lec09 deadlock

7/23/2019 Lec09 Deadlock

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CS162

Operating Systems andSystems ProgrammingLecture 9

Tips for Working in a Project Team/Cooperating Processes and ead!ock

Septem"er 29# 2$1$

Prof% &o'n (u"iato)ic*'ttp+//inst%eecs%"erke!ey%edu/,cs162


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eie)+ efinition of 3onitor

Semap'ores are confusing "ecause dua! purpose+ ot' mutua! e4c!usion and sc'edu!ing constraints C!eaner idea+ .se !ocksfor mutua! e4c!usion andcondition aria"!esfor sc'edu!ing constraints

3onitor+ a !ock and *ero or more condition aria"!esfor managing concurrent access to s'ared data

.se of 3onitors is a programming paradigm Lock+ proides mutua! e4c!usion to s'ared data+

5!)ays acuire "efore accessing s'ared data structure 5!)ays re!ease after finis'ing )it' s'ared data

Condition 7aria"!e+ a ueue of t'reads )aiting forsomet'ing insidea critica! section (ey idea+ a!!o) s!eeping inside critica! section "yatomica!!y re!easing !ock at time )e go to s!eep

Contrast to semap'ores+ Can8t )ait inside critica!

section


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eie)+ Programming )it' 3onitors

3onitors represent t'e !ogic of t'e program Wait if necessary Signa! )'en c'ange somet'ing so any )aiting t'readscan proceed

asic structure of monitor:"ased program+lock

while (need to wait) { condvar.wait();}unlock

do something so no need to wait

lock

condvar.signal();

unlock

C'eck and/or update

state aria"!esWait if necessary

C'eck and/or updatestate aria"!es


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Si!"ersc'at*#


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Tips for Programming in a Project Team

ig projects reuire more t'an oneperson ?or !ong# !ong# !ong time@ ig OS+ t'ousands of person:yearsA

Bt8s ery 'ard to make soft)areproject teams )ork correct!y oesn8t seem to "e as true of "ig

construction projects mpire state "ui!ding finis'ed in

oneyear+ staging iron productiont'ousands of mi!es a)ay

Or t'e Dooer dam+ "ui!t to)ns to

'o!d )orkers Bs it O( to miss dead!inesE

We make it free ?s!ip days@ ea!ity+ t'ey8re ery e4pensie as

time:to:market is one of t'e most

important t'ingsA

FGou just 'aeto get your

sync'roni*ation rig'tAH


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ig Projects

W'at is a "ig projectE Time/)ork estimation is 'ard Programmers are eterna! optimistics?it )i!! on!y take t)o days@A T'is is )'y )e "ug you a"out

starting t'e project ear!y Dad a grad student )'o used to say 'e just needed

F1$ minutesH to fi4 somet'ing% T)o 'ours !aterI Can a project "e efficient!y partitionedE

Partitiona"!e task decreases in time asyou add peop!e

ut# if you reuire communication+

Time reac'es a minimum "ound Wit' comp!e4 interactions# time increasesA

3yt'ica! person:mont' pro"!em+Gou estimate 'o) !ong a project )i!! take Starts to fa!! "e'ind# so you add more peop!e

Project takes een more timeA


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Tec'niues for Partitioning Tasks

0unctiona! Person 5 imp!ements t'reads# Person imp!ementssemap'ores# Person C imp!ements !ocksI

Pro"!em+ Lots of communication across 5PBs Bf c'anges t'e 5PB# 5 may need to make c'anges Story+ Large air!ine company spent K2$$ mi!!ion on a ne)

sc'edu!ing and "ooking system% T)o teams F)orking

toget'er%H 5fter t)o years# )ent to merge soft)are%0ai!edA Bnterfaces 'ad c'anged ?documented# "ut no onenoticed@% esu!t+ )ou!d cost anot'er K2$$ mi!!ion to fi4%

Task Person 5 designs# Person )rites code# Person C tests 3ay "e difficu!t to find rig't "a!ance# "ut can focus oneac' person8s strengt's ?T'eory s systems 'acker@

Since e"ugging is 'ard# 3icrosoft 'as t)otesters foreac'programmer

3ost CS162 project teams are functiona!# "ut peop!e'ae 'ad success )it' task:"ased diisions


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Communication

3ore peop!e mean more communication C'anges 'ae to "e propagated to more peop!e T'ink a"out person )riting code for mostfundamenta! component of system+ eeryone dependson t'emA

3iscommunication is common FBnde4 starts at $E B t'oug't you said 1AH

W'o makes decisionsE Bndiidua! decisions are fast "ut trou"!e


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Coordination

3ore peop!e no one can make a!! meetingsA T'ey miss decisions and associated discussion

4amp!e from ear!ier c!ass+ one person missedmeetings and did somet'ing group 'ad rejected W'y do )e !imit groups to > peop!eE

Gou )ou!d neer "e a"!e to sc'edu!e meetings ot'er)ise W'y do )e reuire ; peop!e minimumE

Gou need to e4perience groups to get ready for rea! )or!d Peop!e 'ae different )ork sty!es

Some peop!e )ork in t'e morning# some at nig't Do) do you decide )'en to meet or )ork toget'erE

W'at a"out project s!ippageE Bt )i!! 'appen# guaranteedA 4+ p'ase ;# eeryone "usy "ut not ta!king% One person)ay "e'ind% =o one kne) unti! ery end M too !ateA

Dard to add peop!e to e4isting group 3em"ers 'ae a!ready figured out 'o) to )ork toget'er


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Do) to 3ake it WorkE

Peop!e are 'uman%


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Suggested ocuments for Gou to 3aintain

Project o"jecties+ goa!s# constraints# and priorities

Specifications+ t'e manua! p!us performance specs T'is s'ou!d "e t'e first document generated and t'e!ast one finis'ed

3eeting notes

ocument a!! decisionsGou can often cut paste for t'e design documents

Sc'edu!e+ W'at is your anticipated timingE T'is document is critica!A

Organi*ationa! C'art W'o is responsi"!e for )'at taskE


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.se Soft)are Too!s

Source reision contro! soft)are ?Su"ersion# C7S# ot'ersI@ asy to go "ack and see 'istory/undo mistakes 0igure out )'ere and )'y a "ug got introduced Communicates c'anges to eeryone ?use C7S8s features@

.se automated testing too!s Write scripts for non:interactie soft)are .se Fe4pectH for interactie soft)are &.nit+ automate unit testing 3icrosoft re"ui!ds t'e 7ista kerne! eery nig't )it' t'eday8s c'anges% eryone is running/testing t'e !atestsoft)are

.se :mai! and instant messaging consistent!y to!eae a 'istory trai!


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Test Continuous!y

Bntegration tests a!! t'e time# not at 11pm

on due dateA Write dummy stu"s )it' simp!e functiona!ity Let8s peop!e test continuous!y# "ut more )ork

Sc'edu!e periodic integration tests


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5dministriia

Project 1 Code ?and fina! design document@ ue Tuesday 1$/> ?ne4t TuesdayA@# ocument Wednesday

Project 2 starts after you are done )it' Project 1 5utograder issues

5utograder not intended to run freuent!y at "eginning 5ssume running eery ; 'ours or so at "eginning of )eek

We did 'ae pro"!ems oer t'e )eekend Dopefu!!y fi4ed "y no)

3idterm B coming up in t'ree )eeks+

3onday# 1$/1# Location+ 1>> )ine!!e Wi!! "e 'our e4am in eening ?>+$:+$ or 6+$$:9+$$@

S'ou!d "e 2 'our e4am )it' e4tra time C!osed "ook# one page of 'and:)ritten notes ?"ot' sides@ Topics+ eryt'ing up to preious Wednesday

=o c!ass on day of 3idterm B )i!! post e4tra office 'ours for peop!e )'o 'ae uestions a"out

t'e materia! ?or !ife# )'ateer@


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C:Language Support for Sync'roni*ation

C !anguage+ Pretty straig'tfor)ard sync'roni*ation

&ust make sure you kno) a!! t'e code pat's out of acritica! sectionint Rtn() {lock.acquire();if (exception) {

lock.release();return errReturnode;}lock.release();return !";}

Watc' out for set#mp/long#mpA Can cause a non:!oca! jump out of procedure Bn e4amp!e# procedure ca!!s !ongjmp# poping stack

"ack to procedure Bf Procedure C 'ad !ock%acuire# pro"!emA

Proc 5

Proc Ca!!s setjmp

Proc C!ock%acuire

Proc

Proc

Ca!!s !ongjmp

Stack

gro

)t'


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CQQ Language Support for Sync'roni*ation

Languages )it' e4ceptions !ike CQQ Languages t'at support e4ceptions are pro"!ematic ?easy

to make a non:!oca! e4it )it'out re!easing !ock@ Consider+

void Rtn() {lock.acquire();

$o%oo();lock.release();

}void $o%oo() {

if (exception) throw err&xception;

}

=otice t'at an e4ception in o0oo?@ )i!! e4it )it'outre!easing t'e !ock


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CQQ Language Support for Sync'roni*ation ?con8t@

3ust catc' a!! e4ceptions in critica! sections Catc' e4ceptions# re!ease !ock# and re:t'ro) e4ception+

void Rtn() {lock.acquire();tr' {

$o%oo();

} catch () { catch exceptionlock.release(); release lockthrow; rethrow the exception

}lock.release();

}

void $o%oo() {if (exception) throw err&xception;

}

en etter+ autoRptrT faci!ity% See CQQ Spec%

Can dea!!ocate/free !ock regard!ess of e4it met'od


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&aa Language Support for Sync'roni*ation

&aa 'as e4p!icit support for t'reads and t'read

sync'roni*ation ank 5ccount e4amp!e+class *ccount {private int +alance; o+#ect constructor

pu+lic *ccount (int initial,alance) {

+alance - initial,alance;}pu+lic synchronizedint get,alance() {

return +alance;}

pu+lic synchronized void deposit(int amount) {+alance - amount;

}}

ery o"ject 'as an associated !ock )'ic' getsautomatica!!y acuired and re!eased on entry and e4itfrom a sync'roni*ed met'od%


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&aa Language Support for Sync'roni*ation ?con8t@

&aa a!so 'as sync'roni*ed statements+s'nchroni/ed (o+#ect) {

}

Since eery &aa o"ject 'as an associated !ock# t'istype of statement acuires and re!eases t'e o"ject8s!ock on entry and e4it of t'e "ody

Works proper!y een )it' e4ceptions+

s'nchroni/ed (o+#ect) {$o%oo();

}void $o%oo() {

throw err&xception;}


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&aa Language Support for Sync'roni*ation ?con8t 2@

Bn addition to a !ock# eery o"ject 'as a sing!econdition aria"!e associated )it' it

Do) to )ait inside a sync'roni*ation met'od of "!ock+ void wait(long timeout); 0ait for timeout void wait(long timeout1 int nanoseconds); variant void wait();

Do) to signa! in a sync'roni*ed met'od or "!ock+

void notif'(); wakes up oldest waiter void notif'*ll(); like +roadcast1 wakes ever'one

Condition aria"!es can )ait for a "ounded !engt' oftime% T'is is usefu! for 'and!ing e4ception cases+

t2 - time.now();while (3*45Request()) {

wait (6&"7&R8!$);t9 - time.new();if (t9 : t2 ?485&) check5achine();

}

=ot a!! &aa 73s euia!entA ifferent sc'edu!ing po!icies# not necessari!y preemptieA


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esources M passie entities needed "y t'reads to do t'eir)ork

CP. time# disk space# memory

T)o types of resources+

Preempta"!e M can take it a)ay

CP.# m"edded security c'ip

=on:preempta"!e M must !eae it )it' t'e t'read

isk space# p!otter# c'unk of irtua! address space

3utua! e4c!usion M t'e rig't to enter a critica! section

esources may reuire e4c!usie access or may "e s'ara"!e

ead:on!y fi!es are typica!!y s'ara"!e

Printers are not s'ara"!e during time of printing

One of t'e major tasks of an operating system is tomanage resources

esources


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Staration s ead!ock

Staration s% ead!ock Staration+ t'read )aits indefinite!y

4amp!e# !o):priority t'read )aiting for resources constant!y inuse "y 'ig':priority t'reads ead!ock+ circu!ar )aiting for resources

T'read 5 o)ns es 1 and is )aiting for es 2T'read o)ns es 2 and is )aiting for es 1

ead!ock Staration "ut not ice ersa Staration can end ?"ut doesn8t 'ae to@

ead!ock can8t end )it'out e4terna! interention

es 2es 1

T'read

T'read5

Wait0or

Wait0or

O)nedy

O)nedy


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Conditions for ead!ock

ead!ock not a!)ays deterministic M 4amp!e 2 mute4es+4hread * 4hread ,

x.7(); '.7();'.7(); x.7();

'.@(); x.@();

x.@(); '.@();

ead!ock )on8t a!)ays 'appen )it' t'is code Dae to 'ae e4act!y t'e rig't timing ?F)rongH timingE@ So you re!ease a piece of soft)are# and you tested it# and

t'ere it is# contro!!ing a nuc!ear po)er p!antI

ead!ocks occur )it' mu!tip!e resources

3eans you can8t decompose t'e pro"!em Can8t so!e dead!ock for eac' resource independent!y

4amp!e+ System )it' 2 disk dries and t)o t'reads ac' t'read needs 2 disk dries to function

ac' t'read gets one disk and )aits for anot'er one


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ridge Crossing 4amp!e

ac' segment of road can "e ie)ed as a resource Car must o)n t'e segment under t'em

3ust acuire segment t'at t'ey are moing into 0or "ridge+ must acuire "ot' 'a!es

Traffic on!y in one direction at a time Pro"!em occurs )'en t)o cars in opposite directions on"ridge+ eac' acuires one segment and needs ne4t

Bf a dead!ock occurs# it can "e reso!ed if one car "acksup ?preempt resources and ro!!"ack@ Seera! cars may 'ae to "e "acked up

Staration is possi"!e ast:going traffic rea!!y fast no one goes )est


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Train 4amp!e ?Worm'o!e:outed =et)ork@

Circu!ar dependency ?ead!ockA@ ac' train )ants to turn rig't

!ocked "y ot'er trains Simi!ar pro"!em to mu!tiprocessor net)orks

0i4E Bmagine grid e4tends in a!! four directions 0orce ordering of c'anne!s?tracks@

Protoco!+ 5!)ays go east:)est first# t'en nort':sout'

Ca!!ed Fdimension orderingH ?U t'en G@

isa!!o)

ed

/y1u!e


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ining La)yers Pro"!em

0ie c'opsticks/0ie !a)yers ?rea!!y c'eap restaurant@ 0ree:for a!!+ La)yer )i!! gra" any one t'ey can =eed t)o c'opsticks to eat

W'at if a!! gra" at same timeE ead!ockA

Do) to fi4 dead!ockE 3ake one of t'em gie up a c'opstick ?Da'A@ entua!!y eeryone )i!! get c'ance to eat

Do) to preent dead!ockE =eer !et !a)yer take !ast c'opstick if no 'ungry

!a)yer 'as t)o c'opsticks after)ards


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0our reuirements for ead!ock

3utua! e4c!usion On!y one t'read at a time can use a resource%

Do!d and )ait T'read 'o!ding at !east one resource is )aiting toacuire additiona! resources 'e!d "y ot'er t'reads

=o preemption

esources are re!eased on!y o!untari!y "y t'e t'read'o!ding t'e resource# after t'read is finis'ed )it' it

Circu!ar )ait T'ere e4ists a set VT1# I# Tn of )aiting t'reads

T1is )aiting for a resource t'at is 'e!d "y T

2 T2is )aiting for a resource t'at is 'e!d "y T I Tnis )aiting for a resource t'at is 'e!d "y T1


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Sym"o!s

esource:5!!ocation


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esource 5!!ocation


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3et'ods for Dand!ing ead!ocks

5!!o) system to enter dead!ock and t'en recoer euires dead!ock detection a!gorit'm

Some tec'niue for forci"!y preempting resourcesand/or terminating tasks

nsure t'at system )i!! neerenter a dead!ock =eed to monitor a!! !ock acuisitions

Se!ectie!y deny t'ose t'at mig't!ead to dead!ock

Bgnore t'e pro"!em and pretend t'at dead!ocksneer occur in t'e system .sed "y most operating systems# inc!uding .=BU


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T1

T2

T

2

1

T;

ead!ock etection 5!gorit'm

On!y one of eac' type of resource !ook for !oops 3ore


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W'at to do )'en detect dead!ockE

Terminate t'read# force it to gie up resources Bn ridge e4amp!e#


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Summary

Suggestions for dea!ing )it' Project Partners Start ar!y# 3eet Often

ee!op


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Summary ?2@

Tec'niues for addressing ead!ock 5!!o) system to enter dead!ock and t'en recoer nsure t'at system )i!! neerenter a dead!ock Bgnore t'e pro"!em and pretend t'at dead!ocks neeroccur in t'e system

ead!ock detection

5ttempts to assess )'et'er )aiting grap' can eermake progress

=e4t Time+ ead!ock preention 5ssess# for eac' a!!ocation# )'et'er it 'as t'epotentia! to !ead to dead!ock

anker8s a!gorit'm gies one )ay to assess t'is

lec09 deadlock

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