Cs2106 Lec7 Deadlock

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Lecture 7

Deadlock

28 September, 2010

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Process 2

:down(T)

down(S)

up(S)

up(T)

semaphore S = T = 1

Process 1

:down(S)

down(T)

up(T)

up(S)

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while (1)

thinkwait till left chopstick is available

pick up left chopstick

wait till right chopstick is availablepick up right chopstick

eatput down left chopstick

put down right chopstick4


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while (1)

thinkdown(chopstick[i])

down(chopstick[(i+1)%N]eat

up(chopstick[i])

up(chopstick[(i+1)%N]

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Resource

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acquire resource

(wait until available)use resource

release resource

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software

vs.hardware

resource

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preemptive

vs.non-preemptive

resource

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single copy

vs.multiple copies

of a resource

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4conditions

for deadlock

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mutual exclusioneach resource must be eitherassigned to exactly one process

or is available

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while (1)



up(chopstick[i])


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hold and waitprocesses holding resources grantedearlier can request for new resource

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while (1)



up(chopstick[i])


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no preemptionresources granted cannot beforcefully taken away

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while (1)



up(chopstick[i])


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circular waitinga circular chain of processes, each waiting for aresource held by the next member of the chain

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http://library.thinkquest.org/08aug/01033/img.htm

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http://library.thinkquest.org/08aug/01033/img.htmhttp://library.thinkquest.org/08aug/01033/img.htm


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A

R

C

T

B

S

A requests RB requests S

C requests TA requests SB requests T

C requests R

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Deadlock

Detectionis the system deadlocked, and if so,

which process are involved?

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1. !periodically build a ! ! ! ! ! resource graph2. !run depth first search on! ! the graph to detect cycle

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Deadlock Detection(if each resource

type has multiple copies)

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0 0 1 0

2 0 0 1

0 1 2 0

4 2 3 1 2 1 0 0

resources in existence resources available

allocation matrix request matrix

2 0 0 1

1 0 1 0

2 1 0 0

A

B

C

A

B

C

R S T UR S T U

R S T UR S T U

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A CB

R TS U

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is there a process

whose requests can

be satisfied?

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A

R TS U

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B

R TS U

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C

R TS U

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C

R TS U

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0 0 1 0

2 0 0 1

0 1 2 0

4 2 3 1 2 1 0 0



2 0 0 1

1 0 1 0

2 1 0 0

A

B

C

A

B

C

R S T UR S T U

R S T UR S T U

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A B

R TS U

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0 0 1 0

2 0 0 1

0 0 0 0

4 2 3 1 2 2 2 0



2 0 0 1

1 0 1 0

0 0 0 0

A

B

C

A

B

C

R S T UR S T U

R S T UR S T U

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0 0 1 0

0 0 0 0

0 0 0 0

4 2 3 1 4 2 2 1



2 0 0 1

0 0 0 0

0 0 0 0

A

B

C

A

B

C

R S T UR S T U

R S T UR S T U

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suppose we have

deadlock,

now what?

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1. preempt

2. rollback

3. terminate

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Deadlock

Avoidanceif we know the resources required by a process,

can we avoid deadlock by careful allocation?

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mutual exclusioneach resource must be either

assigned to exactly one process

or is available

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allow sharing ofresources

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hold and waitprocesses holding resources grantedearlier can request for new resource

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allocate only if allresources are available

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while (1)

think

down(mutex)state[ i ] = HUNGRY

test( i )

up(mutex)

down(semaphore[ i ])eat

down(mutex)

state[ i ] = THINK

test( L )test( R )

up(mutex)

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no preemptionresources granted cannot beforcefully taken away

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allow resources to bepreempted

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circular waitinga circular chain of processes, each waiting for aresource held by the next member of the chain

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order resource

numerically and

acquire in order

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1

2

3 4

5

6

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Livelock

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Starvation

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Mars Pathfinder58
http://library.thinkquest.org/08aug/01033/img.htmhttp://library.thinkquest.org/08aug/01033/img.htmhttp://library.thinkquest.org/08aug/01033/img.htm


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Process A (low):

down(mutex)

:

work:

:

up(mutex)

Process B (high):

down(mutex)

:

important tasks:

:

up(mutex)

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Priority Inversion

Cs2106 Lec7 Deadlock

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