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1 A common project 2 error method Lock () var oldIntStat: int oldIntStat = SetInterruptsTo (DISABLED) if mutex == 0 mutex = 1 heldBy = currentThread oldIntStat = SetInterruptsTo (oldIntStat ) else sleepThreads.AddToEnd(currentThread) currentThread.Sleep() oldIntStat = SetInterruptsTo (oldIntStat ) endIf endMethod method Unlock () var oldIntStat: int t: ptr to Thread oldIntStat = SetInterruptsTo (DISABLED ) mutex = 0 if sleepThreads.IsEmpty() == false t = sleepThreads.Remove() t.status = READY readyList.AddToEnd(t) endIf oldIntStat = SetInterruptsTo (oldIntSt at) endMethod

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Page 1: 1 A common project 2 error method Lock () var oldIntStat: int oldIntStat = SetInterruptsTo (DISABLED) if mutex == 0 mutex = 1 heldBy = currentThread oldIntStat

1

A common project 2 error

method Lock () var

oldIntStat: int

oldIntStat = SetInterruptsTo (DISABLED)if mutex == 0 mutex = 1 heldBy = currentThread oldIntStat = SetInterruptsTo (oldIntStat)else sleepThreads.AddToEnd(currentThread) currentThread.Sleep() oldIntStat = SetInterruptsTo (oldIntStat)endIf

endMethod

 method Unlock ()var oldIntStat: int t: ptr to Thread

oldIntStat = SetInterruptsTo (DISABLED) mutex = 0 if sleepThreads.IsEmpty() == false t = sleepThreads.Remove() t.status = READY readyList.AddToEnd(t) endIf oldIntStat = SetInterruptsTo (oldIntStat)

endMethod

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2

Quiz

What is the difference between a monitor and a semaphore?

Why might you prefer one over the other? How do the wait/signal methods of a

condition variable differ from the wait/signal methods of a semaphore?

What is the difference between Hoare and Mesa semantics for condition variables?

What implications does this difference have for code surrounding a wait() call?

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3

Message Passing

Interprocess Communication via shared memory across machine boundaries

Message passing can be used for synchronization or general communication

Processes use send and receive primitives receive can block (like waiting on a Semaphore) send unblocks a process blocked on receive (just

as a signal unblocks a waiting process)

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4

Producer-consumer with message passing

The basic idea: The producer sends the data to consumer in a

message The system buffers messages

• The producer can out-run the consumer• The messages will be kept in order

But how does the producer avoid overflowing the buffer?

• After consuming the data, the consumer sends back an “empty” message

A fixed number of messages (N=100) The messages circulate back and forth.

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5

Producer-consumer with message passing

thread consumer var c, em: char while true Receive(producer, &c) -- Wait for a char Send(producer, &em) -- Send empty message back // Consume char... endWhileend

const N = 100 -- Size of message buffervar em: charfor i = 1 to N -- Get things started by Send (producer, &em) -- sending N empty messagesendFor

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6

Producer-consumer with message passing

thread producer var c, em: char while true // Produce char c... Receive(consumer, &em) -- Wait for an empty msg Send(consumer, &c) -- Send c to consumer endWhileend

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7

Design choices for message passing Option 1: Mailboxes

System maintains a buffer of sent, but not yet received, messages

Must specify the size of the mailbox ahead of time

Sender will be blocked if the buffer is full Receiver will be blocked if the buffer is empty

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8

Design choices for message passing Option 2: No buffering

If Send happens first, the sending thread blocks If Receive happens first, the receiving thread

blocks Sender and receiver must Rendezvous (ie.

meet) Both threads are ready for the transfer The data is copied / transmitted Both threads are then allowed to proceed

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9

CS 333Introduction to Operating Systems

Class 7 - Deadlock

Jonathan WalpoleComputer Science

Portland State University

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10

Resources and deadlocks

Processes need access to resources in order to make progress

Examples of computer resources printers tape drives kernel data structures (process & file table entries …) locks/semaphores to protect critical sections

Suppose a process holds resource A and requests resource B

at the same time another process holds B and requests A both are blocked and remain so … this is deadlock

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11

Resource usage model

Sequence of events required to use a resource

request the resource (like acquiring a mutex lock) use the resource release the resource (like releasing a mutex lock)

Must wait if request is denied block busy wait fail with error code

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12

Preemptable vs nonpreemptable 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

Deadlocks occur when processes are granted exclusive access to non-preemptable resources and wait when the resource is not available

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13

Definition of deadlock

A set of processes is deadlocked if eachprocess in the set is waiting for an event that onlyanother process in the set can cause

Usually the event is the release of a currently held resource

None of the processes can … be awakened run release resources

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14

Deadlock conditions

A deadlock situation can occur if and only if the following conditions hold simultaneously

Mutual exclusion condition – resource assigned to one process

Hold and wait condition – processes can get more than one resource

No preemption condition

Circular wait condition – chain of two or more processes (must be waiting for resource from next one in chain)

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15

Resource acquisition scenarios

acquire (resource_1)use resource_1release (resource_1)

Thread A:

Example: var r1_mutex: Mutex ... r1_mutex.Lock() Use resource_1 r1_mutex.Unlock()

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16

Resource acquisition scenarios

Thread A:

acquire (resource_1)use resource_1release (resource_1)

Another Example: var r1_sem: Semaphore r1_sem.Signal() ... r1_sem.Wait() Use resource_1 r1_sem.Signal()

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17

Resource acquisition scenarios

acquire (resource_2)use resource_2release (resource_2)

Thread A: Thread B:

acquire (resource_1)use resource_1release (resource_1)

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18

Resource acquisition scenarios

acquire (resource_2)use resource_2release (resource_2)

Thread A: Thread B:

No deadlock can occur here!

acquire (resource_1)use resource_1release (resource_1)

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19

Resource acquisition scenarios: 2 resources

acquire (resource_1)acquire (resource_2)use resources 1 & 2release (resource_2)release (resource_1)

acquire (resource_1)acquire (resource_2)use resources 1 & 2release (resource_2)release (resource_1)

Thread A: Thread B:

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20

Resource acquisition scenarios: 2 resources

acquire (resource_1)acquire (resource_2)use resources 1 & 2release (resource_2)release (resource_1)

acquire (resource_1)acquire (resource_2)use resources 1 & 2release (resource_2)release (resource_1)

Thread A: Thread B:

No deadlock can occur here!

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21

Resource acquisition scenarios: 2 resources

acquire (resource_1)use resources 1release (resource_1)acquire (resource_2)use resource 2release (resource_2)

acquire (resource_2)use resources 2release (resource_2)acquire (resource_1)use resource 1release (resource_1)

Thread A: Thread B:

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22

Resource acquisition scenarios: 2 resources

acquire (resource_1)use resources 1release (resource_1)acquire (resource_2)use resource 2release (resource_2)

acquire (resource_2)use resources 2release (resource_2)acquire (resource_1)use resource 1release (resource_1)

Thread A: Thread B:

No deadlock can occur here!

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23

Resource acquisition scenarios: 2 resources

acquire (resource_1)acquire (resource_2)use resources 1 & 2release (resource_2)release (resource_1)

acquire (resource_2)acquire (resource_1)use resources 1 & 2release (resource_1)release (resource_2)

Thread A: Thread B:

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24

Resource acquisition scenarios: 2 resources

acquire (resource_1)acquire (resource_2)use resources 1 & 2release (resource_2)release (resource_1)

acquire (resource_2)acquire (resource_1)use resources 1 & 2release (resource_1)release (resource_2)

Thread A: Thread B:

Deadlock is possible!

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25

Examples of deadlock

Deadlock occurs in a single program Programmer creates a situation that deadlocks Kill the program and move on Not a big deal

Deadlock occurs in the Operating System Spin locks and locking mechanisms are

mismanaged within the OS Threads become frozen System hangs or crashes Must restart the system and kill all applications

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26

Other examples of deadlock

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27

Resource Allocation Graphs

Resource

R

AProcess/Thread

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28

Resource Allocation Graphs

Resource

R

AProcess/Thread

“is held by”

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29

Resource Allocation Graphs

R

A

“is requesting”

S

Resource

Process/Thread

Resource

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30

Resource Allocation Graphs

R

A S

B

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31

Resource Allocation Graphs

Deadlock

R

A S

B

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32

Resource Allocation Graphs

Deadlock = a cycle in the graph

R

A S

B

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33

Dealing with deadlock

Four general strategies Ignore the problem

• Hmm… advantages, disadvantages?

Detection and recovery

Dynamic avoidance through resource allocation

Prevention, by structurally negating one of the four conditions

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34

Deadlock detection (1 resource of each)

Let the problem happen, then recover

How do you know it happened?

Do a depth-first-search on the resource allocation graph

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35

Deadlock detection (1 resource of each)

Let the problem happen, then recover

How do you know it happened?

Do a depth-first-search on the resource allocation graph

Page 36: 1 A common project 2 error method Lock () var oldIntStat: int oldIntStat = SetInterruptsTo (DISABLED) if mutex == 0 mutex = 1 heldBy = currentThread oldIntStat

36

Deadlock detection (1 resource of each)

Let the problem happen, then recover

How do you know it happened?

Do a depth-first-search on the resource allocation graph

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37

Deadlock detection (1 resource of each)

Let the problem happen, then recover

How do you know it happened?

Do a depth-first-search on the resource allocation graph

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38

Deadlock detection (1 resource of each)

Let the problem happen, then recover

How do you know it happened?

Do a depth-first-search on the resource allocation graph

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39

Mulitple units of a resource

Some resources have only one “unit”. Only one thread at a time may hold the resource.

• Printer• Lock on ReadyQueue

Some resources have several units. All units are considered equal; any one will do.

• Page Frames• Dice in the Gaming Parlor problem

A thread requests “k” units of the resource. Several requests may be satisfied simultaneously.

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40

Deadlock modeling with multiple resources

Theorem: If a graph does not contain a cycle then no processes are deadlocked

A cycle in a RAG is a necessary condition for deadlock

Is it a sufficient condition?

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41

Deadlock modeling with multiple resources

Theorem: If a graph does not contain a cycle then no processes are deadlocked

A cycle in a RAG is a necessary condition for deadlock

Is it a sufficient condition?

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42

Deadlock detection issues

How often should the algorithm run?

On every resource request?

Periodically?

When CPU utilization is low?

When we suspect deadlock because some thread has been asleep for a long period of time?

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43

Recovery from deadlock

What should be done to recover? Abort deadlocked processes and reclaim

resources Temporarily reclaim resource, if possible Abort one process at a time until deadlock cycle

is eliminated

Where to start? Lowest priority process? Shortest running process? Process with fewest resources held? Batch processes before interactive processes? Number of processes that must be terminated?

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44

Other deadlock recovery techniques

Recovery through rollback (preemption) Save state periodically

• take a checkpoint• start computation again from checkpoint

Done for large computation systems

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45

Deadlock avoidance

Detection vs. avoidance…

Detection – “optimistic” approach• Allocate resources• “Break” system to fix the problem

Avoidance – “pessimistic” approach• Don’t allocate resource if it may lead to

deadlock• If a process requests a resource... ... make it wait until you are sure it’s OK

Which one to use depends upon the application

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46

Process-resource trajectories

time

Process At1 t2 t3 t4

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47

Process-resource trajectories

time

Process At1 t2 t3 t4

Requests Printer

Requests CD-RW

Releases Printer

Releases CD-RW

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48

Process-resource trajectoriesti

me

Pro

cess

B

tW

tX

tY

tZ

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49

Process-resource trajectoriesti

me

Pro

cess

B

tW

tX

tY

tZ Requests CD-RW

Requests Printer

Releases CD-RW

Releases Printer

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50

Process-resource trajectoriesP

roce

ss B

tW

tX

tY

tZ

Process At1 t2 t3 t4

tim

e

time

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51

Process-resource trajectoriesP

roce

ss B

tW

tX

tY

tZ

Process At1 t2 t3 t4

tim

e

time

Both processeshold CD-RW

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52

Process-resource trajectoriesP

roce

ss B

tW

tX

tY

tZ

Process At1 t2 t3 t4

tim

e

time

Both processeshold Printer

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53

Process-resource trajectoriesP

roce

ss B

tW

tX

tY

tZ

Process At1 t2 t3 t4

tim

e

time

ForbiddenZone

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54

Process-resource trajectoriesP

roce

ss B

tW

tX

tY

tZ

Process At1 t2 t3 t4

tim

e

time

Trajectory showingsystem progress

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55

Process-resource trajectoriesP

roce

ss B

tW

tX

tY

tZ

Process At1 t2 t3 t4

tim

e

time

B makes progress,A is not running

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56

Process-resource trajectoriesP

roce

ss B

tW

tX

tY

tZ

Process At1 t2 t3 t4

tim

e

time

B requeststhe CD-RW

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57

Process-resource trajectoriesP

roce

ss B

tW

tX

tY

tZ

Process At1 t2 t3 t4

tim

e

time

Request is granted

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58

Process-resource trajectoriesP

roce

ss B

tW

tX

tY

tZ

Process At1 t2 t3 t4

tim

e

time

A runs & makesa request for printer

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59

Process-resource trajectoriesP

roce

ss B

tW

tX

tY

tZ

Process At1 t2 t3 t4

tim

e

time

Request is granted;A proceeds

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60

Process-resource trajectoriesP

roce

ss B

tW

tX

tY

tZ

Process At1 t2 t3 t4

tim

e

time

B runs & requeststhe printer...

MUST WAIT!

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61

Process-resource trajectoriesP

roce

ss B

tW

tX

tY

tZ

Process At1 t2 t3 t4

tim

e

time

A runs & requeststhe CD-RW

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62

Process-resource trajectoriesP

roce

ss B

tW

tX

tY

tZ

Process At1 t2 t3 t4

tim

e

time

A... holds printer requests CD-RWB... holds CD-RW requests printer

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63

Process-resource trajectoriesP

roce

ss B

tW

tX

tY

tZ

Process At1 t2 t3 t4

tim

e

time

A... holds printer requests CD-RWB... holds CD-RW requests printer

DEADLOCK!

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64

Process-resource trajectoriesP

roce

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Process At1 t2 t3 t4

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A danger occurred here.

Should the OS give A the printer, or make it wait???

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65

Process-resource trajectoriesP

roce

ss B

tW

tX

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Process At1 t2 t3 t4

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This area is “unsafe”

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66

Process-resource trajectoriesP

roce

ss B

tW

tX

tY

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Process At1 t2 t3 t4

tim

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Within the “unsafe” area, deadlock is inevitable.

We don’t want toenter this area.

The OS should makeA wait at this point!

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67

Process-resource trajectoriesP

roce

ss B

tW

tX

tY

tZ

Process At1 t2 t3 t4

tim

e

time

B requests the printer,B releases CD-RW,B releases printer,then A runs to completion!

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68

Safe states

The current state:“which processes hold which resources”

A “safe” state: No deadlock, and There is some scheduling order in which every

process can run to completion even if all of them request their maximum number of units immediately

The Banker’s Algorithm: Goal: Avoid unsafe states!!! When a process requests more units, should the

system grant the request or make it wait?

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Avoidance with multiple resource types

Note: These are the max. possiblerequests, which we assumeare known ahead of time

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Banker’s algorithm for multiple resources

Look for a row, R, whose unmet resource needs are all smaller than or equal to A. If no such row exists, the system will eventually deadlock since no process can run to completion

Assume the process of the row chosen requests all the resources that it needs (which is guaranteed to be possible) and finishes. Mark that process as terminated and add all its resources to A vector

Repeat steps 1 and 2, until either all process are marked terminated, in which case the initial state was safe, or until deadlock occurs, in which case it was not

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Avoidance modeling

Available resource vectorTotal resource vector

Maximum Request Vector

Row 2 is what process 2 might need

RUN ALGORITHM ON EVERY RESOURCE REQUEST

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Avoidance algorithm

Max request matrix

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Avoidance algorithm

Max request matrix

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Avoidance algorithm

Max request matrix

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Avoidance algorithm

2 2 2 0

Max request matrix

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Avoidance algorithm

2 2 2 0

Max request matrix

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Avoidance algorithm

4 2 2 1 2 2 2 0

Max request matrix

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Deadlock avoidance

Deadlock avoidance is often impossible because you don’t know in advance what

resources a process will need!

Alternative approach “deadlock prevention”

Make deadlock impossible! Attack one of the four conditions that are

necessary for deadlock to be possible

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Deadlock avoidance

Conditions necessary for deadlock:

Mutual exclusion condition

Hold and wait condition

No preemption condition

Circular wait condition

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Attacking the conditions

Attacking mutual exclusion? a bad idea for some resource types

• Because resource could be corrupted may work for some resources in certain

situations

Attacking no preemption? a bad idea for some resource types

• Because resource may be left in an inconsistent state

may work in some situations

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Attacking the conditions

Attacking hold and wait? Require processes to request all resources

before they begin! Process must know ahead of time Process must tell system its “max potential

needs”

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Attacking the conditions

If a process decides it wants more than its initial declared needs, it must...

Release all resources Give the system a new “max potential needs” Resume execution

Issues: Under-allocation of resources Resource needs not known in advance

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Attacking the conditions

Attacking circular wait? Number each of the resources Require each process to acquire lower

numbered resources before higher numbered resources

More precisely: A process is not allowed to request a resource whose number is lower than the highest numbered resource it currently holds.

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Recall this example of deadlock

Assume that resources are ordered:1. Resource_12. Resource_23. ...etc...

acquire (resource_1)acquire (resource_2)use resources 1 & 2release (resource_2)release (resource_1)

acquire (resource_2)acquire (resource_1)use resources 1 & 2release (resource_1)release (resource_2)

Thread A: Thread B:

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Recall this example of deadlock

Assume that resources are ordered: 1. Resource_1 2. Resource_2 3. ...etc... Thread B violates the ordering!

acquire (resource_1)acquire (resource_2)use resources 1 & 2release (resource_2)release (resource_1)

acquire (resource_2)acquire (resource_1)use resources 1 & 2release (resource_1)release (resource_2)

Thread A: Thread B:

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Why Does Resource Ordering Work?

Assume deadlock has occurred.

Process A holds X requests Y

Process B holds Y requests Z

Process C holds Z requests X

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Why Does Resource Ordering Work?

Assume deadlock has occurred.

Process A holds X requests Y

Process B holds Y requests Z

Process C holds Z requests X

X < Y

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Why Does Resource Ordering Work?

Assume deadlock has occurred.

Process A holds X requests Y

Process B holds Y requests Z

Process C holds Z requests X

X < Y

Y< Z

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Why Does Resource Ordering Work?

Assume deadlock has occurred.

Process A holds X requests Y

Process B holds Y requests Z

Process C holds Z requests X

X < Y

Y< Z

Z < X

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Why Does Resource Ordering Work?

Assume deadlock has occurred.

Process A holds X requests Y

Process B holds Y requests Z

Process C holds Z requests X

X < Y

Y< Z

Z < X

This is impossible!

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Why Does Resource Ordering Work?

Assume deadlock has occurred.

Process A holds X requests Y

Process B holds Y requests Z

Process C holds Z requests X

X < Y

Y< Z

Z < X

This is impossible!

Therefore theassumption mustbe false!

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Resource Ordering

The chief problem: It may be hard to come up with an

acceptable ordering of resources!

Still,this is the most useful approach in an OS

1. ProcessControlBlock2. FileControlBlock3. Page Frames

Also, the problem of resources with multiple units is not addressed.

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A word on starvation

Starvation and deadlock are two different things

With deadlock – no work is being accomplished for the processes that are deadlocked, because processes are waiting for each other. Once present, it will not go away.

With starvation – work (progress) is getting done, however, a particular set of processes may not be getting any work done because they cannot obtain the resource they need

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Quiz

What is deadlock? What conditions must hold for deadlock

to be possible? What are the main approaches for

dealing with deadlock? Why does resource ordering help?


2-Dining Puppies Mutex Failure

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