1. Concurrency: Deadlock and Starvation
Chapter 6

2. Deadlock
Permanent blocking of a set of processes that either compete for system resources or communicate with each other
Involve conflicting needs for resources by two or more processes
No efficient solution to detect or break a deadlock

3. Here it is!...

4. REUSABLE RESOURCES CONSUMABLE RESOURCES
Used by one process at a time and not depleted by that use
Processes obtain resources that they later release for reuse by other processes
Examples: CPU, memory, files, databases, and semaphores
CONSUMABLE RESOURCES
Created (produced) and destroyed (consumed) by a process
Ex: Interrupts, signals, messages, and information in I/O buffers
Deadlock may occur if a Receive message is blocking

5. Deadlock Example (reusable resource)
Space is available for allocation of 200K bytes, and the following sequence of events occur P2 ….
Request 70 KB
Request 85 KB …
Release 70 KB
Release 85 KB P1 ….
Request 80 KB
Request 60 KB …
Release 80 KB
Release 60 KB

6. Deadlock Example (reusable resource)
Space is available for allocation of 200K bytes, and the following sequence of events occur
Deadlock occurs if both processes progress to their second request P2 ….
Request 70 KB
Request 85 KB …
Release 70 KB
Release 85 KB P1 ….
Request 80 KB
Request 60 KB …
Release 80 KB
Release 60 KB

7. Deadlock Example (consumable resource)
Deadlock occurs if Receive is blocking P1 ….
Receive(P2);
Send(P2, Message1) P2 ….
Receive(P1);
Send(P1, Message2)

8. Addressing Deadlock Prevention Avoidance Detection & Recovery
Design the system so that deadlock is impossible
Avoidance
Construct a model of system states, then choose a strategy that, when resources are assigned to processes, will not allow the system to go to a deadlock state
Detection & Recovery
Check for deadlock (periodically or sporadically), then recover
Manual intervention
Have the operator reboot the machine if it seems too slow

9. Deadlock Prevention Deadlock implies Mutual exclusion Hold and wait
No preemption
Circular waiting
If we can ensure that at least one of these conditions is false at all times, then deadlock can be prevented
In other words,
No Mutual Exclusion  No Deadlock
No (Hold & Wait)  No Deadlock
Preemption allowed  No Deadlock
No (Circular Wait)  No Deadlock
Also
(Circular Wait)  Deadlock

10. DEADLOCK PREVENTION “Mutual exclusion” condition
only one process may use a resource at a time
To prevent deadlock
 do not require “mutual exclusion”
If this is a resource constraint, then mutual exclusion must hold at all times.
(i.e. you are not allowed to change it!)

11. DEADLOCK PREVENTION “Hold & wait” condition
To prevent deadlock  avoid hold and wait!
Need to be sure a process does not hold one resource while requesting another
Approach 1: Force a process to request all resources it needs at one time
Approach 2: If a process needs to acquire a new resource, it must first release all resources it holds, then reacquire all it needs

12. DEADLOCK PREVENTION “No preemption” condition
To prevent deadlock  allow preemption !
If a process holding certain resources is denied a further request, that process must release its original resources
( inefficient !)
If a process requests a resource that is held by another process, the OS may force the latter process to release its resources
( waste of CPU and other resources)
Can only be used in very special circumstances

13. DEADLOCK PREVENTION “Circular wait” condition
**here DEADLOCK PREVENTION “Circular Wait” condition
To prevent deadlock  don’t go into a circular wait situation!

14. Circular Wait (cont.)
For deadlock to occur, there must be a cycle in the graph of processes and resources
Choose a resource request strategy by which no cycle will be introduced
Create a total order on all resources.
Then a process can only ask for Rx
if Ri < Rx for Ri the process is currently holding

15. Circular Wait .. Prevented by defining a linear ordering of resources!
i.e R1 < R2 < …. < Rk-1 < Rk Pi .. Ry Rx
Pi+1 Rz Rk P R
P(process) holds R(resource) P R
P requests R

16. Addressing Deadlock Prevention Avoidance Detection & Recovery
Design the system so that deadlock is impossible
Avoidance
Construct a model of system states, then choose a strategy that, when resources are assigned to processes, will not allow the system to go to a deadlock state
Detection & Recovery
Check for deadlock (periodically or sporadically), then recover
Manual intervention
Have the operator reboot the machine if it seems too slow

17. Deadlock Avoidance
Define a model of system states, then choose a strategy that will guarantee that the system will not go to a deadlock state
Requires extra information, e.g. the maximum claim for each process
Resource manager tries to see the worst case that could happen. It does not grant an incremental resource request to a process if this allocation might lead to deadlock
REQUIREMENTS
Max. resource requirement must be stated in advance
There must be a fixed number of resources to allocate
Processes under consideration must be independent; no synchronization requirements
No process may exit while holding resources

18. Resource Allocation Criteria
Referred to as the Banker’s algorithm
State of the system is the current allocation of resources to processes
Safe state : there is at least one sequence of actions (scenario) that does not result in deadlock; i.e. all of the processes could run to completion.
Unsafe state : a state that is not safe
Do not grant a request if it leads to an unsafe state.

19. Is this state Safe ?

20. Is this state Safe ?

21. Is this state Safe ?
Yes, the given state is SAFE ! Because there is at least one scenario
in which all processes can finish: P2, P1, P3, and P4

22. When should we grant a request ?
For one moment, assume that we granted P1’s request for (R1 & R3)  P1
Q: If P1 requests 1 unit each of R1 and R3, should we grant it?
A: No! Because, there is a possibility that a deadlock may occur!

23. Deadlock Detection
1. P4 is not deadlocked because it’s Allocation vector is
2. Available vector = ( )
3. P3’s request vector  ( )
so mark P3 as “not deadlocked”, and release its resources
Now,
Available vector = ( )
4. Unfortunately, no other process has a row  Available vector
5. Therefore ====> P1 and P2 are deadlocked!

24. Strategies once Deadlock is Detected
Abort all deadlocked processes
Back up each deadlocked process to some previously defined checkpoint, and restart.
original deadlock may occur
Successively abort deadlocked processes until deadlock no longer exists
Successively preempt resources until deadlock no longer exists
Some Criteria for terminating a deadlocked process
Least amount of processor time consumed so far
Most estimated time remaining
Least total resources allocated so far
Lowest priority

25. Summary of Approaches: Deadlock Prevention, Avoidance, and Detection
There is no single effective strategy that can deal with all types of deadlock. Table 6.1 summarizes the key elements of the most important approaches that have been developed: prevention, avoidance, and detection. We examine each of these in turn, after first introducing resource allocation graphs and then discussing the conditions for deadlock.