linux internals

LinuxInternalsUnabletohandlekernelpagingrequestatvirtualaddress4d1b65e8 Unabletohandlekernelpagingrequestatvirtualaddress4d1b65e8 Covers pgd=c0280000 versions pgd=c0280000 / 2.6.17.7 2.4.32 [4d1b65e8]*pgd=00000000[4d1b65e8]*pgd=00000000 Internalerror:Oops:f5[#1] Internalerror:Oops:f5[#1] Moduleslinkedin:Moduleslinkedin:hx4700_udchx4700_udcasic3_baseasic3_base CPU:0 CPU:0 PCisatset_pxa_fb_info+0x2c/0x44 PCisatset_pxa_fb_info+0x2c/0x44 LRisathx4700_udc_init+0x1c/0x38[hx4700_udc] LRisathx4700_udc_init+0x1c/0x38[hx4700_udc] pc:[]lr:[]Nottainted sp:c076df78ip:60000093fp:c076df84 pc:[]lr:[]Nottainted

Copyright20062004,MichaelOpdenacker Copyright20032006,OronPeled Copyright20042006CodefidenceLtd.

Based on material by:

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RightstocopyThiskitcontainsoriginalworkbythe followingauthors:AttributionShareAlike2.0 Youarefree tocopy,distribute,display,andperformthework tomakederivativeworks tomakecommercialuseofthework Underthefollowingconditions Attribution.Youmustgivetheoriginalauthorcredit. ShareAlike.Ifyoualter,transform,orbuilduponthiswork, youmaydistributetheresultingworkonlyunderalicense identicaltothisone. Foranyreuseordistribution,youmustmakecleartoothersthe licensetermsofthiswork. Anyoftheseconditionscanbewaivedifyougetpermissionfrom thecopyrightholder. Yourfairuseandotherrightsareinnowayaffectedbytheabove. Licensetext:http://creativecommons.org/licenses/bysa/2.0/legalcodeCopyright20062004,MichaelOpdenacker Copyright20032006,OronPeled Copyright20042006CodefidenceLtd.

Copyright20042006 MichaelOpdenacker FreeElectrons [email protected] http://www.freeelectrons.com Copyright20032006 OronPeled [email protected] http://www.actcom.co.il/~oron Copyright20042006 GiladBenYossef Codefidenceltd. [email protected] http:/www.codefidence.comBased on material by:

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WhatisLinux?Linux is a kernel that implements the POSIX and Single Unix Specification standards which is developed as an Open Source project. Usually when one talks of installing Linux, one is referring to a Linux Distribution. A distribution is a combination of Linux and other programs and library that form an operating system. There exists many such distribution for various purposes, from high end servers to embedded systems. They all share the same interface, thanks to the LSB standard Linux runs on 15 main platforms and supports applications ranging from ccNUMA super clusters to cellular phones and micro controllers. Linux is 11 years old, but is based on the 30 years old Unix design philosophy



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WhatisOpenSource?Open Source is a way to develop software application in a distributed fashion that allows cooperation of multiple bodies to create the end product. They don't have to be from the same company or indeed, any company. With Open Source software the source code is published and any one can use, learn, distribute, adapt and sell the program. An Open Source program is protected under copyright law and is licensed to it's users under a software license agreement. It is NOT software in the public domain. When making use of Open Source software it is imperative to understand what license governs the use of the work and what is and what is not allowed by the terms of the license. The same thing is true for ANY external code used in a product.



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OpenSourceLicenses

BSD MIT X11 ArtisticAs long as you acknowledge by copyright and promise not to sue me you can do what ever you want with the code.Copyright20062004,MichaelOpdenacker Copyright20032006,OronPeled Copyright20042006CodefidenceLtd.

LGPL

GPL CPL APLCustomer receives the same rights I have in the WHOLE work, including source code.

Customer receives the same rights I have in the SPECIFIC PART USED, including source code.


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LayersinaLinuxsystemUserprograms

Kernel Clibrary Systemlibraries Applicationlibraries Userprograms

Kernel Clibrary



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LinuxInternals

KerneloverviewLinuxfeatures



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Studiedkernelversion:2.6Linux2.4Mature Butdevelopmentsstopped;very fewdeveloperswillingtohelp. Nowobsoleteandlacksrecent features. Stillfineifyougetyour sources,toolsandsupportfrom commercialLinuxvendors.

Linux2.62yearsoldstableLinuxrelease! SupportfromtheLinux developmentcommunityandall commercialvendors. Nowmatureandmoreexhaustive. Mostdriversupgraded. Cuttingedgefeaturesand increasedperformance.



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LinuxkernelkeyfeaturesPortabilityandhardwaresupport Runsonmostarchitectures. Scalability Canrunonsupercomputersas wellasontinydevices (4MBofRAMisenough). Compliancetostandardsand interoperability. Exhaustivenetworkingsupport. Security Itcan'thideitsflaws.Itscodeis reviewedbymanyexperts. Stabilityandreliability. Modularity Canincludeonlywhatasystem needsevenatruntime. Easytoprogram Youcanlearnfromexistingcode. Manyusefulresourcesonthenet.Based on material by:


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SupportedhardwarearchitecturesSeethearch/directoryinthekernelsources Minimum:32bitprocessors,withorwithoutMMU 32bitarchitectures(arch/subdirectories) alpha,arm,cris,frv,h8300,i386,m32r,m68k,m68knommu, mips,parisc,ppc,s390,sh,sparc,um,v850,xtensa 64bitarchitectures: ia64,mips64,ppc64,sh64,sparc64,x86_64 Seearch//Kconfig,arch//README,or Documentation//fordetails



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LinuxInternals

KerneloverviewKernelcode



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ImplementedinCImplementedinClikeallUnixsystems. (CwascreatedtoimplementthefirstUnixsystems) AlittleAssemblyisusedtoo: CPUandmachineinitialization,criticallibraryroutines. Seehttp://www.tux.org/lkml/#s153 forreasonsfornotusingC++ (mainreason:thekernelrequiresefficientcode).



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CompiledwithGNUCNeedGNUCextensionstocompilethekernel. So,youcannotuseanyANSICcompiler! SomeGNUCextensionsusedinthekernel:InlineCfunctions Inlineassembly Structurememberinitialization inanyorder(alsoinANSIC99) Branchannotation(seenextpage)



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Helpgcctooptimizeyourcode!Usethelikelyandunlikelystatements (include/linux/compiler.h) Example: if(unlikely(err)){ ... } TheGNUCcompilerwillmakeyourcodefaster forthemostlikelycase. Usedinmanyplacesinkernelcode! Don'tforgettousethesestatements!Copyright20062004,MichaelOpdenacker Copyright20032006,OronPeled Copyright20042006CodefidenceLtd.


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NoClibraryThekernelhastobestandaloneandcan'tuseuserspacecode. Userspaceisimplementedontopofkernelservices,nottheopposite. Kernelcodehastosupplyitsownlibraryimplementations (stringutilities,cryptography,uncompression...) So,youcan'tusestandardClibraryfunctionsinkernelcode. (printf(),memset(),malloc()...). YoucanalsousekernelCheaders. Fortunately,thekernelprovidessimilarCfunctionsforyour convenience,likeprintk(),memset(),kmalloc()...



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KernelStackVerysmallandfixedstack.2pagestack(8k),pertask. Or1pagestack,pertaskandoneforinterrupts. 2.6 Choseninbuildtimeviamenu. Notforallarchitectures

Forsomearchitectures,thekernelprovidesdebugfacilityto detectstackoverruns.



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ManagingendianismLinuxsupportsbothlittleandbigendianarchitectures Eacharchitecturedefines__BIG_ENDIANor__LITTLE_ENDIAN in Canbeconfiguredinsomeplatformssupportingboth. Tomakeyourcodeportable,thekerneloffersconversionmacros (thatdonothingwhennoconversionisneeded).Mostusefulones: u32cpu_to_be32(u32); //CPUbyteordertobigendian u32cpu_to_le32(u32); //CPUbyteordertolittleendian u32be32_to_cpu(u32); //LittleendiantoCPUbyteorder u32le32_to_cpu(u32); //BigendiantoCPUbyteorderCopyright20062004,MichaelOpdenacker Copyright20032006,OronPeled Copyright20042006CodefidenceLtd.


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KernelcodingguidelinesNeverusefloatingpointnumbersinkernelcode.Yourcode mayberunonaprocessorwithoutafloatingpointunit(likeon arm).Floatingpointcanbeemulatedbythekernel,butthisis veryslow. Defineallsymbolsasstatic,exceptexportedones(avoidname spacepollution) Allsystemcallsreturnnegativenumebrs(errorcodes)for errors:#include

SeeDocumentation/CodingStyleformoreguidelinesCopyright20062004,MichaelOpdenacker Copyright20032006,OronPeled Copyright20042006CodefidenceLtd.


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KernellogPrintingtothekernellogisdoneviatheprintkfunction. Thekernelkeepsthemessagesinacircularbuffer (sothatdoesn'tconsumemorememorywithmanymessages) Kernellogmessagescanbeaccessedfromuserspacethroughsystem calls,orthrough/proc/kmsg Kernellogmessagesarealsodisplayedinthesystemconsole.



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printkTheprintkfunction: Similartostdlib'sprintf(3) Nofloatingpointformat. Logmessageareprefixedwitha,wherethenumber denotesseverity,from1(mostsevere)to8. Macrosaredefinedtobeusedforseveritylevels: KERN_EMERG,KERN_ALERT,KERT_CRIT, KERN_ERR,KERN_WARNING,KERN_NOTICE, KERN_INFO,KERN_DEBUG. Usageexample:Copyright20062004,MichaelOpdenacker Copyright20032006,OronPeled Copyright20042006CodefidenceLtd.

printk(KERN_DEBUGHelloWorldnumber%d\n,num);Based on material by:

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AccessingthekernellogManywaysareavailable!Watchthesystemconsole syslogd Daemongatheringkernelmessages in/var/log/messages Followchangesbyrunning: tailf/var/log/messages Caution:thisfilegrows! Uselogrotatetocontrolthis dmesg Foundinallsystems DisplaysthekernellogbufferCopyright20062004,MichaelOpdenacker Copyright20032006,OronPeled Copyright20042006CodefidenceLtd.

logread Same.Oftenfoundinsmall embeddedsystemswithno /var/log/messagesorno dmesg.ImplementedbyBusybox. cat/proc/kmsg Waitsforkernelmessagesand displaysthem. Usefulwhennoneoftheabove userspaceprogramsareavailable (tinysystem)Based on material by:

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LinkedListsManyconstructsusedoublylinkedlists. Listdefinitionandinitialization:structlist_headmylist=LIST_HEAD_INIT(mylist);

orLIST_HEAD(mylist);

orINIT_LIST_HEAD(&mylist);



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ListManipulationListdefinitionandinitialization:voidlist_add(structlist_head*new,structlist_head *head); voidlist_add_tail(structlist_head*new,struct list_head*head); voidlist_del(structlist_head*entry); voidlist_del_init(structlist_head*entry); voidlist_move(structlist_head*list,structlist_head *head); voidlist_add_tail(structlist_head*list,struct list_head*head);



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ListManipulation(cont.)Listsplicingandquery:voidlist_splice(structlist_head*list,struct list_head*head); voidlist_add_splice_init(structlist_head*list, structlist_head*head); voidlist_empty(structlist_head*head);

In2.6,therearevariantsoftheseAPI'sforRCUprotected lists(seesectionaboutLocksahead).



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ListIterationListsalsohaveiteratormacrosdefined:list_for_each(pos,head); list_for_each_prev(pos,head); list_for_each_safe(pos,n,head); list_for_each_entry(pos,head,member);

Example:structmydata*pos; list_for_each_entry(pos,head,dev_list){ pos>some_data=0777; }



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LinuxInternals

KerneloverviewKernelsubsystems



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KernelarchitectureApp1 App2 Clibrary Systemcallinterface Process management Memory management Filesystem support Filesystem types CPUsupport code CPU/MMU supportcode Storage drivers Character devicedrivers Network devicedrivers Hardware CPU RAM StorageBased on material by:

...

User space

Device control

Networking

Kernel space


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KernelModevs.UserModeAllmodernCPUssupportadualmodeofoperation:Usermode,forregulartasks. Supervisor(orprivileged)mode,forthekernel.

ThemodetheCPUisindetermineswhichinstructionsthe CPUiswillingtoexecute:SensitiveinstructionswillnotbeexecutedwhentheCPUis inusermode.

TheCPUmodeisdeterminedbyoneoftheCPUregisters, whichstoresthecurrentRingLevelCopyright20062004,MichaelOpdenacker Copyright20032006,OronPeled Copyright20042006CodefidenceLtd.

0forsupervisormode,3forusermode,12unusedbyLinux.Based on material by:

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TheSystemCallInterfaceWhenauserspacetasksneedstouseakernelservice,itwill makeaSystemCall. TheClibraryplacesparametersandnumberofsystemcallin registersandthenissuesaspecialtrapinstruction. Thetrapatomicallychangestheringleveltosupervisor modeandthesetstheinstructionpointertothekernel. Thekernelwillfindtherequiredsystemcalledviathesystem calltableandexecuteit. Returningfromthesystemcalldoesnotrequireaspecial instruction,sinceinsupervisormodetheringlevelcanbe Copyright20062004,MichaelOpdenacker Based on material by: changeddirectly. Copyright20032006,OronPeledCopyright20042006CodefidenceLtd.

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LinuxSystemCallPathdo_name() sys_name() entry.S Function call Trap

Kernel

Task

Glibc Task



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KernelmemoryconstraintsWhocanlookafterthekernel? Nomemoryprotection Accessingillegalmemory locationsresultin(oftenfatal) kerneloopses. Fixedsizestack(8or4KB) Unlikeinuserspace, nowaytomakeitgrow. Kernelmemorycan'tbeswapped out(forthesamereasons).User process Attempt toaccess Illegal memory location Exception (MMU) Userspacememorymanagement Usedtoimplement: memoryprotection stackgrowth memoryswappingtodisk

SIGSEGV,kill

Kernel



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LinuxInternals

KerneloverviewLinuxversioningschemeanddevelopmentprocess



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LinuxKernelDevelopmentTimeline



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LinuxstablereleasesMajorversions 1majorversionevery2or3years Examples:1.0,2.0,2.4,2.6 Stablereleases 1stablereleaseevery1or2months Examples:2.0.40,2.2.26,2.4.27,2.6.7... Stablereleaseupdates(sinceMarch2005) Updatestostablereleasesuptoseveraltimesaweek Addressonlycriticalissuesinthelateststablerelease Examples:2.6.11.1to2.6.11.7Copyright20062004,MichaelOpdenacker Copyright20032006,OronPeled Copyright20042006CodefidenceLtd.

Evennumber


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LinuxdevelopmentandtestingreleasesTestingreleases Severaltestingreleasespermonth,beforethenextstableone. Youcancontributetomakingkernelreleasesmorestableby testingthem! Example:2.6.12rc1 Developmentversions Unstableversionsusedbykerneldevelopers beforemakinganewstablemajorrelease Examples:2.3.42,2.5.74

Oddnumber



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ContinueddevelopmentinLinux2.6Since2.6.0,kerneldevelopershavebeenabletointroducelots ofnewfeaturesonebyoneonasteadypace,withouthavingto makemajorchangesinexistingsubsystems. OpeninganewLinux2.7(or2.9)developmentbranchwillbe requiredonlywhenLinux2.6isnolongerabletoaccommodate keyfeatureswithoutundergoingtraumaticchanges. Thankstothis,morefeaturesarereleasedtousersatafasterpace. However,theinternalkernelAPIcanundergochangesbetween two2.6.xreleases.Amodulecompiledforagivenversionmay nolongercompileorworkonamorerecentone.Copyright20062004,MichaelOpdenacker Copyright20032006,OronPeled Copyright20042006CodefidenceLtd.


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LinuxKernelDevelopmentProcess



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What'snewineachLinuxrelease?(1)commit 3c92c2ba33cd7d666c5f83cc32aa590e794e91b0 Author: Andi Kleen Date: Tue Oct 11 01:28:33 2005 +0200 [PATCH] i386: Don't discard upper 32bits of HWCR on K8 Need to use long long, not long when RMWing a MSR. I think it's harmless right now, but still should be better fixed if AMD adds any bits in the upper 32bit of HWCR. Bug was introduced with the TLB flush filter fix for i386 Signed-off-by: Andi Kleen Signed-off-by: Linus Torvalds

? ?!

...

TheofficiallistofchangesforeachLinuxreleaseisjusta hugelistofindividualpatches! Verydifficulttofindoutthekeychangesandtogetthe globalpictureoutofindividualchanges.Copyright20062004,MichaelOpdenacker Copyright20032006,OronPeled Copyright20042006CodefidenceLtd.


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What'snewineachLinuxrelease?(2)Fortunately,asummaryofkeychanges withenoughdetailsisavailableon http://wiki.kernelnewbies.org/LinuxChanges

? ?!

Foreachnewkernelrelease,youcanalsogetthe changesinthekernelinternalAPI: http://lwn.net/Articles/2.6kernelapi/ What'snext? Documentation/featureremovalschedule.txt liststhefeatures,subsystemsandAPIsthatare plannedforremoval(announced1yearinadvance).Copyright20062004,MichaelOpdenacker Copyright20032006,OronPeled Copyright20042006CodefidenceLtd.


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LinuxInternals

KerneloverviewKerneluserinterface



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MountingvirtualfilesystemsLinuxmakessystemandkernelinformationavailableinuser spacethroughvirtualfilesystems(virtualfilesnotexistingonany realstorage).Noneedtoknowkernelprogrammingtoaccessthis!

Mounting/proc: mounttprocnone/proc Mounting/sys: mounttsysfsnone/sysFilesystemtype Mountpoint Rawdevice orfilesystemimage Inthecaseofvirtual filesystems,anystringisfineBased on material by:


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KerneluserspaceinterfaceAfewexamples: /proc/cpuinfo:processorinformation /proc/meminfo:memorystatus /proc/version:versionandbuildinformation /proc/cmdline:kernelcommandline /proc//environ:callingenvironment /proc//cmdline:processcommandline ...andmanymore!Seebyyourself!Copyright20062004,MichaelOpdenacker Copyright20032006,OronPeled Copyright20042006CodefidenceLtd.


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UserspaceinterfacedocumentationLotsofdetailsaboutthe/procinterfaceareavailablein Documentation/filesystems/proc.txt (almost2000lines)inthekernelsources. Youcanalsofindotherdetailsintheprocmanualpage: manproc SeetheNewDeviceModelsectionfordetailsabout/sys



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LinuxInternals

CompilingandbootingLinuxGettingthesources



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LinuxkernelsizeLinux2.6.16sources: Rawsize:260MB(20400files,approx7millionlinesofcode) bzip2compressedtararchive:39MB(bestchoice) gzipcompressedtararchive:49MB MinimumcompiledLinuxkernelsize(withLinuxTinypatches) approx300KB(compressed),800KB(raw) Whyarethesesourcessobig? Becausetheyincludethousandsofdevicedrivers,manynetwork protocols,supportmanyarchitecturesandfilesystems... TheLinuxcore(scheduler,memorymanagement...)isprettysmall!Copyright20062004,MichaelOpdenacker Copyright20032006,OronPeled Copyright20042006CodefidenceLtd.


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kernel.org



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GettingLinuxsources:2possibilitiesFullsources Theeasiestway,butlongertodownload. Example: http://kernel.org/pub/linux/kernel/v2.6/linux2.6.14.1.tar.bz2 Orpatchagainstthepreviousversion Assumingyoualreadyhavethefullsourcesofthepreviousversion Example: http://kernel.org/pub/linux/kernel/v2.6/patch2.6.14.bz2(2.6.13to2.6.14) http://kernel.org/pub/linux/kernel/v2.6/patch2.6.14.7.bz2(2.6.14to2.6.14.7)



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DownloadingfullkernelsourcesDownloadingfromthecommandline Withawebbrowser,identifytheversionyouneedonhttp://kernel.org Intherightdirectory,downloadthesourcearchiveanditssignature (copyingthedownloadaddressfromthebrowser):wgethttp://kernel.org/pub/linux/kernel/v2.6/linux2.6.11.12.tar.bz2 wgethttp://kernel.org/pub/linux/kernel/v2.6/linux2.6.11.12.tar.bz2.sign

Checktheelectronicsignatureofthearchive:gpgverifylinux2.6.11.12.tar.bz2.sign

~/.wgetrcconfigfileforproxies:http_proxy=: ftp_proxy=: proxy_user=(ifany) proxy_password=(ifany)

Extractthecontentsofthesourcearchive:tarjxvflinux2.6.11.12.tar.bz2Copyright20062004,MichaelOpdenacker Copyright20032006,OronPeled Copyright20042006CodefidenceLtd.


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Downloadingkernelsourcepatches(1)Assumingyoualreadyhavethelinuxx.y.version Identifythepatchesyouneedonhttp://kernel.orgwithawebbrowser Downloadthepatchfilesandtheirsignature:Patchfrom2.6.10to2.6.11wgetftp://ftp.kernel.org/pub/linux/kernel/v2.6/patch2.6.11.bz2 wgetftp://ftp.kernel.org/pub/linux/kernel/v2.6/patch2.6.11.bz2.sign

Patchfrom2.6.11to2.6.11.12(lateststablefixes)wgethttp://www.kernel.org/pub/linux/kernel/v2.6/patch2.6.11.12.bz2 wgethttp://www.kernel.org/pub/linux/kernel/v2.6/patch2.6.11.12.bz2.sign



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Downloadingkernelsourcepatches(2)Checkthesignatureofpatchfiles:gpgverifypatch2.6.11.bz2.sign gpgverifypatch2.6.11.12.bz2.sign

Applythepatchesintherightorder:cdlinux2.6.10/ bzcat../patch2.6.11.bz2|patchp1 bzcat../patch2.6.11.12.bz2|patchp1 cd.. mvlinux2.6.10linux2.6.11.12



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CheckingtheintegrityofsourcesKernelsourceintegritycanbecheckedthroughOpenPGPdigitalsignatures. Fulldetailsonhttp://www.kernel.org/signature.html Ifneeded,readhttp://www.gnupg.org/gph/en/manual.htmlandcreateanew privateandpublickeypairforyourself. ImportthepublicGnuPGkeyofkerneldevelopers:gpgkeyserverpgp.mit.edurecvkeys0x517D0F0E

Ifblockedbyyourfirewall,lookfor0x517D0F0Eonhttp://pgp.mit.edu/ ,copyandpastethekeytoalinuxkey.txtfile: gpgimportlinuxkey.txt Checkthesignatureoffiles: gpgverifylinux2.6.11.12.tar.bz2.signCopyright20062004,MichaelOpdenacker Copyright20032006,OronPeled Copyright20042006CodefidenceLtd.


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AnatomyofapatchfileApatchfileistheoutputofthediffcommanddiffcommandline diffNrua/Makefileb/Makefile a/Makefile2005030409:27:1508:00 Filedateinfo +++b/Makefile2005030409:27:1508:00 @@1,7+1,7@@ Linenumbersinfiles VERSION=2 Contextinfo:3linesbeforethechange PATCHLEVEL=6 Usefultoapplyapatchwhenlinenumberschanged SUBLEVEL=11 EXTRAVERSION= Removedline(s)ifany +EXTRAVERSION=.1 Addedline(s)ifany NAME=WoozyNumbat

#*DOCUMENTATION*Copyright20062004,MichaelOpdenacker Copyright20032006,OronPeled Copyright20042006CodefidenceLtd.

Contextinfo:3linesafterthechange


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UsingthepatchcommandThepatchcommandapplieschangestofilesinthecurrentdirectory: Makingchangestoexistingfiles Creatingordeletingfilesanddirectories patchusageexamples: patchpops=&acme_fops; acme_cdev>owner=THIS_MODULE;



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Characterdeviceregistration(2)Then,nowthatyourstructureisready,addittothesystem: intcdev_add( structcdev*p, /*Characterdevicestructure*/ dev_tdev, /*Startingdevicemajor/minornumber*/ unsignedcount); /*Numberofdevices*/

Example(continued):if(cdev_add(acme_cdev,acme_dev,acme_count)){ printk(KERN_ERRChardriverregistrationfailed\n);

...



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CharacterdeviceunregistrationFirstdeleteyourcharacterdevice: voidcdev_del(structcdev*p); Then,andonlythen,freethedevicenumber: voidunregister_chrdev_region(dev_tfrom, unsignedcount); Example(continued): cdev_del(acme_cdev); unregister_chrdev_region(acme_dev,acme_count);



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LinuxerrorcodesTrytoreporterrorswitherrornumbersasaccurateaspossible! Fortunately,macronamesareexplicitandyoucanremember themquickly. Genericerrorcodes: include/asmgeneric/errnobase.h Platformspecificerrorcodes: include/asm/errno.h



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Chardriverexamplesummary(1)staticvoid*acme_buf; staticacme_bufsize=8192; staticintacme_count=1; staticdev_tacme_dev; staticstructcdev*acme_cdev; staticssize_tacme_write(...){...} staticssize_tacme_read(...){...} staticstructfile_operationsacme_fops= { .owner=THIS_MODULE, .read=acme_read, .write=acme_write, };



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Chardriverexamplesummary(2)staticint__initacme_init(void) { acme_buf=kmalloc(acme_bufsize, GFP_KERNEL); if(!acme_buf){ err=ENOMEM; gotoerr_exit; } if(alloc_chrdev_region(&acme_dev,0, acme_count,acme)){ err=ENODEV; gotoerr_free_buf; } acme_cdev=cdev_alloc(); if(!acme_cdev){ err=ENOMEM; gotoerr_dev_unregister; } acme_cdev>ops=&acme_fops; acme_cdev>owner=THIS_MODULE; if(cdev_add(acme_cdev,acme_dev, acme_dev_count)){ err=ENODEV; gotoerr_free_cdev; } return0; err_free_cdev: kfree(acme_cdev); err_dev_unregister: unregister_chrdev_region( acme_dev,acme_count); err_free_buf: kfree(acme_buf); err_exit: returnerr; } staticvoid__exitacme_exit(void) { cdev_del(acme_cdev); unregister_chrdev_region(acme_dev, acme_count); kfree(acme_buf); }

Showhowtohandleerrorsanddeallocateresourcesintherightorder!Copyright20062004,MichaelOpdenacker Copyright20032006,OronPeled Copyright20042006CodefidenceLtd.


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CharacterdriversummaryCharacterdriverwriterDefinethefileoperationscallbacksforthedevicefile:read,write,ioctl... Inthemoduleinitfunction,getmajorandminornumberswithalloc_chrdev_region(), initacdevstructurewithyourfileoperationsandaddittothesystemwithcdev_add(). Inthemoduleexitfunction,callcdev_del()andunregister_chrdev_region()

Systemadministration

Systemuser Kernel

Openthedevicefile,read,write,orsendioctl'stoit. Executesthecorrespondingfileoperations



Kernel

Userspace

Loadthecharacterdrivermodule In/proc/devices,findthemajornumberituses. Createthedevicefilewiththismajornumber Thedevicefileisreadytouse!

Kernel

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LinuxInternals

DriverdevelopmentDebugging



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UsefulnessofaserialportMostprocessorsfeatureaserialportinterface(usuallyvery wellsupportedbyLinux).Justneedthisinterfacetobe connectedtotheoutside. Easywayofgettingthefirstmessagesofanearlykernel version,evenbeforeitboots.Aminimumkernelwithonly serialportsupportisenough. Oncethekernelisfixedandhascompletedbooting,possible toaccessaserialconsoleandissuecommands. Theserialportcanalsobeusedtotransferfilestothetarget.Copyright20062004,MichaelOpdenacker Copyright20032006,OronPeled Copyright20042006CodefidenceLtd.


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Whenyoudon'thaveaserialportOnthehost Notanissue.YoucangetaUSBtoserialconverter.Usuallyvery wellsupportedonLinuxandroughlycosts$20.Thedeviceappears as/dev/ttyUSB0onthehost. Onthetarget CheckwhetheryouhaveanIrDAport.It'susuallyaserialporttoo. IfyouhaveanEthernetadapter,trywithit Youmayalsotrytomanuallyhookuptheprocessorserialinterface (checktheelectricalspecificationsfirst!)Copyright20062004,MichaelOpdenacker Copyright20032006,OronPeled Copyright20042006CodefidenceLtd.


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DebuggingwithprintkUniversaldebuggingtechniqueusedsincethebeginningof programming(firstfoundincavemendrawings) Printedornotintheconsoleor/var/log/messages accordingtothepriority.Thisiscontrolledbytheloglevel kernelparameter,orthrough/proc/sys/kernel/printk (seeDocumentation/sysctl/kernel.txt) Availablepriorities(include/linux/kernel.h):#defineKERN_EMERG""/*systemisunusable*/ #defineKERN_ALERT""/*actionmustbetakenimmediately*/ #defineKERN_CRIT""/*criticalconditions*/ #defineKERN_ERR""/*errorconditions*/ #defineKERN_WARNING""/*warningconditions*/ #defineKERN_NOTICE""/*normalbutsignificantcondition*/ #defineKERN_INFO""/*informational*/ #defineKERN_DEBUG""/*debuglevelmessages*/Copyright20062004,MichaelOpdenacker Copyright20032006,OronPeled Copyright20042006CodefidenceLtd.


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DebuggingwiththeMagicKeyYoucanhaveamagickeytocontrolthekernel. Toactiviatethisfeature,makesurethat:KernelconfigurationCONFIG_MAGIC_SYSRQenabled. Enableitatruntime:echo1>/proc/sys/kernel/sysrq

Thekeyis:PCConsole: SerialConsole: Fromshell(2.6only):Copyright20062004,MichaelOpdenacker Copyright20032006,OronPeled Copyright20042006CodefidenceLtd.

SysRq SendaBREAKechot>/proc/sysrqtriggerBased on material by:

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DebuggingwiththeMagicKeyCont.Togetherwiththemagickey,youusethefollowing:b:hardboot(nosync,nounmount) s:sync u:Remountallreadonly. t:tasklist(proccesstable). 18:Setconsoleloglevel. e:ShowInstructionPointer. Andmore...presshforhelp.

Programmerscanaddtheirownhandlersaswell.Copyright20062004,MichaelOpdenacker Copyright20032006,OronPeled Copyright20042006CodefidenceLtd.

SeeDocumentation/sysrq.txtformoredetails.


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Debuggingwith/procor/sys(1)Insteadofdumpingmessagesinthekernellog,youcanhaveyour driversmakeinformationavailabletouserspace Throughafilein/procor/sys,whichcontentsarehandledby callbacksdefinedandregisteredbyyourdriver. Canbeusedtoshowanypieceofinformation aboutyourdeviceordriver. Canalsobeusedtosenddatatothedriverortocontrolit. Caution:anybodycanusethesefiles. Youshouldremoveyourdebugginginterfaceinproduction!Copyright20062004,MichaelOpdenacker Copyright20032006,OronPeled Copyright20042006CodefidenceLtd.


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Debuggingwith/procor/sys(2)Examples cat/proc/acme/stats(dummyexample) Displaysstatisticsaboutyouracmedriver. cat/proc/acme/globals(dummyexample) Displaysvaluesofglobalvariablesusedbyyourdriver.echo600000>/sys/devices/system/cpu/cpu0/cpufreq/scaling_setspeed

AdjuststhespeedoftheCPU(controlledbythecpufreqdriver).



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DebuggingwithioctlCanusetheioctl()systemcalltoqueryinformation aboutyourdriver(ordevice)orsendcommandstoit. Thiscallstheioctlfileoperationthatyoucanregisterin yourdriver. Advantage:yourdebugginginterfaceisnotpublic. Youcouldevenleaveitwhenyoursystem(oritsdriver)isin thehandsofitsusers.



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DebuggingwithgdbSchrdingerpenguinprinciple. Ifyouexecutethekernelfromadebuggeronthesamemachine, thiswillinterferewiththekernelbehavior. However,youcanaccessthecurrentkernelstatewithgdb: gdb/usr/src/linux/vmlinux/proc/kcore uncompressedkernelkerneladdressspace Youcanaccesskernelstructures,followpointers...(readonly!) RequiresthekerneltobecompiledwithCONFIG_DEBUG_INFO (Kernelhackingsection)



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kgdbkernelpatchhttp://kgdb.linsyssoft.com/ Theexecutionofthepatchedkernelisfullycontrolledby gdbfromanothermachine,connectedthroughaserialline. Candoalmosteverything,includinginsertingbreakpointsin interrupthandlers. Supportedarchitectures:i386,x86_64,ppcands390.



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Kernelcrashanalysiswithkexeckexecsystemcall:makesitpossibleto callanewkernel,withoutrebootingand goingthroughtheBIOS/firmware. Idea:afterakernelpanic,makethe kernelautomaticallyexecuteanew, cleankernelfromareservedlocationin RAM,toperformpostmortemanalysis ofthememoryofthecrashedkernel. SeeDocumentation/kdump/kdump.txt inthekernelsourcesfordetails.Copyright20062004,MichaelOpdenacker Copyright20032006,OronPeled Copyright20042006CodefidenceLtd.

1.Copydebug kernelto reserved RAM 3.Analyze crashed kernelRAM

Standardkernel 2.kernel panic,kexec debugkernel Debugkernel

RegularRAM


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DecryptingoopsmessagesYouoftengetkerneloopsmessageswhen youdevelopdrivers(dereferencingnull pointers,illegalaccessestomemory...). Theygiverawinformationaboutthe functioncallstackandCPUregisters. Youcanmakethesemessagesmore explicitinyourdevelopmentkernel,for examplebyreplacingrawaddressesby symbolnames,bysetting: #GeneralSetup CONFIG_KALLSYMS=y Replacestheksymoopstoolwhich shouldn'tbeusedanymorewithLinux2.6Copyright20062004,MichaelOpdenacker Copyright20032006,OronPeled Copyright20042006CodefidenceLtd.Unable to handle kernel paging req uest at virtual address 4d 1b65e8 Unable to handle kernel pag ing req uest at virtual address 4d1b6 5e8 pgd = c0280000 pgd = c0280000 [4d1b65e8] *pgd=00000000[4d1b65e8] *pgd=00000000 Internal error: Oops: f5 [#1] Internal error: Oops: f5 [#1] Modules linked in:Modules linked in: hx4700_udc hx4700_udc asic3_base asic3_base CPU: 0 CPU: 0 PC is at set_pxa_fb_info+0x2c/0x44 PC is at set_pxa_fb_info+0x2c/0x44 LR is at hx4700_udc_init+0x1c/0x38 [hx4700_udc] LR is at hx4700_udc_init+0x1c/0x38 [hx4700_udc] pc : [] lr : [] Not tainted sp : c076df78 ip : 60000093 fp : c076df84 pc : [] lr : [] Not tainted sp : c076df78 ip : 60000093 fp : c076df84 r10: 00000002 r9 : c076c000 r8 : c001c7e4 r10: 00000002 r9 : c076c000 r8 : c001c7e4 r7 : 00000000 r6 : c0176d40 r5 : bf007500 r4 : c0176d58 r7 : 00000000 r6 : c0176d40 r5 : bf007500 r4 : c0176d58 r3 : c0176828 r2 : 00000000 r1 : 00000f76 r0 : 80004440 r3 : c0176828 r2 : 00000000 r1 : 00000f76 r0 : 80004440 Flags: nZCvFlags: nZCv IRQs on FIQs on Mode SVC_32 Segme nt user


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DebuggingwithKprobeshttp://sourceware.org/systemtap/kprobes/ Fairlysimplewayofinsertingbreakpointsinkernelroutines Unlikeprintkdebugging,youneitherhavetorecompilenorrebootyour kernel.Youonlyneedtocompileandloadadedicatedmoduletodeclarethe addressoftheroutineyouwanttoprobe. Nondisruptive,basedonthekernelinterrupthandler Kprobesevenletsyoumodifyregistersandglobalkernelinternals. Supportedarchitectures:i386,x86_64,ppc64andsparc64 Niceoverviews:http://lwn.net/Articles/132196/ andhttp://www106.ibm.com/developerworks/library/lkprobes.htmlCopyright20062004,MichaelOpdenacker Copyright20032006,OronPeled Copyright20042006CodefidenceLtd.


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KerneldebuggingtipsIfyourkerneldoesn'tbootyetorhangswithoutanymessage,it canhelptoactivateLowLeveldebugging (KernelHackingsection,onlyavailableonarm): CONFIG_DEBUG_LL=y Moreaboutkerneldebugginginthefree LinuxDeviceDriversbook(Referencessection)!



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LinuxInternals

DriverdevelopmentConcurrentaccesstoresources



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SourcesofconcurrencyissuesThesameresourcescanbeaccessedbyseveralkernelprocessesin parallel,causingpotentialconcurrencyissues Severaluserspaceprogramsaccessingthesamedevicedataor hardware.Severalkernelprocessescouldexecutethesamecodeon behalfofuserprocessesrunninginparallel. Multiprocessing:thesamedrivercodecanberunningonanother processor.ThiscanalsohappenwithsingleCPUswithhyperthreading. Kernelpreemption,interrupts:kernelcodecanbeinterruptedatany time(justafewexceptions),andthesamedatamaybeaccessbyanother processbeforetheexecutioncontinues.Copyright20062004,MichaelOpdenacker Copyright20032006,OronPeled Copyright20042006CodefidenceLtd.


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AvoidingconcurrencyissuesAvoidusingglobalvariablesandshareddatawheneverpossible (cannotbedonewithhardwareresources) Don'tmakeresourcesavailabletootherkernelprocessesuntil theyarereadytobeused. Usetechniquestomanageconcurrentaccesstoresources. SeeRustyRussell'sUnreliableGuideToLocking Documentation/DocBook/kernellocking/ inthekernelsources.



226

ConcurrencyprotectionwithsemaphoresProcess1Failed Acquirelock Success Criticalcodesection Tryagain SuccessWaitlockrelease

Process2

Sharedresource

ReleaselockCopyright20062004,MichaelOpdenacker Copyright20032006,OronPeled Copyright20042006CodefidenceLtd.


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KernelsemaphoresAlsocalledmutexes(MutualExclusion)1 (free) P (down)P:Probeer Try(todecrement)inDutch V:Verhoog IncrementinDutch

V (up) 0 (locked)



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InitializingasemaphoreStatically DECLARE_MUTEX(name); DECLARE_MUTEX_LOCKED(name); Dynamically voidinit_MUTEX(structsemaphore*sem); voidinit_MUTEX_LOCKED(structsemaphore*sem);



229

lockingandunlockingsemaphoresvoiddown(structsemaphore*sem); Decrementsthesemaphoreifsetto1,waitsotherwise. Caution:can'tbeinterrupted,causingprocessesyoucannotkill! intdown_interruptible(structsemaphore*sem); Same,butcanbeinterrupted.Ifinterrupted,returnsanonzerovalue anddoesn'tholdthesemaphore.Testthereturnvalue!!! intdown_trylock(structsemaphore*sem); Neverwaits.Returnsanonzerovalueifthesemaphoreisnot available. voidup(structsemaphore*sem); Releasesthesemaphore.Makesureyoudoitassoonaspossible!Copyright20062004,MichaelOpdenacker Copyright20032006,OronPeled Copyright20042006CodefidenceLtd.


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Reader/writersemaphoresAllowsharedaccessbyunlimitedreaders,orbyonly1writer.Writersgetpriority. voidinit_rwsem(structrw_semaphore*sem); voiddown_read(structrw_semaphore*sem); intdown_read_trylock(structrw_semaphore*sem); intup_read(structrw_semaphore*sem); voiddown_write(structrw_semaphore*sem); intdown_write_trylock(structrw_semaphore*sem); intup_write(structrw_semaphore*sem); Wellsuitedforrarewrites,holdingthesemaphorebriefly.Otherwise,readersget starved,waitingtoolongforthesemaphoretobereleased.



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WhentousesemaphoresBeforeandafteraccessingsharedresources Beforeandaftermakingotherresourcesavailabletoother partsofthekernelortouserspace(typicallyandmodule initialization). Insituationswhensleepingisallowed.



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SpinlocksLockstobeusedforcodethatcan'tsleep(criticalsections, interrupthandlers...Beverycarefulnottocallfunctionswhich cansleep! Intendedformultiprocessorsystems Spinlocksarenotinterruptible, don'tsleepandkeepspinninginaloop untilthelockisavailable.Spinlock

Stilllocked?

SpinlockscausekernelpreemptiontobedisabledontheCPU executingthem. Mayrequireinterruptstobedisabledtoo.Copyright20062004,MichaelOpdenacker Copyright20032006,OronPeled Copyright20042006CodefidenceLtd.


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InitializingspinlocksStatic spinlock_tmy_lock=SPIN_LOCK_UNLOCKED; Dynamic voidspin_lock_init(spinlock_t*lock);



234

Usingspinlocksvoidspin_[un]lock(spin_lock_t*lock); voidspin_[un]lock_irqsave(spin_lock_t*lock, unsignedlongflags); DisablesIRQsonthelocalCPU voidspin_[un]lock_irq(spin_lock_t*lock); DisablesIRQswithoutsavingflags.Whenyou'resurethatnobody alreadydisabledinterrupts. voidspin_[un]lock_bh(spin_lock_t*lock); Disablessoftwareinterrupts,butnothardwareones Notethatreader/writerspinlocksalsoexist.Copyright20062004,MichaelOpdenacker Copyright20032006,OronPeled Copyright20042006CodefidenceLtd.


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DeadlocksituationsTheycanlockupyoursystem.Makesuretheyneverhappen!Don'tcallafunctionthatcantry togetaccesstothesamelock Holdingmultiplelocksisrisky!

Getlock1 Getlock1 call Waitforlock1Dead Lock!Copyright20062004,MichaelOpdenacker Copyright20032006,OronPeled Copyright20042006CodefidenceLtd.

Getlock2

Getlock2

Dead Lock!

Getlock1


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AlternativestolockingAswehavejustseen,lockingcanhaveastrongnegativeimpacton systemperformance.Insomesituations,youcoulddowithoutit. ByusinglockfreealgorithmslikeReadCopyUpdate(RCU). RCUAPIavailableinthekernel (Seehttp://en.wikipedia.org/wiki/RCU). Whenavailable,useatomicoperations.



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AtomicvariablesUsefulwhenthesharedresourceisan integervalue Evenaninstructionliken++isnot guaranteedtobeatomiconallprocessors! Header #include Type atomic_t containsasignedinteger(atleast24bits)

Operationswithoutreturnvalue:voidatomic_inc(atomic_t*v); voidatomic_dec(atomic_*v); voidatomic_add(inti,atomic_t*v); voidatomic_sub(inti,atomic_t*v);

Simularfunctionstestingtheresult:intatomic_inc_and_test(...); intatomic_dec_and_test(...); intatomic_sub_and_test(...);

Functionsreturningthenewvalue:intatomic_inc_and_return(...); intatomic_dec_and_return(...); intatomic_add_and_return(...); intatomic_sub_and_return(...);

Atomicoperations(mainones) Setorreadthecounter:atomic_set(atomic_t*v,inti); intatomic_read(atomic_t*v);Copyright20062004,MichaelOpdenacker Copyright20032006,OronPeled Copyright20042006CodefidenceLtd.


238

AtomicbitoperationsSupplyveryfast,atomicoperations Onmostplatforms,applytoanunsignedlongtype. Applytoavoidtypeonafewothers. Set,clear,toggleagivenbit: voidset_bit(intnr,unsignedlong*addr); voidclear_bit(intnr,unsignedlong*addr); voidchange_bit(intnr,unsignedlong*addr); Testbitvalue: inttest_bit(intnr,unsignedlong*addr); Testandmodify(returnthepreviousvalue): inttest_and_set_bit(...); inttest_and_clear_bit(...); inttest_and_change_bit(...);Copyright20062004,MichaelOpdenacker Copyright20032006,OronPeled Copyright20042006CodefidenceLtd.


239

LinuxInternals

DriverdevelopmentProcessesandscheduling



240

ProcessesandThreadsAprocessisaninstanceofarunningprogramMultipleinstancesofthesameprogramcanberunning. Programcode(textsection)memoryisshared. Eachprocesshasitsowndatasection,addressspace,open filesandsignalhandlers.

AthreadisasingletaskinaprogramItbelongstoaprocessandsharesthecommondatasection, addressspace,openfilesandpendingsignals. Ithasit'sownstack,pendingsignalsandstate.

It'scommontorefertosinglethreadedprogramsas Copyright20062004,MichaelOpdenacker Based on material by: processes. Copyright20032006,OronPeledCopyright20042006CodefidenceLtd.

241

TheKernelandThreadsThe2.4kerneldidnothaveanotionofthreads.Allthreadswereimplementedasprocessesthathappento sharethesameaddressspace,filesystemresources,file descriptorsandsignalhandlersastheirparentprocess.

In2.6anexplicitnotionofprocessesandthreadswas introducedtothekernel. Schedulingisstilldoneonathreadbythreadbasis.



242

Threadvs.Processvs.TaskTask 123 Task 124 Task 125 Task 126 Task 127 Task 128 Linux Kernel

Memory/Files Process 123 T 1 T 2 T 3

M/F 126 T 1

M/F 127 T 1

M/F 128 T 1 POSIX API



243

task_structEachtaskisrepresentedbyatask_struct. Thetaskislinkedinthetasktreevia:parent Pointertoit'sparent children sibling Alinkedlist Alinkedlist

task_structcontainsapidfieldpidismappedtotask_structpointerviaahashtable



244

TaskIdentifiersEachtask_structhasthefollowingidentities:PID Globallyunique.Differentoneforeachthread. TGID ThreadGroupId.Returnedtouserspaceas getpid()Sharedbyallthreadsofaprocess. Forsinglethreadedprocess==PID.

PGID ProccessGroupId.(Posix.1). SID SessionId(Posix.1).

currentpointstothecurrentprocesstask_structCopyright20062004,MichaelOpdenacker Copyright20032006,OronPeled Copyright20042006CodefidenceLtd.

Whenapplicablenotvalidininterruptcontext.


245

AprocesslifeParentprocessCallsfork() andcreates anewprocess Theprocessiselected bythescheduler Taskterminatedbutits resourcesarenotfreedyet. Waitingforitsparent toacknowledgeitsdeath.

TASK_ZOMBIE

TASK_RUNNINGReadybut notrunning Theprocessispreempted bytoschedulertorun ahigherprioritytask

TASK_RUNNINGActuallyrunning

Theeventoccurs ortheprocessreceives asignal.Processbecomes runnableagain

TASK_INTERRUPTIBLE orTASK_UNINTERRUPTIBLEWaiting

Decidestosleep onawaitqueue foraspecificevent



246

ProcesscontextUserspaceprogramsandsystemcallsarescheduledtogether

Processexecutinginuserspace... (canbepreempted) Systemcall orexception

Processcontinuinginuserspace... (orreplacedbyahigherpriorityprocess) (canbepreempted)

Kernelcodeexecuted onbehalfofuserspace (canbepreemptedtoo!)

Stillhasaccesstoprocess data(openfiles...)



247

KernelthreadsThekerneldoesnotonlyreactfromuserspace(systemcalls,exceptions) orhardwareevents(interrupts).Italsorunsitsownprocesses. Kernelspacearestandardprocessesscheduledandpreemptedinthesame way(youcanviewthemwithtoporps!)Theyjusthavenospecial addressspaceandusuallyrunforever. Kernelthreadexamples: pdflush:regularlyflushesdirtymemorypagestodisk(file changesnotcommittedtodiskyet). ksoftirqd:managessoftirqs.



248

ProcessprioritiesRegularprocesses Prioritiesfrom20(maximum)to19(minimum) Onlyrootcansetnegativepriorities (rootcangiveanegativeprioritytoaregularuserprocess) Usethenicecommandtorunajobwithagivenpriority: nicen Usetherenicecommandtochangeaprocesspriority: renicep



249

RealtimeprocessesRealtimeprocessescanbestartedbyrootusingthePOSIXAPI Availablethrough(seemansched.hfordetails) 100realtimeprioritiesavailable SCHED_FIFOschedulingclass: TheprocessrunsuntilcompletionunlessitisblockedbyanI/O,voluntarily relinquishestheCPU,orispreemptedbyahigherpriorityprocess. SCHED_RRschedulingclass: Difference:theprocessesarescheduledinaRoundRobinway. Eachprocessisrununtilitexhaustsamaxtimequantum.Thenother processeswiththesamepriorityarerun,andsoandso...



250

TimerfrequencyTimerinterruptsareraisedeveryHZthofsecond(=1jiffy) HZisnowconfigurable(inProcessortypeandfeatures): 100,250(i386default)or1000. Supportedoni386,ia64,ppc,ppc64,sparc64,x86_64 Seekernel/Kconfig.hz. Compromisebetweensystemresponsivenessandglobalthroughput. Caution:notanyvaluecanbeused.Constraintsapply! AnotherideaistocompletelyturnoffCPUtimerinterruptswhenthe systemisidle(dynamictick):seehttp://muru.com/linux/dyntick. Thissavespower.Supportsarmandi386sofar.Copyright20062004,MichaelOpdenacker Copyright20032006,OronPeled Copyright20042006CodefidenceLtd.


251

O(1)schedulerThekernelmaintains2priorityarrays: theactiveandtheexpiredarray. Eacharraycontains140entries(100realtimepriorities+40 regularones),1foreachpriority,eachcontainingalistof processeswiththesamepriority. Thearraysareimplementedinawaythatmakesitpossible topickaprocesswiththehighestpriorityinconstanttime (whateverthenumberofrunningprocesses).



252

ChoosingandexpiringprocessesTheschedulerfindsthehighestprocesspriority Itexecutesthefirstprocessinthepriorityqueueforthis priority. Oncetheprocesshasexhausteditstimeslice,itismovedto theexpiredarray. Theschedulergetsbacktoselectinganotherprocesswiththe highestpriorityavailable,andsoon... Oncetheactivearrayisempty,the2arraysareswapped! Again,everythingisdoneinconstanttime!Copyright20062004,MichaelOpdenacker Copyright20032006,OronPeled Copyright20042006CodefidenceLtd.


253

Whenisschedulingrun?Eachprocesshasaneed_reschedflagwhichisset: Afteraprocessexhausteditstimeslice. Afteraprocesswithahigherpriorityisawakened. Thisflagischecked(possiblycausingtheexecutionofthescheduler) Whenreturningtouserspacefromasystemcall Whenreturningfromaninterrupthandler(includingthecputimer) Schedulingalsohappenswhenkernelcodeexplicitelyruns schedule()orexecutesanactionthatsleeps.



254

TimeslicesThescheduleralsoprioritizeshighpriorityprocessesbygiving themabiggertimeslice. Initialprocesstimeslice:parent'stimeslicesplitin2 (otherwiseprocesswouldcheatbyforking). Minimumpriority:5msor1jiffie(whicheverislarger) Defaultpriorityinjiffies:100ms Maximumpriority:800ms Note:actuallydependsonHZ. Seekernel/sched.cfordetails.Copyright20062004,MichaelOpdenacker Copyright20032006,OronPeled Copyright20042006CodefidenceLtd.


255

DynamicprioritiesOnlyappliestoregularprocesses Forabetteruserexperience,theLinuxschedulerbootsthepriority ofinteractiveprocesses(processeswhichspendmostoftheirtime sleeping,andtaketimetoexhausttheirtimeslices).Such processesoftensleepbutneedtorespondquicklyafterwakingup (example:wordprocessorwaitingforkeypresses). Prioritybonus:upto5points. Conversely,theLinuxschedulerreducesthepriorityofcompute intensivetasks(whichquicklyexhausttheirtimeslices). Prioritypenalty:upto5points.Copyright20062004,MichaelOpdenacker Copyright20032006,OronPeled Copyright20042006CodefidenceLtd.


256

LinuxInternals

DriverdevelopmentSleeping



257

Howtosleep(1)Sleepingisneededwhenauserprocessiswaitingfordatawhich arenotreadyyet.Theprocessthenputsitselfinawaitingqueue. Staticqueuedeclaration DECLARE_WAIT_QUEUE_HEAD(module_queue); Dynamicqueuedeclaration wait_que_head_tqueue; init_waitqueue_head(&queue);



258

Howtosleep(2)Severalwaystomakeakernelprocesssleepwait_event(queue,condition); Sleepsuntilthegivenbooleanexpressionistrue. Caution:can'tbeinterrupted(i.e.bykillingtheclientprocessinuserspace) wait_event_interruptible(queue,condition); Canbeinterrupted wait_event_timeout(queue,condition,timeout); Sleepsandautomaticallywakesupafterthegiventimeout.wait_event_interruptible_timeout(queue,condition,timeout);

Sameasabove,interruptible.



259

Wakingup!Typicallydonebyinterrupthandlerswhendatasleeping processesarewaitingforareavailable. wake_up(&queue); Wakesupallthewaitingprocessesonthegivenqueue wake_up_interruptible(&queue); Doesthesamejob.Usuallycalledwhenprocesseswaited usingwait_event_interruptible.



260

LinuxInternals

DriverdevelopmentInterruptmanagement



261

NeedforinterruptsInternalprocessorinterruptsusedbytheprocessor,for exampleformultitaskscheduling. Externalinterruptsneededbecausemostinternalandexternal devicesareslowerthantheprocessor.Betternotkeepthe processorwaitingforinputdatatobereadyordatatobe output.Whenthedeviceisreadyagain,itsendsaninterrupt togettheprocessorattentionagain.



262

InterrupthandlerconstraintsNotrunfromausercontext: Can'ttransferdatatoandfromuserspace (needtobedonebysystemcallhandlers) InterrupthandlerexecutionismanagedbytheCPU,notby thescheduler.Handlerscan'trunactionsthatmaysleep, becausethereisnothingtoresumetheirexecution. Inparticular,needtoallocatememorywithGFP_ATOMIC Havetocompletetheirjobquicklyenough: theyshouldn'tblocktheirinterruptlinefortoolong.



263

Registeringaninterrupthandler(1)Definedininclude/linux/interrupt.h intrequest_irq( unsignedintirq, Requestedirqchannel irqreturn_t(*handler)(...), Interrupthandler unsignedlongirq_flags, Optionmask(seenextpage) constchar*devname, Registeredname void*dev_id); Pointertosomehandlerdata CannotbeNULLandmustbeuniqueforsharedirqs! voidfree_irq(unsignedintirq,void*dev_id);

?

Whydoesdev_idhavetobeunique?Answer...



264

Registeringaninterrupthandler(2)irq_flagsbitvalues(canbecombined,noneisfinetoo) SA_INTERRUPT "Quick"interrupthandler.Runwithallinterruptsdisabledonthecurrentcpu. Shouldn'tneedtobeusedexceptinspecificcases(suchastimerinterrupts) SA_SHIRQ Runwithinterruptsdisabledonlyonthecurrentirqlineandonthelocalcpu. Theinterruptchannelcanbesharedbyseveraldevices. RequiresahardwarestatusregistertellingwhetheranIRQwasraisedornot. SA_SAMPLE_RANDOM Interruptscanbeusedtocontributetothesystementropypoolusedby /dev/randomand/dev/urandom.Usefultogenerategoodrandom numbers.Don'tusethisiftheinterruptbehaviorofyourdeviceispredictable!Copyright20062004,MichaelOpdenacker Copyright20032006,OronPeled Copyright20042006CodefidenceLtd.


265

WhentoregisterthehandlerEitheratdriverinitializationtime: consumeslotsofIRQchannels! Oratdeviceopentime(firstcalltotheopenfileoperation): betterforsavingfreeIRQchannels. Needtocountthenumberoftimesthedeviceisopened,to beabletofreetheIRQchannelwhenthedeviceisnolonger inuse.



266

Informationoninstalledhandlers/proc/interruptsCPU0 0:5616905XTPICtimer#Registeredname 1:9828XTPICi8042 2:0XTPICcascade 3:1014243XTPICorinoco_cs 7:184XTPICIntel82801DBICH4 8:1XTPICrtc 9:2XTPICacpi 11:566583XTPICehci_hcd,uhci_hcd, uhci_hcd,uhci_hcd,yenta,yenta,radeon@PCI:1:0:0 12:5466XTPICi8042 14:121043XTPICide0 15:200888XTPICide1 NMI:0#NonMaskableInterrupts ERR:0Copyright20062004,MichaelOpdenacker Copyright20032006,OronPeled Copyright20042006CodefidenceLtd.


267

Totalnumberofinterruptscat/proc/stat|grepintr intr819076760929671037701102775520196...Totalnumber ofinterrupts IRQ1 total IRQ2 IRQ3 total ...



268

Interruptchanneldetection(1)Usefulwhenadrivercanbeusedindifferentmachines/architectures SomedevicesannouncetheirIRQchannelinaregister ManualdetectionRegisteryourinterrupthandlerforallpossiblechannels Askforaninterrupt LetthecalledinterrupthandlerstoretheIRQnumberinaglobal variable. Tryagainifnointerruptwasreceived Unregisterunusedinterrupthandlers.



269

Interruptchanneldetection(2)Kerneldetectionutilities mask=probe_irq_on(); Activateinterruptsonthedevice Deactivateinterruptsonthedevice irq=probe_irq_off(mask);>0:uniqueIRQnumberfound =0:nointerrupt.Tryagain! Pseudofilesystems) Doesn'twasteRAM:growsandshrinkstoaccommodatestoredfiles SavesRAM:noduplication;canswapoutpagestodiskwhenneeded. SeeDocumentation/filesystems/tmpfs.txtinkernelsources.



320

LinuxInternals

TheNetworksubsystem andNetworkdevicedrivers



321

AviewoftheLinuxnetworkingsubsystemStack App App 1 App2 Socket Layer UDP Networking Stack Driver Stack Driver Hardware TCP IP Networking Stack Driver ICMP Bridge App3



322

NetworkDeviceDriverHardwareInterfacexxx TxSend Send Send SentOK SendErr Free

xxx

xxx

xxx

xxxMemory Access

Driver

Rx

Free

Free

RcvOk

RcvErr RecvCRC RcvOK

xxxDMA

xxx

xxx

xxx

Driver allocates Ring Buffers. Driver resets descriptors to initial state. Driver puts packet to be sent in Tx buffers. Device puts received packet in Rx buffers. Driver/Device update descriptors to indicate state. Usually, device indicates Rx and end of Tx with interrupt, unless polling or NAPI. Based on material by:


323

NetworkDeviceDriverStackInterfaceAnetworkdevicedriverprovidesinterfacetothenetwork stack. Itdoesnothaveorusemajor/minornumbers,likecharacter devices. Anetworkdriverisrepresentedbya:structnet_device

Andisregisteredvia:intregister_netdev(structnet_device*dev); intunregister_netdev(structnet_devicedev);Copyright20062004,MichaelOpdenacker Copyright20032006,OronPeled Copyright20042006CodefidenceLtd.

Afterfillinginsomeimportantbits...


324

SocketBuffersWeneedtomanipulatepacketsthroughthestack Thismanipulationinvolvesefficiently:Addingprotocolheaders/trailersdownthestack. Removingprotocolheaders/trailersupthestack.

Packetscanbechainedtogether. Eachprotocolshouldhaveconvenientaccesstoheader fields. Todoallthisthekernelprovidesthesk_buffstructure.Copyright20062004,MichaelOpdenacker Copyright20032006,OronPeled Copyright20042006CodefidenceLtd.


325

sk_buffAnsk_buffrepresentsasinglepacket. Thisstructispassedthroughtheprotocolstack. Itholdspointerstoabufferwiththepacketdata:sk_buff mac head data nh tcp h tail telnet tailroom 326 end

headroom mac ipCopyright20062004,MichaelOpdenacker Copyright20032006,OronPeled Copyright20042006CodefidenceLtd.


sk_buffManipulationManipulatesk_buff:unsignedchar*skb_put(structsk_buff*skb, unsignedintlen); tail+=len unsignedchar*skb_push(structsk_buff*skb, unsignedintlen); data=len unsignedchar*skb_pull(structsk_buff*skb, unsignedintlen); data+=len



327

sk_buffManipulationManipulatesk_buff:intskb_headroom(conststructsk_buff*skb); datahead intskb_tailroom(conststructsk_buff*skb); endtail intskb_reserve(conststructsk_buff*skb,unsignedintlen); tail=(data+=len)



328

sk_buffAllocationLowlevelallocationisdonevia:structsk_buff*alloc_skb(unsignedint size,intgfp_mask);

Butitisbettertousethewrapper:structsk_buff*dev_alloc_skb(unsignedint size); Whichreservessomespaceforoptimization.



329

sk_buffAllocationExampleImmediatelyafterallocation,weshouldreservetheneeded headroom:structsk_buff*skb; skb=dev_alloc_skb(1500); if(unlikely(!skb))break;

/*Markasbeingusedbythisdevice*/ skb>dev=dev; /*AlignIPon16byteboundaries*/ skb_reserve(skb,2);Copyright20062004,MichaelOpdenacker Copyright20032006,OronPeled Copyright20042006CodefidenceLtd.


330

SoftnetWasintroducedinkernel2.4.x ParallelizepackethandlingonSMPmachines Packettransmit/receiveishandledviatwosoftirqs:NET_TX_SOFTIRQFeedspacketsfromnetworkstackto driver. NET_RX_SOFTIRQFeedspacketsfromdrivertonetwork stack.

Thetransmit/receivequeuesarestoredinpercpu softnet_data.Copyright20062004,MichaelOpdenacker Copyright20032006,OronPeled Copyright20042006CodefidenceLtd.


331

PacketReceptionThedriver:Allocatesanskb. setsupadescriptorintheringbuffersforthehardware.

ThedriverRxinterrupthandlercallsnetif_rx(skb). netif_rx(skb)Depositsthesk_buffinthepercpuinputqueue. MarkstheNET_RX_SOFTIRQtorun.

Laternet_rx_action()iscalledbyNET_RX_SOFTIEQ, whichcallsthedriverpoll()methodtofeedthepacketup.Copyright20062004,MichaelOpdenacker Based on material by: Normallypoll()issettoproccess_backlog()bynet_dev_init() Copyright20032006,OronPeled Copyright20042006CodefidenceLtd.

332

PacketReceptionOverview



333

PacketTransmissionEachnetworkdevicedefinesamethod:int(*hard_start_xmit)(structsk_buff*skb,struct net_device*dev);

Thismethodisindirectlycalledfromthe NET_TX_SOFTIRQ Callstothismethodareserializedviadev>xmit_lock_owner Thedrivercanmanagethetransmitqueue:voidnetif_start_queue(structnet_device*net); voidnetif_stop_queue(structnet_device*net); voidnetif_wake_queue(structnet_device*net);Copyright20062004,MichaelOpdenacker Copyright20032006,OronPeled Copyright20042006CodefidenceLtd.

intnetif_queue_stopped(structnet_device*net);Based on material by:

334

PacketReceptionOverview



335

NetworkDeviceAllocationEachnetworkdeviceisrepresentedbyastructnet_device Theyareallocatedusing:structnet_device*alloc_netdev(size,mask, setup_func);

sizesizeofourprivdatapart maskanamingpattern(e.g.eth%d) setup_funcAfunctiontopreparetherestofnet_device.

Anddeallocatedwithvoidfree_netdev(struct*net_device);



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NetworkDeviceAllocation(cont.)ForEthernetwehaveashortversion:structnet_device*alloc_etherdev(size);

whichcallsalloc_netdev(size,eth%d,ether_setup);



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NetworkDeviceInitializationThenet_deviceshouldbefilledwithnumerousmethods:openrequestresources,registerinterrupts,startqueues. stopdeallocatesresources,unregisterirq,stopqueue. get_statsreportstatistics set_multicast_listconfiguredeviceformulticast do_ioctldevicespecificIOCTLfunction change_mtuControldeviceMTUsetting hard_start_xmitcalledbythestacktoinitiateTx.



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NetworkDeviceInitialization(Cont.)Also,thedev>flagsshouldbesetaccordingtodevice capabilities:IFF_MULTICASTDevicesupportmulticast IFF_NOARPDevicedoesnotsupportARPprotocol



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NAPINetnetworkAPI Optionalprovidesinterruptmitigationunderhighload Requirements:ADMAringbuffer. Abilitytoturnoffreceiveinterrupts.

Itisusedbydefininganewmethod:int(*poll)(structnet_device*dev,int*budget);

Calledbythenetworkstackperiodicallywhensignaledby thedrivertodoso.Copyright20062004,MichaelOpdenacker Copyright20032006,OronPeled Copyright20042006CodefidenceLtd.


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NAPI(cont.)Whenareceiveinterruptoccurs,driver:Turnsoffreceiveinterrupts. Callsnetif_rx_schedule(dev)togetstacktostart callingit'spollmethod.

PollmethodScansreceiveringbuffers,feedingpacketstothestackvia: netif_receive_skb(skb). Ifworkfinishedwithinbudgetparameter,reenablesinterrupts andcallsnetif_rex_complete(dev) Else,stackwillcallpollmethodagain.Copyright20062004,MichaelOpdenacker Copyright20032006,OronPeled Copyright20042006CodefidenceLtd.


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AdviceandresourcesGettinghelpandcontributions



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SolvingissuesIfyoufaceanissue,anditdoesn'tlookspecifictoyourworkbut rathertothetoolsyouareusing,itisverylikelythatsomeoneelse alreadyfacedit. SearchtheInternetforsimilarerrorreports Onwebsitesormailinglistarchives (usingagoodsearchengine) Onnewsgroups:http://groups.google.com/ Youhavegreatchancesoffindingasolutionorworkaround,orat leastanexplanationforyourissue. Otherwise,reportingtheissueisuptoyou!Copyright20062004,MichaelOpdenacker Copyright20032006,OronPeled Copyright20042006CodefidenceLtd.


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GettinghelpIfyouhaveasupportcontract,askyourvendor Otherwise,don'thesitatetoshareyourquestionsandissues onmailinglistsEithercontacttheLinuxmailinglistforyourarchitecture(likelinux armkernelorlinuxshdev...) Orcontactthemailinglistforthesubsystemyou'redealingwith (linuxusbdevel,linuxmtd...).Don'taskthemaintainerdirectly! MostmailinglistscomewithaFAQpage.Makesureyoureadit beforecontactingthemailinglist RefrainfromcontactingtheLinuxKernelmailinglist,unlessyou're anexperienceddeveloperandneedadviceCopyright20062004,MichaelOpdenacker Copyright20032006,OronPeled Copyright20042006CodefidenceLtd.


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AdviceandresourcesBugreportandpatchsubmission



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ReportingLinuxbugsFirstmakesureyou'reusingthelatestversion Makesureyouinvestigatetheissueasmuchasyoucan: seeDocumentation/BUGHUNTING Makesurethebughasnotbeenreportedyet.Abugtrackingsystem (http://bugzilla.kernel.org/)existsbutveryfewkerneldevelopersuseit. Besttousewebsearchengines(accessingpublicmailinglistarchives) Ifthesubsystemyoureportabugonhasamailinglist,useit. Otherwise,contacttheofficialmaintainer(seetheMAINTAINERSfile). Alwaysgiveasmanyusefuldetailsaspossible.



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HowtosubmitpatchesordriversDon'tmergepatchesaddressingdifferentissues Youshouldidentifyandcontacttheofficialmaintainerforthe filestopatch. SeeDocumentation/SubmittingPatchesfordetails. Fortrivialpatches,youcancopytheTrivialPatchMonkey. Specialsubsystems:ARMplatform:it'sbesttosubmityourARMpatchestoRussell King'spatchsystem: http://www.arm.linux.org.uk/developer/patches/Copyright20062004,MichaelOpdenacker Copyright20032006,OronPeled Copyright20042006CodefidenceLtd.


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Howtobecomeakerneldeveloper?GregKroahHartmangatheredusefulreferencesandadvicefor peopleinterestedincontributingtokerneldevelopment: Documentation/HOWTO(inkernelsourcessince2.6.15rc2) Donotmissthisveryusefuldocument!



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AdviceandresourcesReferences



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Informationsites(1)LinuxWeeklyNews http://lwn.net/ TheweeklydigestoffallLinuxandfreesoftware informationsources Indepthtechnicaldiscussionsaboutthekernel Subscribetofinancetheeditors($5/month) Articlesavailablefornonsubscribers after1week.



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Informationsites(2)KernelTrap http://kerneltrap.org/ Forumwebsiteforkerneldevelopers News,articles,whitepapers,discussions,polls,interviews Perfectifadigestisnotenough!



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Usefulreading(1)LinuxDeviceDrivers,3rdedition,Feb2005 ByJonathanCorbet,AlessandroRubini,GregKroahHartman,O'Reilly http://www.oreilly.com/catalog/linuxdrive3/ Freelyavailableonline! Greatcompaniontotheprintedbookforeasyelectronicsearches! http://lwn.net/Kernel/LDD3/(1PDFfileperchapter) http://freeelectrons.com/community/kernel/ldd3/(singlePDFfile) AmusthavebookforLinuxdevicedriverwriters!



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Usefulreading(2)LinuxKernelDevelopment,2ndEdition,Jan2005 RobertLove,NovellPress http://rlove.org/kernel_book/ Averysyntheticandpleasantwaytolearnaboutkernel subsystems(beyondtheneedsofdevicedriverwriters) UnderstandingtheLinuxKernel,3rdedition,Nov2005 DanielP.Bovet,MarcoCesati,O'Reilly http://oreilly.com/catalog/understandlk/ AnextensivereviewofLinuxkernelinternals,coveringLinux2.6atlast. Unfortunately,onlycoversthePCarchitecture.



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UsefulonlineresourcesLinuxkernelmailinglistFAQ http://www.tux.org/lkml/ CompleteLinuxkernelFAQ Readthisbeforeaskingaquestiontothemailinglist KernelNewbies http://kernelnewbies.org/ Glossaries,articles,presentations,HOWTOs, recommendedreading,usefultoolsforpeople gettingfamiliarwithLinuxkernelordriver development.Copyright20062004,MichaelOpdenacker Copyright20032006,OronPeled Copyright20042006CodefidenceLtd.


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Internationalconferences(1)UsefulconferencesfeaturingLinuxkernelpresentations OttawaLinuxSymposium(July):http://linuxsymposium.org/ Rightafterthe(private)kernelsummit. Lotsofkerneltopics.Manycorekernelhackersstillpresent. Fosdem:http://fosdem.org(Brussels,February) Fordevelopers.Kernelpresentationsfromwellknownkernelhackers. CELinuxForum:http://celinuxforum.org/ Organizesseveralinternationaltechnicalconferences,inparticularin California(SanJose)andinJapan.NowopentononCELFmembers! Veryinterestingkerneltopicsforembeddedsystemsdevelopers.Copyright20062004,MichaelOpdenacker Copyright20032006,OronPeled Copyright20042006CodefidenceLtd.


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Internationalconferences(2)linux.conf.au:http://conf.linux.org.au/(Australia/NewZealand) Featuresafewpresentationsbykeykernelhackers. LinuxKongress(Germany,September/October) http://www.linuxkongress.org/ Lotsofpresentationsonthekernelbutveryexpensiveregistrationfees. Don'tmissourfreeconferencevideoson http://freeelectrons.com/community/videos/conferences/!



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AdviceandresourcesLastadvice



357

UsetheSource,Luke!ManyresourcesandtricksontheInternetfindyouwill,but solutionstoalltechnicalissuesonlyintheSourcelie.

ThankstoLucasArtsCopyright20062004,MichaelOpdenacker Copyright20032006,OronPeled Copyright20042006CodefidenceLtd.


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AnnexesQuizanswers



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Quizanswersrequest_irq,free_irq Q:Whydoesdev_idhavetobeuniqueforsharedIRQs? A:Otherwise,thekernelwouldhavenowayofknowingwhichhandlerto release.Alsoneededformultipledevices(disks,serialports...)managed bythesamedriver,whichrelyonthesameinterrupthandlercode. Interrupthandling Q:Whydidthekernelsegfaultatmoduleunload(forgettingtounregister ahandlerinasharedinterruptline)? A:Kernelmemoryisallocatedatmoduleloadtime,tohostmodulecode. Thismemoryisfreedatmoduleunloadtime.Ifyouforgettounregistera handlerandaninterruptcomes,thecpuwilltrytojumptotheaddressof thehandler,whichisinafreedmemoryarea.Crash!Copyright20062004,MichaelOpdenacker Copyright20032006,OronPeled Copyright20042006CodefidenceLtd.


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AnnexesInitrunlevels



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SystemVinitrunlevels(1)IntroducedbySystemVUnix MuchmoreflexiblethaninBSD Makeitpossibletostartorstop differentservicesforeach runlevel Correspondtotheargumentgiven to/sbin/init. Runlevelsdefinedin /etc/inittab.

/etc/initabexcerpt:id:5:initdefault: #Systeminitialization. si::sysinit:/etc/rc.d/rc.sysinit l0:0:wait:/etc/rc.d/rc0 l1:1:wait:/etc/rc.d/rc1 l2:2:wait:/etc/rc.d/rc2 l3:3:wait:/etc/rc.d/rc3 l4:4:wait:/etc/rc.d/rc4 l5:5:wait:/etc/rc.d/rc5 l6:6:wait:/etc/rc.d/rc6



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SystemVinitrunlevels(2)Standardlevels init0 Haltthesystem init1 Singleusermodeformaintenance init6 Rebootthesystem initS Singleusermodeformaintenance. Mountingonly/.Oftenidenticalto1 Customizablelevels:2,3,4,5 init3 Oftenmultiusermode,withonly commandlinelogin init5 Oftenmultiusermode,with graphicallogin



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initscriptsAccordingto/etc/inittabsettings,initruns: First/etc/rc.d/rc.sysinitforallrunlevels Thenscriptsin/etc/rc.d/ Startingservices(1,3,5,S): runsS*scriptswiththestartoption Killingservices(0,6): runsK*scriptswiththestopoption Scriptsareruninfilenamelexicalorder Justuselsltofindouttheorder!Copyright20062004,MichaelOpdenacker Copyright20032006,OronPeled Copyright20042006CodefidenceLtd.


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/etc/init.dRepositoryforallavailableinitscripts /etc/rc.d/onlycontainslinkstothe/etc/init.d/ scriptsneededforrunleveln /etc/rc1.d/example(fromFedoraCore3)K01yum>../init.d/yum K02cupsconfigdaemon>../init.d/cups configdaemon K02haldaemon>../init.d/haldaemon K02NetworkManager> ../init.d/NetworkManager K03messagebus>../init.d/messagebus K03rhnsd>../init.d/rhnsd K05anacron>../init.d/anacron K05atd>../init.d/atd S00single>../init.d/single S01sysstat>../init.d/sysstat S06cpuspeed>../init.d/cpuspeed



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HandlinginitscriptsbyhandSimplycallthe/etc/init.dscripts! /etc/init.d/sshdstart Startingsshd:[OK] /etc/init.d/nfsstop ShuttingdownNFSmountd:[FAILED] ShuttingdownNFSdaemon: [FAILED]ShuttingdownNFSquotas: [FAILED] ShuttingdownNFSservices:[OK] /etc/init.d/pcmciastatus cardmgr(pid3721)isrunning... /etc/init.d/httpdrestart Stoppinghttpd:[OK] Startinghttpd:[OK]Copyright20062004,MichaelOpdenacker Copyright20032006,OronPeled Copyright20042006CodefidenceLtd.


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LinksCodeexamples,additionalresources andupdatesareavailableat: http://www.codefidence.com/sourcedrop/course

Codefidencespecialistswillbedelightedtoprovideonetoone handsonconsultationandsupport.Giveusacall: http://www.codefidence.com



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Documents

open source software

including source code

struct list head head

data len

acme cdev

oronpeled copyright20042006codefidenceltd

tainted sp

unsignedlong addr