The ostrich algorithm
Deadlock detection and recovery
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