chap#06 deadlocks

7/26/2019 Chap#06 Deadlocks

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MODERN OPERATING SYSTEMS

Chapter 6

Deadlocks

Tanenbaum, Modern Operating Systems 3 e, (c) 2008 Prentice-Ha, !nc" # rig$ts reser%ed" 0-&3-6006639


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Preemptable and Nonpreemptable

Resorces

Se!ence o" e#ents re!$red to se a

resorce%

&' Re!est the resorce'(' )se the resorce'

*' Release the resorce'



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+$,re 6-&' )s$n, a semaphore to protect resorces'

.a/ One resorce' .b/ T0o resorces'

Resorce Ac!$s$t$on .&/



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+$,re 6-(' .a/

Deadlock-"ree

code'

Resorce Ac!$s$t$on .(/



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+$,re 6-(' .b/ Code 0$th a

potent$al deadlock'

Resorce Ac!$s$t$on .*/



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Introdct$on To Deadlocks

Deadlock can be de"$ned "ormall1 as

"ollo0s%

A set of processes is deadlocked if eachprocess in the set is waiting for an event

that only another process in the set can

cause.



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Cond$t$ons "or Resorce Deadlocks

&' Mtal e2cls$on cond$t$on

(' 3old and 0a$t cond$t$on'

*' No preempt$on cond$t$on'4' C$rclar 0a$t cond$t$on'



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Deadlock Model$n,

Strate,$es "or deal$n, 0$th deadlocks%

&' 5st $,nore the problem'

(' Detect$on and reco#er1' et deadlocks

occr7 detect them7 take act$on'

*' D1nam$c a#o$dance b1 care"l resorce

allocat$on'

4' Pre#ent$on7 b1 strctrall1 ne,at$n, oneo" the "or re!$red cond$t$ons'



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+$,re 6-8' .a/ A resorce ,raph' .b/ A c1cle e2tracted "rom .a/'

Deadlock Detect$on 0$th

One Resorce o" Each T1pe .&/



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One Resorce o" Each T1pe .(/

Al,or$thm "or detect$n, deadlock%

&' +or each node7 N $n the ,raph7 per"orm the

"ollo0$n, "$#e steps 0$th N as the start$n, node'

(' In$t$al$9e to the empt1 l$st7 des$,nate all arcsas nmarked'

*' Add crrent node to end o" 7 check to see $"

node no0 appears $n t0o t$mes' I" $t does7

,raph conta$ns a c1cle .l$sted $n /7 al,or$thmterm$nates'

:



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One Resorce o" Each T1pe .*/

4' +rom ,$#en node7 see $" an1 nmarkedot,o$n, arcs' I" so7 ,o to step 8; $" not7 ,o to

step 6'

8' P$ck an nmarked ot,o$n, arc at random andmark $t' Then "ollo0 $t to the ne0 crrent node

and ,o to step *'

6' I" th$s $s $n$t$al node7 ,raph does not conta$n an1

c1cles7 al,or$thm term$nates' Other0$se7 deadend' Remo#e $t7 ,o back to pre#$os node7

make that one crrent node7 ,o to step *'



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+$,re 6-6' The "or data strctres needed

b1 the deadlock detect$on al,or$thm'

Deadlock Detect$on 0$th Mlt$ple

Resorces o" Each T1pe .&/



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Deadlock Detect$on 0$th Mlt$ple

Resorces o" Each T1pe .(/

Deadlock detect$on al,or$thm%

&' ook "or an nmarked process7 Pi7 "or 0h$ch

the $-th ro0 o" R$s less than or e!al toA'

(' I" sch a process $s "ond7 add the i-thro0 o" C

toA7 mark the process7 and ,o back to step &'

*' I" no sch process e2$sts7 the al,or$thm

term$nates'



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+$,re 6-


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Reco#er1 "rom Deadlock

= Reco#er1 thro,h preempt$on

= Reco#er1 thro,h rollback

= Reco#er1 thro,h k$ll$n, processes



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+$,re 6->' T0o process resorce tra?ector$es'

Deadlock A#o$dance



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+$,re 6-@' Demonstrat$on that the state $n .a/ $s sa"e'

Sa"e and )nsa"e States .&/



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+$,re 6-&' Demonstrat$on that the state $n .b/ $s not sa"e'

Sa"e and )nsa"e States .(/


Th B k Al $th


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+$,re 6-&&' Three resorce allocat$on states%

.a/ Sa"e' .b/ Sa"e' .c/ )nsa"e'

The Bankers Al,or$thm

"or a S$n,le Resorce


Th B k Al $th


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+$,re 6-&(' The bankers al,or$thm 0$th mlt$ple resorces'


"or Mlt$ple Resorces .&/


Th B k Al $th


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"or Mlt$ple Resorces .(/

Al,or$thm "or check$n, to see $" a state $s sa"e%

&' ook "or ro07 R7 0hose nmet resorce needs all

A' I" no sch ro0 e2$sts7 s1stem 0$ll e#entall1

deadlock s$nce no process can rn to complet$on

(' Assme process o" ro0 chosen re!ests all resorces$t needs and "$n$shes' Mark process as term$nated7 add

all $ts resorces to the A #ector'

*' Repeat steps & and ( nt$l e$ther all processes marked

term$nated .$n$t$al state 0as sa"e/ or no process le"t0hose resorce needs can be met .there $s a

deadlock/'



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Deadlock Pre#ent$on

= Attack$n, the mtal e2cls$on cond$t$on

= Attack$n, the hold and 0a$t cond$t$on

= Attack$n, the no preempt$on cond$t$on

= Attack$n, the c$rclar 0a$t cond$t$on


Att k$ th


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+$,re 6-&*' .a/ Nmer$call1 ordered resorces'

.b/ A resorce ,raph'

Attack$n, the

C$rclar a$t Cond$t$on



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+$,re 6-&4' Smmar1 o" approaches to deadlock pre#ent$on'

Approaches to Deadlock Pre#ent$on



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Other Isses

= T0o-phase lock$n,

= Commn$cat$on deadlocks

= $#elock= Star#at$on



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+$,re 6-&8' A resorce deadlock $n a net0ork'

Commn$cat$on Deadlocks



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+$,re 6-&6' Bs1 0a$t$n, that can lead to l$#elock'

$#elock

chap#06 deadlocks

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