deadlocks - uio.no · Deadlocks Formal definition : A set of processes is deadlocked if each...

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Deadlocks

Thomas Plagemann

With slides from C. Griwodz, K. Li,A. Tanenbaum and M. van Steen

Preempting Scheduler Activations

Scheduler activations are completelypreemptable

Kernelspace

Userspace

2


Maintaining the run queue needs to be a protected critical sectionLet’s use spin locks for protection

Thread 1 Upcall

Acquire spin lock

Start queue maintenance

Interrupt Thread 1

Acquire spin lock -> block

Resources

Examples of computer resourcesCPUMemoryDisk driveTape drivesPrintersPlotterLoudspeaker

3

ResourcesProcesses

Need access to resources in reasonable order

Typical way to use a resourceRequestUseRelease

Suppose a process holds resource A and requests resource B

At same time another process holds B and requests ABoth are blocked and remain so

ResourcesActive resource

Provides a serviceE.g. CPU, network adaptor

Passive resourceSystem capabilities that are required by active resourcesE.g. memory, network bandwidth

Exclusive resourceOnly one process at a time can use itE.g. loudspeaker, processor

Shared resourceCan be used by multiple processesE.g. memory, bandwidth

4

ResourcesSingle resource

Exists only once in the systemE.g. loudspeaker

Multiple resourceExists several time in the systemE.g. processor in a multiprocessor system

Preemptable resourceResource that can be taken away from a processE.g. CPU can be taken away from processes in user space

Non-preemptable resourceTaking it away will cause processes to failE.g. Disk, files

ResourcesProcess must wait if request is denied

Requesting process may be blockedMay fail with error code

DeadlocksOccur only whenprocesses are granted exclusive access to resources

blockacquire

use

acquire

use

5

DeadlocksFormal definition :

A set of processes is deadlockedif each process in the set is waiting for an eventthat only another process in the set can cause

Usually the event is release of a currently held resourceNone of the processes can …

RunRelease resourcesBe awakened

Four Conditions for Deadlock1. Mutual exclusion condition

Each resource assigned to 1 process or is available

2. Hold and wait conditionProcess holding resources can request additional

3. No preemption conditionPreviously granted resources cannot forcibly taken away

4. Circular wait conditionMust be a circular chain of 2 or more processesEach is waiting for resource held by next member of the chain

6

Deadlock ModelingModeled with directed graphs

Resource R assigned to process AProcess B is requesting/waiting for resource SProcess C and D are in deadlock over resources T and U

A

R B

S C

U D

T

Deadlock ExampleA utility program

Copies a file from a tape to diskPrints the file to a printer

ResourcesTapeDiskPrinter

A deadlock

tape disk printer

A

B

7

Deadlock ModelingHow deadlock occurs

ARequests RRequests SReleases SReleases R

BRequests SRequests TReleases TReleases S

CRequests TRequests RReleases RReleases T

A requests RB requests SC requests TA requests SB requests TC requests R

Resources

Processes A B C

R S T

Deadlock ModelingHow deadlock can be avoided

ARequests RRequests SReleases SReleases R

BRequests SRequests TReleases TReleases S

CRequests TRequests RReleases RReleases T

A requests RC requests TA requests SB requests SB requests TC requests RA releases SA releases RC releases RC releases T

Resources

Processes A B C

R S T

8

Deadlocks: Strategies

Ignore the problemIt is user’s fault

Detection and recoveryFix the problem afterwards

Dynamic avoidanceCareful allocation

PreventionNegate one of the four conditions

The Ostrich Algorithm

Pretend there is no problem

Reasonable if Deadlocks occur very rarely Cost of prevention is high

UNIX and Windows take this approachIt is a trade-off between

ConvenienceCorrectness

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Deadlock Detection and RecoveryOne Resource of Each Type

A cycle can be found within the graph, denoting deadlock

A B

C

R

S TD E

F

G

U V

W

init notebook L

add current node CN to L

is CN two times in L?cycle detected

is an unmarkedoutgoing arc at CN?

follow the arcCN = next node

backtrackingCN = previous node

Deadlock Detection and RecoveryOne Resource of Each Type

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Deadlock Detection and RecoveryMultiple Resources of Each Type

⎥⎥⎥⎥

⎦

⎤

⎢⎢⎢⎢

⎣

⎡

nmnnn

m

m

CCCC

CCCCCCCC

..................

...

...

321

2232221

1131211

⎥⎥⎥⎥

⎦

⎤

⎢⎢⎢⎢

⎣

⎡

nmnnn

m

m

RRRR

RRRRRRRR

..................

...

...

321

2232221

1131211

Existing resources Available resources

( )mEEEE ,...,,, 321 ( )mAAAA ,...,,, 321

Current allocation matrix Request matrix

Deadlock Detection and RecoveryMultiple Resources of Each Type

4 2 3 1 2 1 0 0

0 0 1 02 0 0 10 1 2 0

2 0 0 11 0 1 02 1 0 0

E=(

Current allocation matrix Request matrix

Tape d

rivers

Plotter

s

CD-Rom

s

Scann

ers

A=() )Tap

e driv

ers

Plotter

s

CD-Rom

s

Scann

ers

R=C=

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Deadlock Detection and RecoveryRecovery

Recovery through preemptionTake a resource from some other processDepends on nature of the resource

Recovery through rollbackCheckpoint a process periodicallyUse this saved state Restart the process if it is found deadlocked

Recovery through killing processesCrudest but simplest way to break a deadlockKill one of the processes in the deadlock cycleThe other processes get its resources Choose process that can be rerun from the beginning

Deadlock AvoidanceResource Trajectories

finished

request releaserequest release

Printer

Plotter

A

Prin

ter

release

release

request

request

Plo

tter

B

start

Two processresource trajectories

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Deadlock AvoidanceSafe and Unsafe States

state is safe

ABC

322

947

Free: 3

has maxABC

342

947

Free: 1

has maxABC

302

9

7Free: 5

has maxABC

307

9

7Free: 0

has maxABC

300

9

Free: 7

has max

Deadlock AvoidanceSafe and Unsafe States

state is safe

ABC

322

947

Free: 3

has maxABC

422

947

Free: 2

has maxABC

302

9

7Free: 4

has maxABC

442

947

Free: 0

has max

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Deadlock AvoidanceBanker’s Algorithm for a Single Resource

Each process has a creditSystem knows how many resources a process requests at most before releasing resources

Total resources may not satisfy all creditsKeep track of resources assigned and neededCheck on each allocation whether it is safe

Safe: there exists a sequence of other states that all processes can terminate correctly

Deadlock AvoidanceBanker's Algorithm for a Single Resource

Free: 10

has max

0000

6547

Free: 2

has max

1124

6547

Free: 1

has max

1224

6547

650 50 6

40 470 7

42 4

ABCD

33551 561 6

74 7

ABCD

1224

ABCD

Free: 4Free: 5Free: 6Free: 2

Resource allocation state

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Deadlock Detection and RecoveryBanker’s Algorithm for Multiple Resources

An example for the deadlockdetection algorithm

A 3 0 1B 0 1 0C 1 1 1

Assigned resources Resources still needed

6 3 4 25 3 2 2

E=(Tap

e driv

ers

Plotter

s

CD-Rom

s

Scann

ers

P=())

1 0 2 0A=( )100

D 1 1 0 1E 0 0 0 0

A 1 1 0B 0 1 1C 3 1 0

020

D 0 0 1 0E 2 1 1 0



A 3 0 1B 0 1 0C 1 1 1

6 3 4 24 2 2 1

E=(Tap

e driv

ers

Plotter

s

CD-Rom

s

Scann

ers

P=())

2 1 2 1A=( )100

D 0 0 0 0E 0 0 0 0

A 1 1 0B 0 1 1C 3 1 0

020

D - - - -E 2 1 1 0


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A 0 0 0B 0 1 0C 1 1 1

6 3 4 21 2 1 0

E=(Tap

e driv

ers

Plotter

s

CD-Rom

s

Scann

ers

P=())

5 1 3 2A=( )000

D 0 0 0 0E 0 0 0 0

A - - -B 0 1 1C 3 1 0

-20

D - - - -E 2 1 1 0




A 0 0 0B 0 0 0C 1 1 1

6 3 4 21 1 1 0

E=(Tap

e driv

ers

Plotter

s

CD-Rom

s

Scann

ers

P=())

5 2 3 2A=( )000

D 0 0 0 0E 0 0 0 0

A - - -B - - -C 3 1 0

--0

D - - - -E 2 1 1 0


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A 0 0 0B 0 0 0C 0 0 0

6 3 4 20 0 0 0

E=(Tap

e driv

ers

Plotter

s

CD-Rom

s

Scann

ers

P=())

6 3 4 2A=( )000

D 0 0 0 0E 0 0 0 0

A - - -B - - -C - - -

---

D - - - -E 2 1 1 0




A 3 0 1B 0 1 0C 1 1 1

6 3 4 25 3 2 2

E=(Tap

e driv

ers

Plotter

s

CD-Rom

s

Scann

ers

P=())

1 0 2 0A=( )100

D 1 1 0 1E 0 0 0 0

A 1 1 0B 0 1 1C 3 1 0

020

D 0 0 1 0E 2 1 1 0

SAFE


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Deadlock AvoidancePractical Avoidance

Two Phase LockingPhase I

Process tries to lock all resources it needs, one at a timeIf needed resources found locked, start over(no real work done in phase one)

Phase IIRunReleasing locks

Note similarity to requesting all resources at onceAlgorithm works where programmer can arrange

Deadlock PreventionR: Conditions for Deadlock

1. Mutual exclusion conditionEach resource assigned to 1 process or is available

2. Hold and wait conditionProcess holding resources can request additional

3. No preemption conditionPreviously granted resources cannot forcibly taken away

4. Circular wait conditionMust be a circular chain of 2 or more processesEach is waiting for resource held by next member of the chain

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Deadlock PreventionMutual Exclusion Condition

Some resources are not sharablePrinter, tape, etc

Some resources can be made sharableSome resources can be made virtual

Spooling - PrinterDoes spooling apply to all non-sharable resources?

Mixing - Soundcard

Principle:Avoid assigning resource when not absolutely necessaryA few processes as possible actually claim the resource

Deadlock PreventionHold and Wait Condition

Require processes to request resources before starting

A process never has to wait for what it needsTelephone companies do this

ProblemsMay not know required resources at start of runAlso ties up resources other processes could be using

Variation: Process must give up all resourcesThen request all immediately needed

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Deadlock PreventionNo Preemption Condition

This is not a viable optionConsider a process given the printer

Halfway through its jobNo forcibly take away printer!!??

Deadlock PreventionCircular Wait Condition

A

1 5432

Normally ordered resourcesA resource graph

1. CD Rom drive2. Tape drive3. Plotter4. Scanner5. Imagesetter

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Deadlock PreventionCircular Wait Condition

Impose an order of requests for all resourcesMethod

Assign a unique id to each resourceAll resource requests must be in an ascending order of the idsRelease resources in a descending order

Can you prove this method has no circular wait?Is this generally feasible?

Deadlock PreventionOverview

Order resources numericallyCircular waitTake resources awayNo preemptionRequest all resource initiallyHold and waitSpool everythingMutual exclusion

ApproachCondition

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Non-resource Deadlocks

Possible for two processes to deadlockEach is waiting for the other to do some task

Can happen with semaphoresEach process required to do a down() on two semaphores (mutex and another)If done in wrong order, deadlock results


So how do they handle this deadlock?

Thread 1 Upcall

Acquire spin lock

Start queue maintenance

Interrupt Thread 1

Acquire spin lock -> block

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Preempting Scheduler ActivationsDetection and recovery (like in Mach)Basic idea:

Upcall handler checks first the state of each interruptedthreadIf it is in a critical section allow it to finish this

Implementation:Protect critical sections with spin locksAcquire_spin_lock() increments a counter in thethread’s descriptorUpcall handler checks spin lock count of all interruptedthreadsIf there are threads that hold spin locks set flagSwitch to the context of these threadsAfterwards switch back to upcall handler

Summary

ResourceIntroduction to deadlocksStrategies

Ostrich algorithmDeadlock detection and recoveryDeadlock avoidanceDeadlock prevention

Non-resource deadlocks

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