lecture14 locking

7/21/2019 Lecture14 Locking

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CSE 332 Data Abstractions:

Data Races and Memory,

Reordering, Deadlock,Readers/Writer Locks, andCondition Variables (oh my!)

Kate DeibelSummer 2012

August 6, 2012 CSE 332 Data Abstractions, Summer 2012 1


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THE FINAL EXAM

*ominous music*



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The FinalIt is next Wenesa!, August 1"

It #ill ta$e u% t&e entire class %erio

Is it com%re&ensi'e( )es an o

Will %rimaril! call u%on onl! #&at #e co'ere sincet&e miterm +starting at sorting u% t&roug& next

ona!-s lecture on minimum s%anning trees. Still, !ou #ill nee to unerstan algorit&mic

anal!sis, big/&, an best#orst/case or an! atastructures #e &a'e iscusse

)ou #ill be oing tree or &ea% mani%ulationsbut !ou ma! +i4e4, #ill. o some gra%& algorit&ms

5ul! 11, 2012 CSE 332 Data Abstractions, Summer 2012 3


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Specific TopicsAlt&oug& t&e inal is b! no means inalie, $no#ingt&e ollo#ing #oul be goo7 8o# to o 9ig/& +!es, again:.

9est an #orst case or all ata structures an algorit&ms #eco'ere

Sorting algorit&m %ro%erties +in/%lace, stable.

;ra%& re%resentations o%ological sorting

Dieuctions

=arallel =reix, =ac$, an Sorting

?or$5oin @ibrar! coe

Ke! ieas &ig&/le'el notions o concurrenc!

5ul! 11, 2012 CSE 332 Data Abstractions, Summer 2012


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Book, Calculator, and Notes

&e exam is close boo$

)ou can bring a calculator i !ou #ant

)ou can bring a limite set o notes7

ne 3x" inex car +bot& sies.

ust be &an#ritten +no t!%ing:.

)ou must turn in t&e car #it& !our exam

5ul! 11, 2012 CSE 332 Data Abstractions, Summer 2012 "


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MORE ON RACECONDITIONS

Some &orses li$e #et trac$s or r! trac$s or mu! trac$sB



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RacesA race conitionoccurs #&en t&e com%utation result e%enson sc&euling +&o# t&reas are interlea'e on 1 %rocessors.

nl! occurs i 1 an 2 are sc&eule in a %articular #a! As %rogrammers, #e cannot control t&e sc&euling o t&reas

=rogram correctness must be ine%enent o sc&euling

>ace conitions are bugs t&at exist onl! ue to concurrenc! o interlea'e sc&euling #it& 1 t&rea

!%icall!, t&e %roblem is some intermediate statet&at messesu% a concurrent t&rea t&at sees t&at state

We #ill istinguis& bet#een ata races an ba interlea'ings,bot& o #&ic& are t!%es o race conition bugs

August 6, 2012 CSE 332 Data Abstractions, Summer 2012


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Data RacesA ata race is a t!%e o race conditiont&at can&a%%en in t#o #a!s7 #o t&reaspotentiall#rite a 'ariable at t&e same time

ne t&reapotentiall#rite a 'ariable #&ile anot&er reas

ot a race7 simultaneous reas %ro'ie no errors

=otentiall! is im%ortant We claim t&at coe itsel &as a ata race ine%enent o an!

%articular actual execution

Data races are ba, but t&e! are not t&e onl! orm orace conitions We can &a'e a race, an ba be&a'ior, #it&out an! ata race

August 6, 2012 CSE 332 Data Abstractions, Summer 2012 F


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Stack Example

August 6, 2012 CSE 332 Data Abstractions, Summer 2012 G

classStack {

privateE[] array= (E[])newObject[SIZE]; int ine!= "#;

sync$r%ni&eb%%lean isE'pty() {

retrnine!=="#;

sync$r%ni&e v%ips$(E val) {

array[**ine!] = val;

sync$r%ni&e Ep%p() {

i+(isE'pty()) t$r%w newStackE'ptyE!cepti%n();

retrnarray[ine!""];


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A Race Condition: But Not a Data Race

In a seHuential #orl,t&is coe is o i!,ugl!, an Huestionablestle, but correct

&e algorit&m is t&eonl! #a! to #rite a

peek&el%er met&o it&is interace is all !ou

&a'e to #or$ #it&

classStack {

,

sync$r%ni&eb%%lean isE'pty() {,

sync$r%ni&e v%ips$(E val) {,

sync$r%ni&e Ep%p(E val) {,

Epeek() {

E ans= p%p(); ps$(ans);

retrnans;


ote t&at %ee$+. t&ro#st&e Stac$Em%t! exce%tion'ia its call to %o%+.


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peekin a Concurrent Contextpeek&as no o!eralleect on t&e s&are ata It is a reaer not a #riter

State s&oul be t&e same ater it executes as beore

&is im%lementation creates an inconsistentintermediate state Calls tops$anp%pare s!nc&ronie,so t&ere are no

!ata raceson t&e unerl!ing arra!

9ut t&ere is still a race con!ition

&is intermeiate state

s&oul not be ex%ose @eas to se'eral

"ad interlea!in#s

Epeek() { E ans= p%p();

ps$(ans);

retrnans;



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Example $: peek and isEmpt

=ro%ert! #e #ant7

I t&ere &as been aps$+an nop%p),t&en isE'ptys&oul return +alse

Wit&peekas #ritten, %ro%ert! can be

'iolate &o#(

E ans= p%p();

ps$(ans);

retrnans;

ps$(!)

b%%leanb= isE'pty()3ime

&rea 2&rea 1 +peek.



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Example %: peek and push

=ro%ert! #e #ant7

Jalues are returne rom %o% in @I? orer

Wit&peekas #ritten, %ro%ert! can be'iolate &o#(

E ans= p%p();

ps$(ans);

retrnans;

3ime

&rea 2&rea 1 +peek.


ps$(!)

ps$(y)E e= p%p()


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=ro%ert! #e #ant7

Jalues are returne rom %o% in @I? orer

Wit&peekas #ritten, %ro%ert! can be'iolate &o#(

Example %: peek and push

E ans= p%p();

ps$(ans);

retrnans;

3ime

&rea 2&rea 1 +peek.

August 6, 2012 CSE 332 Data Abstractions, Summer 2012 1"

ps$(!)

ps$(y)E e= p%p()

Race ca"ses error #it$: "# $ush(%) "& $o$() "# $ush(%)

"& $ush(%)


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Example &: peek and peek

=ro%ert! #e #ant7

peekoes not t&ro# an exce%tion unlesst&e stac$ is em%t!

Wit&peekas #ritten, %ro%ert! can be

'iolate &o#(

E ans= p%p();

ps$(ans);

retrnans;

3ime

&rea 2&rea 1 +peek.


E ans= p%p();

ps$(ans);

retrnans;


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The Fixpeeknees s!nc&roniation to isallo# interlea'ings

&e $e! is to ma$e a lar#er critical section &is %rotects t&e intermeiate state o peek

se re/entrant loc$sL #ill allo# calls tops$anp%p

Can be one in stac$ +let. or an external class +rig&t.


classStack {

,

sync$r%ni&eEpeek(){

E ans= p%p();

ps$(ans); retrnans;

class-{

E'y.eek(Stack s){

sync$r%ni&e (s) {

E ans= s/p%p();

s/ps$(ans); retrnans;


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An 'ncorrect (Fix(So ar #e &a'e ocuse on %roblems create #&en

peek%erorms #ritest&at lea to an incorrect

intermeiate state

A tem%ting but incorrect %ers%ecti'e I an im%lementation opeekoes not #rite an!t&ing,

t&en ma!be #e can s$i% t&e s!nc&roniation(

Does not#or$ ue to data races#it&ps$anp%p Same issue a%%lies #it& ot&er reaers, suc& as isE'pty

August 6, 2012 CSE 332 Data Abstractions, Summer 2012 1F


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Another 'ncorrect Example

classStack {

privateE[] array= (E[])newObject[SIZE]; int ine!= "#;

b%%lean isE'pty() { 00 nsync$r%ni&e1 wr%n234

retrnine!=="#;

sync$r%ni&e v%ips$(E val) {

array[**ine!] = val;

sync$r%ni&e Ep%p() {

retrnarray[ine!""];

Epeek() { 00 nsync$r%ni&e1 wr%n24

retrnarray[ine!];

August 6, 2012 CSE 332 Data Abstractions, Summer 2012 1G


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)h )ron#*It looks likeisE'ptyanpeekcan get a#a! #it&t&is becauseps$anp%pa


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ettin# 't Ri#ht

A'oiing race conitions on s&are resources

is iicult Decaes o bugs &a'e le to some con'entional

#isom an general tec&niHues $no#n to #or$

We #ill iscuss some $e! ieas an trae/os ore a'ailable in t&e suggeste aitional reaings

one o t&is is specificto 5a'a or a %articular boo$

a! be &ar to a%%reciate in beginning

Come bac$ to t&ese guielines o'er t&e !ears

Do not tr! to be anc!



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%OIN% F&RTHER 'ITHEXCL&SION AND LOC(IN%

)ale ni'ersit! is t&e best %lace to stu! loc$sB



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Three Choices for -emor

?or e'er! memor! locationin !our %rogram

+e4g4, ob


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Thread./ocalW&ene'er %ossible, o not s&are resources: Easier or eac& t&rea to &a'e its o#n t&rea/local co%! o a

resource instea o one #it& s&are u%ates Correct onl! i t&reas o not communicate t&roug& resource

In ot&er #ors, multi%le co%ies are correct a%%roac&

Exam%le7 5an%'ob


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'mmuta"leW&ene'er %ossible, o not u%ate ob


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E!erthin# Else: 1eep it Snchroni0ed

Ater minimiing t&e amount o memor! t&at is bot&+1. t&rea/s&are an +2. mutable, #e nee to ollo#

guielines or using loc$s to $ee% t&at ata consistent

;uieline N07 o ata races

e'er allo# t#o t&reas to rea#rite or

#rite#rite t&e same location at t&e same timeNecessar7

In 5a'a or C, a %rogram #it& a ata race is almostal#a!s #rong

But Not Sufficient7

urpeekexam%le &a no ata races



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Consistent /ockin#;uieline N17 Consistent @oc$ing

?or eac& location t&at reHuires s!nc&roniation, #es&oul &a'e a loc$ t&at is al#a!s &el #&en reaingor #riting t&e location We sa! t&e loc$ guarst&e location

&e same loc$ can guar multi%le locations +an oten s&oul.

Clearl! ocument t&e guar or eac& location

In 5a'a, t&e guar is oten t&e ob


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Consistent /ockin#&e ma%%ing rom locations to guaring loc$s is conceptual,anmust be enorce b! !ou as t&e %rogrammer

It %artitions t&e s&are/O/mutable locations into #&ic& loc$

Consistent loc$ing is7

ot Suicient7It %re'ents all ata races, but still allo#s ba interlea'ings

ur %ee$ exam%le use consistent loc$ing, but &a ex%oseintermeiate states an ba interlea'ings

ot ecessar!7

Can !namicall! c&ange t&e loc$ing %rotocol

August 6, 2012 CSE 332 Data Abstractions, Summer 2012 2F


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Beond Consistent /ockin#Consistent loc$ing is an excellent guieline

A eault assum%tion about %rogram esign

)ou #ill sa'e !oursel man! a &eaac&e using t&is guieline

9ut it is not reHuire or correctness7Dierentpro#ram phasescan use ierent loc$ing tec&niHues

=ro'ie all t&reas coorinate mo'ing to t&e next %&ase

Exam%le rom =ro


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LOC( %RAN&LARIT)

W&ole/grain loc$s are better t&an o'erl! %rocesse loc$sB


k l


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/ock ranularitCoarserained?e#er loc$s +more ob


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Trade.2ffsCoarse/graine a'antages Sim%ler to im%lement

?astereasier to im%lement o%erations t&at access multi%lelocations +because all guare b! t&e same loc$.

Easier to im%lement moiications o ata/structure s&a%e

?ine/graine a'antages ore simultaneous access +im%ro'es %erormance

#&en coarse/graine #oul lea to unnecessar! bloc$ing.

;uieline N27 @oc$ ;ranularit!Start #it& coarse/graine +sim%ler., mo'e to ine/graine+%erormance. onl! i contentionon coarse loc$s is an issue4Alas, oten leas to bugs4


E l S Ch i i 3 h "l


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Example: Separate Chainin# 3ashta"le

Coarse/graine7 ne loc$ or entire &as&table

?ine/graine7 ne loc$ or eac& buc$et

W&ic& su%%orts more concurrenc! or insertan l%%kp(

?ine/graineL allo#s simultaneous access to ierentbuc$ets

W&ic& ma$es im%lementing resi&eeasier(

Coarse/graineL


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Critical.Section ranularitA secon, ort&ogonal granularit! issue is t&e sie ocritical/sections

8o# muc& #or$ s&oul #e o #&ile &oling loc$+s.

I critical sections run or too long7

=erormance loss as ot&er t&reas are bloc$e

I critical sections are too s&ort7

9ugs li$el! as !ou bro$e u% somet&ing #&ere ot&ert&reas s&ouln-t be able to see intermeiate state

;uieline N37 ;ranularit!Do not o ex%ensi'e com%utations or I in criticalsections, but also o not introuce race conitions


E l C iti l S ti l it


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Example: Critical.Section ranularit

Su%%ose #e #ant to c&ange t&e 'alue or a $e!in a &as&table #it&out remo'ing it rom t&e table

Assume l%ckguars t&e #&ole table

August 6, 2012 CSE 332 Data Abstractions, Summer 2012 3"

sync$r%ni&e(l%ck) {

v# = table/l%%kp(k);

v6 = e!pensive(v#);

table/re'%ve(k);

table/insert(k7v6);

4apa Bear5s criticalsection +as too lon#

Ta"le is locked durin#

the expensi!e call



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sync$r%ni&e(l%ck) {

v# = table/l%%kp(k);

v6 = e!pensive(v#);

sync$r%ni&e(l%ck) { table/re'%ve(k);

table/insert(k7v6);

-ama Bear5s critical section+as too short

'f another thread updated

the entr, +e +ill lose theinter!enin# update



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%ne = +alse;

w$ile(4%ne) {

sync$r%ni&e(l%ck) { v# = table/l%%kp(k);

v6 = e!pensive(v#);

sync$r%ni&e(l%ck) {

i+(table/l%%kp(k)==v#) {

%ne = tre;

table/re'%ve(k);

table/insert(k7v6);

Ba" Bear5s criticalsection +as 6ust ri#ht

if another updateoccurred, +e +ill trour update a#ain

At i it


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AtomicitAn o%eration is atomici no ot&er t&rea can see it%artl! execute Atomic as in a%%ears ini'isible We t!%icall! #ant AD o%erations atomic, e'en to ot&er

t&reas running o%erations on t&e same AD

;uieline N7 Atomicit! &in$ in terms o #&at o%erations nee to be atomic

a$e critical sections


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Do Not Roll 7our 2+nIn real lie, !ou rarel! #rite !our o#n ata structures Excellent im%lementations %ro'ie in stanar libraries

=oint o CSE 332 is to unerstan t&e $e! trae/os,abstractions, an anal!sis o suc& im%lementations

Es%eciall! true or concurrent ata structures

?ar too iicult to %ro'ie ine/graine s!nc&roniation#it&out race conitions

Stanar t&rea/saelibraries li$e -%ncrrent8as$9apare#ritten b! #orl ex%erts an been extensi'el! 'ette

;uieline N"7 @ibraries8se "uilt.in li"raries +hene!er the meet our needs

August 6, 2012 CSE 332 Data Abstractions, Summer 2012 3G

-oti!atin# -emor -odel 'ssues


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-oti!atin# -emor.-odel 'ssues

ric$! an surprisin#l +ron#uns!nc&ronie

concurrent coe ?irst unerstan #&! it loo$sli$e t&e assertion cannot ail7

Eas! case7

A call to g ens beore an! callto starts

Eas! case7At least one call to com%letes

beore call to g starts

I calls to an g interlea'eB


classC{

privateint x = 0;

privateint y= 0;

void f() {

x = 1;

y = 1;

}

void g() {

int a= y;

int b= x;

assert(b >= a);

}

'nterlea!in#s Are Not Enou#h


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'nterlea!in#s Are Not Enou#h&ere is no interlea'ing o +an 2suc& t&at t&eassertion ails

=roo N17Ex&austi'el! consier all %ossible orerings o accessto s&are memor! +t&ere are 6.


'nterlea!in#s Are Not Enou#h


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'nterlea!in#s Are Not Enou#h=roo N27Ex&austi'el! consier all %ossible orerings o access

to s&are memor! +t&ere are 6.

I 4(b>=a), t&en a==#anb==:49ut i a==#, t&en y=#&a%%ene beore a=y49ecause %rograms execute in orer7

a=y&a%%ene beoreb=!an !=#&a%%ene beore y=#

So b! transiti'it!,b==#4

Contraiction4


! = #;

y = #;

int a= y;

intb= !;

assert(b >= a);

&rea 17 f &rea 27 g

)ron#


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)ron#

8o#e'er, t&e coe &as a data race ns!nc&ronie rea#rite or #rite#rite o t&e

memor! same location

I coe &as ata races, !ou cannot reason

about it #it& interlea'ings &is is sim%l! t&e rules o 5a'a +an C, CPP,

CN, ot&er languages.

t&er#ise #e #oul slo# o#n all %rograms


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)h?or %erormance reasons, t&e com%iler an t&e&ar#are #ill oten reorer memor! o%erations a$e a com%iler or com%uter arc&itecture course to learn

more as to #&! t&is is goo t&ing

course, com%ilers cannot = a);

&rea 17 f &rea 27 g

The rand Compromise


45/80

The rand Compromise&e com%iler&ar#are #ill EJE>7

=erorm a memor! reorering t&at aects t&e result o a

single/t&reae %rogram =erorm a memor! reorering t&at aects t&e result o a

ata/race/reemulti/t&reae %rogram

So7 I no interlea'ing o !our %rogram &as a ata race,

t&en !ou can for#et a"out all this reorderin# nonsense:t&e result #ill be eHui'alent to some interlea'ing

&e 9ig =icture7

)our


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Fixin# 2ur Exampleaturall!, #e can use s!nc&roniation to a'oi ataraces an t&en, inee, t&e assertion cannot ail


class-{

privateint ! = :;

privateint y= :;

v%i +() {

sync$r%ni&e(t$is) { ! = #;

sync$r%ni&e(t$is) { y = #;

v%i 2() {

int a7b; sync$r%ni&e(t$is) { a = y;

sync$r%ni&e(t$is) { b = !;

assert(b >= a);

A Second Fix: Sta A+a from This


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A Second Fix: Sta A+a from This5a'a &as v%latileiels7 accesses o not count as ata races

9ut !ou cannot rea/u%ate/#rite

Im%lementation Details

Slo#er t&an regular iels but aster t&an loc$s

>eall! or ex%erts7 a'oi t&emL use stanar libraries instea

An #&! o !ou nee coe li$e t&is an!#a!(

August 6, 2012 CSE 332 Data Abstractions, Summer 2012

class-{

private v%latileint ! = :;

private v%latileint y= :;

v%i +() {

! = #; y = #;

v%i 2() {

int a= y; intb= !;

assert(b >= a);

Code That is )ron#


48/80

Code That is )ron#8ere is a more realistic exam%le o coe t&at is #rong

o #uarantee&rea 2 #ill e!ersto% +ue to ata race.

9ut &onestl! it #ill li$el! #or$ in %ractice

August 6, 2012 CSE 332 Data Abstractions, Summer 2012 F

class-{

b%%lean st%p = +alse;

v%i +() {

w$ile(4st%p) {

00 raw a '%nster

v%i 2() {

st%p = iserit();

"hread & +()

"hread # 2()


49/80

DEADLOC(

ot nearl! as sill! as Deat&lo$ rom ar'el comicsB

August 6, 2012 CSE 332 Data Abstractions, Summer 2012 G

-oti!atin# Deadlock 'ssues


50/80

-oti!atin# Deadlock 'ssuesConsier t&e ollo#ing met&o or transering mone!bet#een ban$ accounts

During call to a4e%osit, t&e t&rea &ols t#o loc$s

@et-s in'estigate #&en t&is ma! be a %roblem

August 6, 2012 CSE 332 Data Abstractions, Summer 2012 "0

classank?cc%nt{

,

sync$r%ni&e v%iwit$raw(int a't) {,

sync$r%ni&e v%i ep%sit(int a't) {,

sync$r%ni&e v%i trans+er@%(int a't7ank?cc%nta) {

t$is/wit$raw(a't);

a/ep%sit(a't);

The Deadlock


51/80

The Deadlock

Su%%ose !an yare iels &oling accounts


acquire lock for !

do withdraw from!

block on lock fory

acquire lock for ydo withdraw fromy

block on lock for !

&rea 17x.transfero(1!y)

3ime

&rea 27y.transfero(1!x)

The Dinin# 4hilosophers


52/80

The Dinin# 4hilosophers?i'e %&iloso%&ers go out to innertoget&er at an Italian restaurant

&e! sit at a roun tableL oneor$ %er %late setting

?or etiHuette reasons, t&e%&iloso%&ers nee t#o or$s

to eat s%ag&etti %ro%erl!

W&en t&e s%ag&etti comes,eac& %&iloso%&er %rocees tograb t&eir rig&t or$, t&en

t&eir let or$

Q@oc$ing- or eac& or$ results in a !ea!loc*


Deadlock


53/80

DeadlockA ealoc$ occurs #&en t&ere are t&reas "&, B, "n

suc& t&at7

?or iR1 to n/1, "iis #aiting or at least one

resource &el b! "i+&

"nis #aiting or a resource &el b! "&

In ot&er #ors, t&ere is a ccleo #aiting

ore ormall!, a gra%& o e%enencies is c!clic

Dealoc$ a'oiance in %rogramming amounts totec&niHues to ensure a c!cle can ne'er arise


Back to 2ur Example


54/80

Back to 2ur Example%tions or ealoc$/%roo transer7

14 a$e a smaller critical section7trans+er@%not s!nc&ronie

Ex%oses intermeiate state aterwit$rawbeore ep%sit

a! be o$a!, but ex%oses #rong total amount to ban$

24 Coarsen loc$ granularit!7ne loc$ or all accounts allo#ing transers bet#een t&em

Wor$s, but sacriices concurrent e%osits#it&ra#als

34 ;i'e e'er! ban$/account a uniHue number an al#a!s

acHuire loc$s in t&e same orer Entire pro#ram s&oul obe! t&is orer to a'oi c!cles

Coe acHuiring onl! one loc$ can ignore t&e orer

August 6, 2012 CSE 332 Data Abstractions, Summer 2012 "

2rderin# /ocks


55/80

2rderin# /ocks

August 6, 2012 CSE 332 Data Abstractions, Summer 2012 ""

classank?cc%nt{

,

privateint acctA'ber; 00 'st be niBe v%itrans+er@%(int a't7ank?cc%nta) {

i+(t$is/acctA'ber < a/acctA'ber)

sync$r%ni&e(t$is) {

sync$r%ni&e(a) {

t$is/wit$raw(a't);

a/ep%sit(a't);

else

sync$r%ni&e(a) { sync$r%ni&e(t$is) {

t$is/wit$raw(a't);

a/ep%sit(a't);

Strin#Buffer Example


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Strin#Buffer Example?rom t&e 5a'a stanar librar!


classStrin2++er{privateint c%nt;

privatec$ar[] vale;

,

sync$r%ni&eappen(Strin2++er sb) {

int len= sb/len2t$(); i+(t$is/c%nt * len > t$is/vale/len2t$)

t$is/e!pan(,);

sb/2et-$ars(:7len7t$is/vale7t$is/c%nt);

,

sync$r%ni&e2et-$ars(int !7 int7 y7

c$ar[] a7 int &) {

"copy t$is/vale[!//y]into astarting at &"

T+o 4ro"lems


57/80

T+o 4ro"lems=roblem N17

@oc$ or sbnot &el bet#een calls to sb/len2t$an sb/2et-$ars

So sbcoul get longer

Woul cause appento t&ro# an?rray%nsE!cepti%n

=roblem N27

Dealoc$ %otential i t#o t&reas tr! to appenin o%%osite

irections, ientical to t&e ban$/account irst exam%le

ot eas! to ix bot& %roblems #it&out extra co%!ing7

Do not #ant uniHue is on e'er! Strin2++er

Do not #ant one loc$ or all Strin2++erob


58/80

4erspecti!eCoe li$e account/transer an string/buer a%%enare iicult to eal #it& or ealoc$

Easier case7 ierent t!%es o ob


59/80

IM+RO,IN% LITERAC):READER-'RITER LOC(S

We encourage multi%le reaersB

August 6, 2012 CSE 332 Data Abstractions, Summer 2012 "G

Readin# !s )ritin#


60/80

Readin# !s9 )ritin#>ecall7 ulti%le concurrent reas o same memor!7 Nota %roblem

ulti%le concurrent #rites o same memor!7 =roblem ulti%le concurrent rea O #rite o same memor!7 =roblem

So ar7 I concurrent #rite#rite or rea#rite mig&t occur, use

s!nc&roniation to ensure one/t&rea/at/a/time

9ut t&is is unnecessaril! conser'ati'e7

Coul still allo# multi%le simultaneous reaers:

Example


61/80

ExampleConsier a &as&table #it& one coarse/graine loc$

nl! one t&rea can %erorm o%erations at a time

9ut su%%ose7

&ere are man! simultaneous l%%kpo%erations

An inserto%erations are 'er! rare

ote7Criticall! im%ortant t&at l%%kpoes not actuall!

mutate s&are memor!, li$e a mo'e/to/ront list ors%la! tree o%eration #oul

Readers)riter locks


62/80

Readers)riter locksA ne# s!nc&roniation AD7 t&e reaers#riter loc$

A loc$s states all into t&ree categories7 Tnot &elU

T&el or #ritingU b! one t&rea

T&el or reaingU b! one or moret&reas

%erations7

new1 ma$e a ne# loc$, initiall! Tnot &elU acBireCwrite1 bloc$ i currentl! T&el or reaingU or i T&el

or #ritingU, else ma$e T&el or #ritingU

releaseCwrite1 ma$e Tnot &elU

acBireCrea1 bloc$ i currentl! T&el or #ritingU, elsema$e$ee% T&el or reaingU an increment readers count

releaseCrea1 ecrement reaers count, i 0, ma$e Tnot&elU

AD In'ariants7

0 #riters 10 reaers#riters reaersRR0

4seudocode Example ;not


63/80

4seudocode Example ;not


64/80

Readers)riter /ock DetailsA reaers#riter loc$ im%lementation +#&ic& is Tnot our%roblemU. usuall! gi'esprioritto #riters7

Ater a #riter bloc$s, no reaers arri!in# later#ill get t&eloc$ beore t&e #riter

t&er#ise an insertcoul star!e

>e/entrant +same t&rea acHuires loc$ multi%le times.(

ostl! an ort&ogonal issue 9ut some libraries su%%ort up#radin#rom reaer to #riter

W&! not use reaers#riter loc$s #it& more ine/graineloc$ing( @i$e on eac& buc$et(

ot #rong, but li$el! not #ort& it ue to lo# contention

'n


65/80

'n


66/80

CONDITION ,ARIA.LES

&e natural successor to s&am%oo 'ariables


-oti!atin# Condition >aria"les


67/80

-oti!atin# Condition >aria"les

o moti'ate conition 'ariables, consier t&e canonical exam%le

o a boune bueror s&aring #or$ among t&reas

9oune buer7 A Hueue #it& a ixe sie

nl! slig&tl! sim%ler i unboune, core nee still arises

?or s&aring #or$ t&in$ an assembl! line7 =roucer t&rea+s. o some #or$ an enHueue result ob


68/80

First Attempt

class++er {

E[] array= (E[])newObject[SIZE]; , 00 +r%nt7 back +iels7 isE'pty7 isHll 'et$%s

sync$r%ni&ev%i enBee(E elt) {

i+(isHll())

333

else

, add to array and adjust back

sync$r%ni&eE eBee()

i+(isE'pty()) 333

else

, take from array and adjust front

)aitin#


69/80

)aitin#enBeeto a ull buer s&oul notraise an exce%tionbut s&oul #aituntil t&ere is room

eBeerom an em%t! buer s&oul notraise anexce%tion but s&oul #aituntil t&ere is ata

ne baa%%roac&is to spin.+ait+#aste #or$ an

$ee% grabbing loc$.v%i enBee(E elt) {

w$ile(tre) {

sync$r%ni&e(t$is) {

i+(isHll()) c%ntine; , add to array and adjust back

retrn;

00 eBee si'ilar

)hat +e )ant


70/80

at e a t9etter #oul be or a t&rea to sim%l!+aituntil it can %rocee

It s&oul not s%in%rocess continuousl!

Instea, it s&oul be notified#&en it s&oul tr! again In t&e meantime, let ot&er t&reas run

@i$e loc$s, not somet&ing !ou can im%lement on !our o#n

@anguage or librar! gi'es it to !ou, t!%icall! im%lemente#it& o%erating/s!stem su%%ort

An AD t&at su%%orts t&is7 conition 'ariable

Inorms #aiter+s. #&en t&e conditiont&at causes itt&em to

#ait &as !aried

erminolog! not com%letel! stanarL #ill mostl! stic$ #it& 5a'a


71/80

pp ? #

class++er {

,


i+(isHll())

t$is/wait(); 00 releases l%ck an waits

add to array and adjust back

i+(buffer was empty) t$is/n%ti+y();00 wake s%'eb%y p

sync$r%ni&eE eBee() {

i+(isE'pty())

t$is/wait(); 00 releases l%ck an waits take from array and adjust front

i+(buffer was full)

t$is/n%ti+y();00 wake s%'eb%y p

1e 'deas 7ou Should 1no+


72/80

5a'a is a bit #eir7

E'er! ob


73/80

#

9et#een t&e time a t&rea is notiie an it re/acHuires

t&e loc$, t&e conition can become alse again:

sync$r%ni&ev%i enBee(E elt){

i+(isHll())

t$is/wait();

add to array and adjust back

,

i+(isHll())

t$is/wait();

add to array

3ime

&rea 2 +eHueue.&rea 1 +enHueue.

take from array

i+(was full)

t$is/n%ti+y();

make full again

&rea 3 +enHueue.

Bu# Fix


74/80

#

;uieline7Al+asre/c&ec$ t&e conition ater re/gaining t&e loc$?or obscure +::. reasons, 5a'a is tec&nicall! allo#e to noti! at&rea spuriousl+i4e4, or no reason an #it&out actuall!ma$ing a call to n%ti+y.


w$ile(isHll())

t$is/wait();


w$ile(isE'pty())

t$is/wait();

,

Another Bu#


75/80

#

I multi%le t&reas are #aiting, #e #a$e u% onl! one

Sure onl! one can o #or$ no+, but #e cannotorget t&e ot&ers:

3ime

w$ile(isHll()) t$is/wait();

,

&rea 1 +enHueue.

00 eBee #

i+(buffer was full)

t$is/n%ti+y();

00 eBee 6

i+(buffer was full)

t$is/n%ti+y();

&rea 3 +eHueues.&rea 2 +enHueue.

w$ile(isHll()) t$is/wait();

,

Bu# Fix


76/80

#

n%ti+y?ll#a$es u% all current #aiters on t&e conition 'ariable

;uieline7 I in an! oubt, use n%ti+y?ll

Wasteul #a$ing is muc& better t&an ne'er #a$ing u%+because !ou alrea! nee to re/c&ec$ conition.

So #&! oes n%ti+yexist( Well, it is aster #&en correctB


,

i+(buffer was empty) t$is/n%ti+y?ll();00 wake everyb%y p


,

i+(buffer was full

) t$is/n%ti+y?ll();00 wake everyb%y p

Alternate Approach


77/80

ppAn alternati'e is to call n%ti+y+not n%ti+y?ll. one'er! enBee eBee, not


78/80

ppAn alternate a%%roac& #or$s i t&e enHueuers aneHueuers #ait on differentconition 'ariables

9ut or mutual exclusion bot& conition 'ariablesmust be associate #it& t&e same loc$

5a'as Te'er!t&ing is a loc$ conition 'ariableU oes notsu%%ort t&is7 eac& conition 'ariable is associate #it& itsel

Instea, 5a'a &as classes in java/til/c%ncrrent/l%cksor#&en !ou #ant multi%le conitions #it& one loc$

class 5eentrantG%ck&as a met&o new-%niti%nt&atreturns a ne# -%niti%nobaria"le


79/80

n%ti+y0n%ti+y?lloten calle si2nal0br%acastorplse0plse?ll

Conition 'ariables are subtle an &arer to use t&an loc$s

9ut #&en !ou nee t&em, !ou nee t&em

S%inning an ot&er #or$arouns o not #or$ #ell

?ortunatel!, li$e most t&ings !ou see in a ata/structurescourse, t&e common use/cases are %ro'ie in libraries #rittenb! ex%erts an &a'e been t&oroug&l! 'ette

Exam%le7 java/til/c%ncrrent/?rrayl%ckin2ee

All conition 'ariables &ienL


80/80

Access to s&are resources introuces ne# $ins o bugs

Data races

Critical sections too small Critical sections use #rong loc$s

Dealoc$s

>eHuires s!nc&roniation

@oc$s or mutual exclusion +common, 'arious la'ors.

Conition 'ariables or signaling ot&ers +less common.

;uielines or correct use &el% a'oi common %italls

ot al#a!s clear s&are/memor! is #ort& t&e %ain 9ut ot&er moels not a %anacea +e4g4, message %assing.

lecture14 locking

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