1. DEADLOCK

2. Resources 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

3. Resources (1) 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

4. Resources (2) 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

5. 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
Usually the event is release of a currently held resource
None of the processes can …
run
release resources
be awakened

6. Deadlock Modeling 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

7. THE DEADLOCK PHENOMENON
Resource allocation graphs. (a) Holding a resource. (b) Requesting a resource. (c) Deadlock.
CONDITIONS FOR DEADLOCK:
1. Mutual exclusion
No private copy of resource; each resource assigned to 1 process or is available
2. Hold and wait
May hold a resource and ask for more
3. Non preemption
Serial usage of every resource; previously granted resources cannot be taken away
4. Circular wait
Must be a circular chain of 2 or more processes; each is waiting for resource held by next member of the chain

8. Deadlock Modeling
A B C
How deadlock occurs

9. How deadlock can be avoided
Deadlock Modeling
(o) (p) (q)
How deadlock can be avoided

10. An example of how deadlock occurs and how it can be avoided.
(A,B) (B,C) (C,A) have only one resource in common. By allowing any pair to proceed and blocking the third, no deadlock is possible.

11. HOW TO DEAL WITH DEADLOCK
User/System administrator intervention
Detection and recovery by O.S.
Prevention
Break any of the four conditions for deadlock by administrative rules; must work for any set of user resource requirements
Avoidance
Exploit user resource usage information to formulate deadlock-free allocation policy

12. Summary of approaches to deadlock prevention.
Spooling not always possible because of spooler memory limitations and transaction serializability requirements.
Resource needs may not be known a priori.

13. Attacking the Mutual Exclusion Condition
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

14. Attacking the Hold and 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
also ties up resources other processes could be using
Variation:
process must give up all resources
then request all immediately needed

15. Attacking the No Preemption Condition
This is not a viable option
Consider a process given the printer
halfway through its job
now forcibly take away printer
!!??

16. Detection with One Resource of Each Type (1)
Note the resource ownership and requests
A cycle can be found within the graph, denoting deadlock

17. Detection with One Resource of Each Type (2)
Data structures needed by deadlock detection algorithm

18. Detection with One Resource of Each Type (3)
An example for the deadlock detection algorithm

19. Attacking the Circular Wait Condition
(a) (b)
Numerically ordered resources
A resource graph

20. Requests must be consistent with numerical order
(a) Numerically ordered resources. (b) A resource graph. i' j
May request only resources ranking lower than (have bigger numbers) those already acquired A B i j'
i'>i
j'>j
A loop of increasing resource indices is impossible!

21. Attacking the Circular Wait Condition (1)
Summary of approaches to deadlock prevention

22. Two process resource trajectories.
DEADLOCK AVOIDANCE
Two process resource trajectories.
Both using printer
Both using plotter
Unsafe region bound to end up in deadlock
S is safe because the trajectory can be extended to u without going through deadlock region

23. Safe and Unsafe States (1)
(a) (b) (c) (d) (e)
Demonstration that the state in (a) is safe

24. Safe and Unsafe States (2)
(a) (b) (c) (d)
Demonstration that the sate in b is not safe

25. The Banker's Algorithm for a Single Resource
(a) (b) (c)
Three resource allocation states
safe
unsafe

26. Banker's Algorithm for Multiple Resources
Example of banker's algorithm with multiple resources

27. Three resource allocation states: (a) Safe. (b) Safe. (c) Unsafe.
BANKER’S ALGORITHM
Andy
Pete 1 …
Available: 1
Three resource allocation states: (a) Safe. (b) Safe. (c) Unsafe.
A state is safe if there exists a sequence of states extending from it which ends in all processes completing.
(b) is safe because Marvin can proceed and then Barbara (or Suzanne).
(c) is unsafe because none can proceed if maximum is required for each.
Notice that look-ahead until everybody finishes is required to avoid unsafe states. Ex. suppose in state (b), a new process Pete joins the system, and then Barbara asks for 1 more unit. Pete can still proceed (one-step lookahead) if Barbara’s request is granted, but we know that granting Barbara’s request is unsafe.

28. The banker’s algorithm with multiple resources.
Existing Equipment
Possessed
E-P MP MN
The banker’s algorithm with multiple resources.
Find a row in MN that is no greater than the availability vector A.
Allow the corresponding process to complete and release its resources.
Update the tables and go to step 1.
If every process completes, there is no deadlock.

29. The banker’s algorithm with multiple resources.

30. The banker’s algorithm with multiple resources.

31. The banker’s algorithm with multiple resources.

32. Deadlock Handling in Distributed Systems
Detector Site
Reports R1 Status
Reports R2 Status R1 R2
Site 1
Site 1 A B
Process A
Lock R1
Lock R2
Release R1
Release R2
Process B
Lock and release requests are performed by message passing. Site performs lock and release operations on local resources.

33. Detection Techniques Path pushing Edge tracing
Global state determination
Diffusion computation

34. Centralized Path Pushing
Each site tracks lock and release operations of resources on the site
Each site reports the Resource Allocation Graph (RAG) fragment it tracks to central detector
Central detector detects cycles from the RAG fragments reported to it

35. Commanded Edge Tracing
On command of central controller, detector on each site propagates “probes” along the out-edges of the RAGs it tracks. Each probe carries originating site's Id.
Unblocked processes on the site discard the probes targeted at them. Blocked process waiting for a resource propagates probe backward to site that currently holds the resource
If probe returns to its originating site, detector has found a cycle
Central controller initiates edge tracing when deadlock is suspected.

36. Notice that A and B both follow prevention protocol
In distributed systems, centralized detection may have false alarms
Detector Site
Reports R1 Status
Reports R2 Status R1 R2
Site 1
Site 1 A B
Process A
Lock R1
Release R1
Lock R2
Release R2
Process B
Notice that A and B both follow prevention protocol

37. Site 1 Site 2 Detector False Alarm! Cure: Edge Tracing A A A1 R1 R1 B1
R1 released A A B3 R1 R2 R1 R2 A3 B B
False Alarm!
Cure: Edge Tracing

38. Efficiency Considerations
Deadlock detection incurs message and time overheads in distributed systems. Overhead may be high especially if application processes are mobile
Hierarchical allocation of resources may help if hierarchy is consistent with the spatial locality of processes to resources they use

39. Recovery from Deadlock (1)
Recovery through preemption
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

40. Recovery from Deadlock (2)
Recovery through killing processes
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

41. Two-Phase Locking in Database Systems Uses Rollback
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 only if system can allow processes to be stopped and rolled back

42. Nonresource 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

43. 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:
fair schedulers