Linux设备驱动第五章并发和竞争读书笔记Linux device drivers fifth chapters concurrency and compe.docx

Linux设备驱动第五章并发和竞争读书笔记Linux device drivers fifth chapters concurrency and compe.docx

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Linux设备驱动第五章并发和竞争读书笔记Linuxdevicedriversfifthchaptersconcurrencyandcompe

Linux设备驱动第五章（并发和竞争）读书笔记（Linuxdevicedrivers,fifthchapters（concurrencyandcompetition）,readingnotes）

Thefifthchapterisconcurrencyandcompetition

Referencearticle:

Id=408682?

5.3flagsandmutex

1）flagisthecoreofasingleintegervalue.Combinedwithapairoffunctions,alsoknownasPVoperations".

OnewantstoenterthecriticalsectionofthePprocesswiththeflagraised,iftheflagvalueisgreaterthan0,thisvaluedecreased1,andtheprocesscontinues.Iftheflagissmallthanorequalto0,indicatingthattheflaghasbeenoccupiedbyotherprocesses,theprocessmustwaituntiltheflagisreleased.

Unlocktheflag,flagisreleased,tocompletetheoperationbycallingtheVVoperation,increasingthevalueoftheflag,andtheflagcanwakeupwaitingprocess.

2）mutexfunctionis:

mutualexclusion,topreventmultipleprocessessimultaneouslyinthesamecriticalarea.Themutexisinitializedto1flags.Becausethatisinitializedto1,whenthefirstprocessofP,1>0,sotheprocesscanrunatthesametime,theflagvalueis0,1decline,thesecondprocesstoobtaintheflagisusedtoaccessthecriticalsection,butfoundthattheflagvalueis0,sotheprocesscannotrununtilthe2releaseoftheflagprocess1.

5.3.1implementationoftheLINUXflag

1）tousetheflag,musttypecontains.isrelatedtothestructsemaphore;

2）theflag:

createaflag,thenusesema_inittosetit

Suchas:

Structsemaphore*sem;//defineflag

Sema_init（SEM,Val）;//initializeValtoinitializethevalueofflagflag.

3）usually,flagisusedtomodelthemutex.Asaresult,thekernelprovidesaseriesofmacrostoinitialize.

DECLARE_MUTEX（name）;//initializeamutex,avalueof1

DECLARE_MUTEX_LOCKED（name）;//initializeamutex,avalueof0is:

flagafterinitializationisnotavailable.

Tousethisflaganyprocessmustfirstunlockitagain.

Whenweneedtoinitializeatruntime（thatis,dynamiccreation）,weusethe:

Voidinit_MUTEX（structsemaphore*sem）;//initializeamutex,avalueof1

Voidinit_MUTEX_LOCKED（structsemaphore*sem）;initializesamutexwiththevalueof0

Forexample,wewanttocreateamutexthatisinitializedto0:

Structsemaphore*sem;//defineflag

ECLARE_MUTEX_LOCKED（SEM）;//initializeflagvalueis0.

Theaboveistheinitializationflag,afterinitializationofcoursewemustusetheflag,orcreateitwhatisthesignificance?

4）flagacquisitionandrelease（canalsobeamutex,becausebasicallyallflagsareusedtoLINUXmutex）

GettheflagalsoknownasP,alsocalleddown.Downliterallymeans"drop".Theflagisminus1.

ThePfunctionisasfollows:

Voiddown（structsemaphore*sem）;/*isnotrecommended,willestablishnotkillprocess*/

Intdown_interruptible（structsemaphore*sem）;/*recommendedtheuseofdown_interruptibleneedtobecareful,iftheoperationisinterrupted,thefunctionreturnsanonzerovalue,anditwillnothavethecallsignal.Theproperuseofdown_interruptiblerequiresalwayscheckingthereturnvalueandrespondingaccordingly.*/

Intdown_trylock（structsemaphore*sem）;/*with"_trylock"neversleeps,ifthesignalisnotavailableinthecall,willreturnanonzerovalue.*/

Weusuallyusethedown_interruptiblefunction.Recommendeddropsinthebook.

Oncetheflag,theflagoftheobtainingprocesscanaccessthecriticalareatoprotecttheflag.Whatdoyoudowhenyouuseit?

Ofcourseistocontrolthediscardedinthecriticalregion,isalsothereleaseflag.

ThereleaseflagalsoknownasV,alsocalledUPup,literallyrise.

Theflagis1.（downandupdescriptionoftheimageoftheflagoperation）.

TheVfunctionisasfollows:

Voidup（struct,semaphore,*sem）;

OncecalledUP,willnothavetheflagintheprocess.

5）useflagsareeasytomakemistakes

GettheprocesstoreleasetheflagflagusingthecalltoUP,butnotmanycallUP,thatis:

adownforaUP.

Intheholdflagwhenanerrorisencountered,wemustbeinthereturn（errorstate）istocallUPreleaseflag,otherwisethecriticalregionhasbeentheprocessofpossession,butperhapstheprocesshasbeenkill,andtheothertotheuseofcriticalareaswillbedueprocesshasnotbeensuspendedcriticalregion.

5.3.2.inscullusingtheflag

Thekeytothecorrectuseofthelockprimitiveistospecifyexactlywhatresourcestoprotectandconfirmthateachaccesstotheseresourcesusesthecorrectlockingmethod

Firstofall,lookatascullstructure:

Structscull_dev{

Structscull_qset*dataPointertofirstquantumset;/**/

Intquantumthecurrentquantumsize;/**/

Intqsetthecurrentarraysize;/**/

Unsignedlongsizeamountofdatastoredhere;/**/

Unsignedintaccess_keyusedbysculluidandscullpriv;/**/

StructsemaphoreSEMmutualexclusionsemaphore;/**/

StructCDEVCDEVChardevicestructure;/**/

};

StructsemaphoreSEMisourflag,andthisstructureistheobjectwewanttoprotectthe.

Flagmustbeinitializedbeforeuse.Scullthisinitializationatloadtime,inthiscycle:

For（I=0;I

Scull_devices[i].quantum=scull_quantum;

Scull_devices[i].qset=scull_qset;

Init_MUTEX（&scull_devices[i].sem）;

Scull_setup_cdev（&scull_devices[i],I）;

}

Wedefineascull_nr_devscharacterdriven,sowesetupascull_nr_devsflag,peopleask:

whynotsetaglobalflagonit?

Theansweristhateachcharacterdriverdoesnotshareresources.Forefficiency,thereisnoreasonforustouseoneoftheSCULLdevicesandotherprocessescannotuseothersculldevices.

Thislooptypicallyappearsinthe__initloadingfunctionthatdrivestheinitializationofvariablesandtheloadingofcharacterdevices.

Init_MUTEXiscalledinscull_setup_cdev.Intheoppositeorderofthisoperationmayproduceacompetitivesituation,theflagmaybeaccessedbeforeitisready.（note）.

Whentheinitializationfinished,wewanttousethedriver,wemustconfirmthatthereisnoaccesstothedatastructureofscull_devintheabsenceofholdingflag.Inotherwords:

onlyintheholdwhentheflagcanaccessthedata.

Inthescull_writecode,wecansee:

If（down_interruptible（&dev->sem））

Return-ERESTARTSYS;

Notethatthereturnvalueoftheexaminationofthedown_interruptible;ifitreturnsnon-zero,theoperationwasinterrupted.Usuallydointhiscaseistoreturnto-ERESTARTSYS.toseethereturnvalue,thekernelorrestartthecallfromthetoporreturntheerrortotheuser.Ifyoureturn-ERESTARTSYS,youmustfirstrestoreanyuservisiblehasbeendonetochange,toensurethatwhentherightthingshappenagainwhenthesystemcall.Ifyoucannotrecoverinthisway,youshouldbebackfor-EINTR.

Whenwefinishedusingthecriticalarea,thewirtefunctionmustreleasetheflag,

Whetheritsucceedsorfailswhenweusethewritefunction,forexample,kmallocfailstoallocatememory,andcopy_form_uesrfailsfromuserspacecopydata.Remember:

besuretoreleasetheflag.

Code：

Out:

Up（&dev->sem）;

Returnretval;

Whenyouencounteradifferenterror,usethegotostatementtojumptoout.

Also,youmustmakesurethatyoudonotaccessthescull_devstructurewhenyoudon'thavesemaphores.

5.3.3reader/writerflag

Flagforallcallerexclusive,butsometimeswecanthink:

whatistheflag?

Ingeneral,weusedintheaccesstothecriticalsection,becauseifthereisnoflag,canprocess1hasjustcompletedtherevisionofaprocessvariable,2andmodifiedthesamevariable,leadingtothelaterdataonthefrontofthedatatocover.Justliketheexamplesin5.1.scull.Butwhenwereadonly,weonlyallowoneprocesstoread,whichmakesitinefficient.Therefore,thereader/writerflag.

Thereader/writeflag:

wecanrealizetheconcurrentreadexclusivewrite,butonly.Improvedefficiency.Theread-onlytaskscanworkinparallelwithoutwaitingforotherreaderstoexitthecriticalarea

1）userwsemtoinclude.

2）initializearwsem.

Voidinit_rwsem（struct,rw_semaphore,*sem）;

3）functionasfollows:

Theinterfacethatrequiresread-onlyaccesstothecodeis:

Voiddown_read（struct,rw_semaphore,*sem）;

Intdown_read_trylock（struct,rw_semaphore,*sem）;

Voidup_read（struct,rw_semaphore,*sem）;

Providesread-onlyaccesstoprotectedresourcesofthedown_readcallcanaccessconcurrentlywithotherreaders.Notethatthedown_readmaybesettothecallingprocesscannotbeinterruptedsleep.Ifthedown_read_trylockisnotwaitingforthereadaccessisnotavailable;ifallowedtoaccessitreturnsnon-zero,otherwiseitis0.down_read_trylockconventiondifferentfrommostofthekernelfunction,areturnvalueof0indicatessuccess.Theuseofadown_readtoobtaintherwsemmusteventuallyuseup_readrelease.

Thereader'sinterfaceissimilar:

Voiddown_write（struct,rw_semaphore,*sem）;

Intdown_write_trylock（struct,rw_semaphore,*sem）;

Voidup_write（struct,rw_semaphore,*sem）;

Voiddowngrade_write（struct,rw_semaphore,*sem）;

Down_write,down_write_trylock,andup_writeallliketheirreaderscorrespondingpart,besides,ofcourse,theyprovidewriteaccess.Ifyouareinsuchasituation,theneedforawritelocktodoaquickchange,thenread-onlyaccessforalongtime,youcanusedowngrade_writeonceyouhavecompletedthechangeinallowotherreaderstoenter.

4）rwsemusesrelativelylittleindriving,butsometimestheyareuseful.Rwsemallowsmultiplereaderstoholdtheflag,andwriteapriority,whenawritertryingtogettheflag,itdoesnotallowreaderstoenteruntilthewritecompletesthework.Butitcanlead