1/7/2015 LinuxKernelArchive:Re:Randompanicinload_balance()with3.16rc

http://lkml.iu.edu//hypermail/linux/kernel/1407.3/00650.html 1/2

Re:Randompanicinload_balance()with3.16rcFrom:LinusTorvaldsDate:ThuJul24201414:47:25EST

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OnWed,Jul23,2014at6:43PM,MichelDnzerwrote:>>>>Michel,minddoing>>>>makekernel/sched/fair.s>>>>andsendingustheresultingfile?>>Hereitis,gzipped,hopethat'sokay.>>Notethatmytreeisnowbasedon3.16rc6.

Ok,soI'mlookingatthecodegenerationandyourcompilerispureandutter*shit*.

AddingJakubtothecc,becausegcc4.9.0seemstobeterminallybroken.

Lookiehere,yourcompilerdoessomeabsolutelyinsanethingswiththespilling,includingspillinga*constant*.Forchrissake,thatcompilershouldn'thavebeenallowedtograduatefromkindergarten.We'retalking"sloththatwasdroppedontheheadasababy"levelretardationlevelshere:

...movq$load_balance_mask,136(%rbp)#,%sfpsubq$184,%rsp#,movq(%rdx),%rax#sd_22(D)>parent,sd_parentmovl%edi,144(%rbp)#this_cpu,%sfpmovl%ecx,140(%rbp)#idle,%sfpmovq%r8,200(%rbp)#continue_balancing,%sfpmovq%rax,184(%rbp)#sd_parent,%sfpmovq136(%rbp),%rax#%sfp,tcp_ptr__#APPadd%gs:this_cpu_off,%rax#this_cpu_off,tcp_ptr__#NO_APP...

Notethecontentsof136(%rbp).Seriously.That'san_immediate_constant_thatthecompilerisspilling.

Somebodyneedstoraisethatasagccbug.Becauseitdamnwellissomeseriouslycrazyshit.

However,thatconstantspillingpartjustcountsas"toostupidtolive".Therealbugisthis:

movq$load_balance_mask,136(%rbp)#,%sfpsubq$184,%rsp#,

wheregcccreatesthestackframe*after*havingalreadyusedittosavethatconstant*deep*belowthestackframe.

Thex8664ABIspecifiesa128byteredzoneunderthestackpointer,andthisisokbythatlimit.Itlookslikeit'sillegal(136>128),butthefactis,we'vehadfour"pushq"stoupdate%rspsinceloadingtheframepointer,soit'sjust*barely*legalwiththeredzoning.

Butwebuildthekernelwithmnoredzone.Wedo*not*followthex8664ABIwrtredzoning,becausewe*cannot*:interruptswhileinkernelmode*will*usethestackwithoutaredzone.Sothat"mnoredzone"isnotsome"optionalguideline".It'sahardandharshrequirementforthekernel,andgcc4.9isabuggypieceofshitforignoringit.Andyourbughappensbecuaseyouhappentohitaninterrupt_just_inthatsingleinstructionwindow(orperhapshitsomeothersimilarcaseandcorruptedkerneldatastructuresearlier).

1/7/2015 LinuxKernelArchive:Re:Randompanicinload_balance()with3.16rc

http://lkml.iu.edu//hypermail/linux/kernel/1407.3/00650.html 2/2

Now,Isuspectthatthisredzoningbugmightactuallyberelatedtothefactthatgccisstupidinspillingaconstant.Iwouldnotbesurprisedifthereissomelivenessanalysisgoingontodecide*when*toinsertthestackdecrement,andconstantsarebeingignoredbecauseclearlylivenessisn'tanissueforaconstantvalue.Sothetwobugs("stupidconstantspilling"and"invaliduseorredzonestack")gohandinhand.Butwhoknows.

Anyway,thisisnotakernelbug.Thisisyourcompilercreatingcompletelybrokencode.Wemayneedtoaddawarningtomakesurenobodycompileswithgcc4.9.0,andtheDebianpeopleshouldprobablydowngratetheirshinynewcompiler.

Jakub,anyideas?

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