1. Deadlock and Starvation
Deadlock: A set of Processes holding resources and blocked waiting for others held by a processes in the same wait set.
Starvation: Resource is always granted to other waiting processes
Example: 2 tape drives, P1 and P2 each hold one and need both
Example: P0: wait(A); wait(B); P1: wait(B); wait(A);
Resource: The narrow bridge
Deadlock Resolution: Cars back up (preemption and rollback)
Starvation is possible.

2. Conditions Necessary for Deadlock
Hold and wait: Processes partially allocate resources
Limited Resources: There is a limited number of resources of a particular type. Processes will block if requesting a limited that is unavailable.
No preemption: A resource can be released only voluntarily by the process holding it.
Circular wait: there exists a process set {P0, P1, …, P0} where P0 is waiting for a resource that is held by P1, P1 is waiting for a resource that is held by P2, …, Pn–1 is waiting for a resource that is held by P0.
P0 P1
wait (A); wait(B)
wait (B); wait(A)

3. Exercise Refer to the above figure
How are all four conditions present?
How can deadlock be prevented, avoided, and detected?

4. Directed Resource Allocation Graph
Process
Resource Type with 4 instances
Pi requests Rj instance
Pi holding Rj instance
Graph: A set of vertices & edges.
Two vertex categories:
Circles: represent Processes {P1, P2, …, Pn}
Rectangles: represent Resource types {R1,R2,…,Rm}
Two edge categories
Pi requests resource type Rj: PiRj
Resource Rj instance assigned to Pi: RjPi Pi Pi
Resource categories: CPU cycles, memory files, I/O devices
Access: request, utilize, then release

5. Graph Cycles No cycles  no deadlock. cycles 
Deadlock if there is only one instance per resource type
Deadlock possibility several instances per resource type and all processes are waiting for a resource
No Deadlock
Deadlock

6. Possible Deadlock class B implements Runnable;
{ private Lock one, two;
public(Lock one,Lock two)
{ this.one = one;
this.two = two; }
public void run()
{ try
{ two.lock();
doSomething();
one.lock();
doSomethingElse();
finally
{ two.unlock();
one.unlock();
} } }
class A implements Runnable;
{ private Lock one, two;
public(Lock one,Lock two)
{ this.one = one;
this.two = two; }
public void run()
{ try
{ one.lock();
doSomething();
two.lock();
doSomethingElse();
finally
{ two.unlock();
one.unlock();
} } }
Deadlock possible depending on timing, but sometimes will not occur

7. Handling Deadlocks
Prevention: Eliminate one of the four necessary conditions
Avoidance: Ensure that the system will never enter a deadlock state.
Recovery: Allow the system to enter a deadlock state and then recover.
Ignore: Most operating systems, including UNIX and WINDOWS, pretend that deadlocks never occur. It is up to application programs to handle.

8. Deadlock Prevention Hold and Wait Limited Resources No Preemption
Eliminate one of the necessary conditions
Hold and Wait
– sharable resources like read-only files are deadlock free
Limited Resources
Resources with unlimited instances are deadlock free.
No Preemption
Release all resources instead of blocking on a resource request. The preempted resources are added to the resource wait list.
Restart when all requested resources are available.
Circular Wait (Disadvantage: Low resource utilization)
Impose a total ordering of resource types. Processes must request resources in an increasing order of enumeration.
Processes cannot request resources when already holding others.

9. Deadlock Avoidance
The system must have advance knowledge of total needed resources
Processes will not hold a resource forever
Safe state: A sequence of requests exist that can be satisfied without deadlock
Safe state  no deadlocks
Unsafe state  possibility of deadlock
Avoidance disallows all unsafe states
Algorithm Pseudo code
System is initially safe
At each request for a resource
IF allocation causes an unsafe state THEN block the process
ELSE grant the allocation
At each resource release
WHILE a blocked request can be granted safely
Grant the allocation; unblock the process

10. Resource Allocation Graph for Deadlock Avoidance
Dashed line: Pi to Rj Pi may request Rj
Solid line: Pi to Rj Pi has requested Rj.
Solid line: Rj to Pi: Rj is assigned to Pi.
If resources with a single instance, implement a cycle detection algorithm
(Recall Union Find algorithm from Data Structures)

11. Dijkstra Banker’s Algorithm
Original algorithm’s purpose: Prevent runs on a bank
Notation
request[p, r] = k means process p wants k instances of resource type r
available[r] = k means there are k resources of type r available
need[p,r] = k means process p potentially needs k resources of type r
allocated[p,r] = k means process p owns k resources of type r
Pseudo code upon processor p request for resource r
Throw Exception IF request[p,r[ > need[p,r]
IF request[p,r] > available[r], block process p
Tentatively allocate requested resources to process p: available[r] -= request[p,r]; allocated[p,r] += request[p,r]; need[p] -= request[p,r]
IF system state is safe THEN allocation is okay ELSE restore original state and block process p

12. Safe State Algorithm Safety Algorithm
Let work and finish be arrays of length R and P, respectively.
Copy available to work
FOR p = 1 to P, finish [p] = false
WHILE a p exists such that for all r, finish [p]=false AND need[p, r]  work[r] THEN for all r, work[r] += allocated[p, r] finish[i]=true
IF finish[p]==true for all p, then safe system state (allocate resources) ELSE block process
Data structures for this algorithm: P processes, R resources
Available: available[r]=k k instances of resource r are available
Max: max[p, r]=k Process p may request up to k of resource r
Allocated: allocated[p, r]=k Process p owns k instances of resource r
Need: need[p, r] = k Process p may need k more of resource r
Note: need [p,r] = max[p, r] – allocation [p,r]

13. Deadlock Avoidance Example
5 processes P0 through P4 and 3 resources (A,B,C)
Resource A has 10 instances Resource B has 5 instances
Resource C has 7 instances.
The system state: Safe
< P1, P3, P4, P2, P0> satisfies the safety criteria.
P1 now requests (1,0,2).
Request  Need
Request  Available
Execute safety algorithm:
<P1, P3, P4, P0, P2> satisfies the safety criteria
Questions:
Is P4 request for (3,3,0) safe?
Is P2 request for (0,2,0) safe?
Need = Max – Allocated
A B C A B C A B C
P = –
P = –
P = –
P = –
P = –
Available = 3 3 2

14. Deadlock Detection Allow Deadlock, detect its occurrence, recover
Design Issues
When, and how often, to invoke the detection algorithm?
Is it possible to detect the cause?
How often is a deadlock is likely to occur?
How many processes are part of the deadlock?
Which group of processes should we abort?
All involved in the deadlock?
One at a time till the deadlock cycle is eliminated?
Can we roll back to a safe state and try again?
What is the same process is always the victim
How many rollbacks should we allow?
How to determine the cost of recovery?
Process priority and type (interactive or batch)
Process run time, and the estimated time to completion.
Resources the process has used and those still needed

15. Deadlock Detection Single resource instances
Single Instance
Maintain wait graph
Nodes are processes.
Pi  Pj if Pi is waiting for Pj
Periodically invoke an algorithm to detect a cycle in the graph
The text claims cycle detection requires O(|V|2). Is that true?
Resource allocation wait-for
graph graph

16. Deadlock Detection Multiple Resource Instance Algorithm
Data structures for this algorithm: P processes, R resources
Available: available[r]=k k instances of resource r are available
Allocated: allocated[p, r]=k Process p owns k instances of resource r
Request: request[p, r] = k Process p requests k instances of resource r
Let work and finish be arrays of length R and P, respectively
Copy available to work
FOR all p and r // no circular wait if process doesn’t hold any resources IF allocated[p,r]  0, then finish[p] = false ELSE finish[p] = true
// No circular wait for a process that can be fully satisfied
WHILE p exists where for all r, finish[p] == false and request[p,r]  work[r]
SET work[p] += allocated[p,r] AND finish[p] = true
IF finish[p] == false, for some p, 1  p  P
THEN the system and process p is in deadlocked

17. Deadlock Detection Example
Description of the system state
Five processes: P0 through P4
Three resource types A has 7 instances B has 2 instances C has 6 instances Allocated Request Available
A B C A B C A B C P P P P P
Sequence <P0, P2, P3, P1, P4> causes finish[p] = true for all p
If P2 requests a type C resource, the request array becomes:
A B C P P P P P
We can reclaim P0 resources, but there are not sufficient resources to fulfill the other requests
Deadlock involves P1, P2, P3, and P4

18. Conclusion
Most current operating systems ignore deadlocks. It is left to the application developer to handle
There is not one approach that is sufficient
The eventual solution is to combine the three basic approaches of prevention, avoidance, and detection
Different resource classes would employ a selected solution