Is Parallel Programming Hard, And, If So, What Can You Do About It?

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216 APPENDIX D. READ-COPY UPDATE IMPLEMENTATIONSmynode pointer in this CPU’s rcu_data structure(see Figure D.26). Line 14 picks up a mask withthisCPU’sbitsetforuseontheleafrcu_nodestructure’sqsmask field.The loop spanning lines 15-25 then clears thisCPU’s bits up the rcu_node hierarchy, starting withthisCPU’sleafrcu_nodestructure. Line16acquiresthe current rcu_node structure’s ->lock field, andline 17 clears the bit corresponding to this CPU(or group, higher up in the hierarchy) from the->qsmaskinitfield, sothatfuturegraceperiodswillnotwaitonquiescentstatesfromthisCPU.Iftheresulting->qsmaskinit value is non-zero, as checkedby line 18, then the current rcu_node structure hasother online CPUs that it must track, so line 19 releasesthe current rcu_node structure’s ->lock andline 20 exits the loop. Otherwise, we need to continuewalking up the rcu_node hierarchy. In thiscase, line 22 picks up the mask to apply to the nextlevel up, line 23 releases the current rcu_node structure’s->lock, and line 24 advances up to the nextlevel of the hierarchy. Line 25 exits the loop shouldwe exit out the top of the hierarchy.Quick Quiz D.38: But does line 25 of FigureD.32 ever really exit the loop? Why or whynot?Line 26 picks up the the specified rcu_statestructure’s ->completed field into the local variablelastcomp, line 27 releases ->onofflock (but leavesirqs disabled), and line 28 invokes cpu_quiet() inorder to note that the CPU being offlined is now inan extended quiescent state, passing in lastcomp toavoid reporting this quiescent state against a differentgrace period than it occurred in.Quick Quiz D.39: Suppose that line 26 gotexecuted seriously out of order in Figure D.32, sothat lastcomp is set to some prior grace period, butso that the current grace period is still waiting onthe now-offline CPU? In this case, won’t the call tocpu_quiet() fail to report the quiescent state, thuscausing the grace period to wait forever for this nowofflineCPU?Quick Quiz D.40: Given that an offline CPUis in an extended quiescent state, why does line 28of Figure D.32 need to care which grace period it isdealing with?Lines 29-39 move any RCU callbacks from theCPU going offline to the currently running CPU.This operation must avoid reordering the callbacksbeing moved, as otherwise rcu_barrier() will notwork correctly. Line 29 puts a pointer to the currentlyrunning CPU’s rcu_data structure into localvariable rdp_me. Line 30 then checks to see if theCPU going offline has any RCU callbacks. If so,lines 31-38 move them. Line 31 splices the list ofcallbacks onto the end of the running CPU’s list.Lines 32-33 sets the running CPU’s callback tailpointer to that of the CPU going offline, and thenlines34-36initializethegoing-offlineCPU’slisttobeempty. Line 37 adds the length of the going-offlineCPU’s callback list to that of the currently runningCPU, and, finally, line 38 zeroes the going-offlineCPU’s list length.Quick Quiz D.41: But this list movement inFigureD.32makesallofthegoing-offlineCPU’scallbacksgo through another grace period, even if theywere ready to invoke. Isn’t that inefficient? Furthermore,couldn’t an unfortunate pattern of CPUsgoingofflinethencomingbackonlinepreventagivencallback from ever being invoked?Finally, line 40 re-enables irqs.D.3.5 Miscellaneous FunctionsThis section describes the miscellaneous utility functions:1. rcu_batches_completed2. rcu_batches_completed_bh3. cpu_has_callbacks_ready_to_invoke4. cpu_needs_another_gp5. rcu_get_rootFigure D.33 shows a number of miscellaneousfunctions. Lines 1-9 shown rcu_batches_completed() and rcu_batches_completed_bh(),whichareusedbythercutorturetestsuite. Lines11-15 show cpu_has_callbacks_ready_to_invoke(),which indicates whether the specified rcu_datastructure has RCU callbacks that have passedthrough their grace period, which is indicated bythe “done” tail pointer no longer pointing to thehead of the list. Lines 17-24 show cpu_needs_another_gp(), which indicates whether the CPUcorresponding to the specified rcu_data structurerequires an additional grace period during a timewhen no grace period is in progress. Note that thespecified rcu_data structure is required to be associatedwith the specified rcu_state structure. Finally,lines 26-30 show rcu_get_root(), which returnsthe root rcu_node structure associated withthe specified rcu_state structure.
D.3. HIERARCHICAL RCU CODE WALKTHROUGH 2171 long rcu_batches_completed(void)2 {3 return rcu_state.completed;4 }56 long rcu_batches_completed_bh(void)7 {8 return rcu_bh_state.completed;9 }1011 static int12 cpu_has_callbacks_ready_to_invoke(struct rcu_data *rdp)13 {14 return &rdp->nxtlist != rdp->nxttail[RCU_DONE_TAIL];15 }1617 static int18 cpu_needs_another_gp(struct rcu_state *rsp,19 struct rcu_data *rdp)20 {21 return *rdp->nxttail[RCU_DONE_TAIL] &&22 ACCESS_ONCE(rsp->completed) ==23 ACCESS_ONCE(rsp->gpnum);24 }2526 static struct rcu_node27 *rcu_get_root(struct rcu_state *rsp)28 {29 return &rsp->node[0];30 }1 static void note_new_gpnum(struct rcu_state *rsp,2 struct rcu_data *rdp)3 {4 rdp->qs_pending = 1;5 rdp->passed_quiesc = 0;6 rdp->gpnum = rsp->gpnum;7 rdp->n_rcu_pending_force_qs = rdp->n_rcu_pending +8 RCU_JIFFIES_TILL_FORCE_QS;9 }1011 static int12 check_for_new_grace_period(struct rcu_state *rsp,13 struct rcu_data *rdp)14 {15 unsigned long flags;16 int ret = 0;1718 local_irq_save(flags);19 if (rdp->gpnum != rsp->gpnum) {20 note_new_gpnum(rsp, rdp);21 ret = 1;22 }23 local_irq_restore(flags);24 return ret;25 }Figure D.34: Noting New Grace PeriodsFigure D.33: Miscellaneous FunctionsD.3.6 Grace-Period-Detection FunctionsThis section covers functions that are directly involvedin detecting beginnings and ends of grace periods.This of course includes actually starting andending grace periods, but also includes noting whenother CPUs have started or ended grace periods.D.3.6.1 Noting New Grace PeriodsThe main purpose of Hierarchical RCU is to detectgrace periods, and the functions more directlyinvolved in this task are described in this section.Section D.3.6.1 covers functions that allow CPUsto note that a new grace period has begun, SectionD.3.6.2 covers functions that allow CPUs tonote that an existing grace period has ended, SectionD.3.6.3 covers rcu_start_gp(), which starts anew grace period, and Section D.3.6.4 covers functionsinvolved in reporting CPUs’ quiescent statesto the RCU core.Figure D.34 shows the code for note_new_gpnum(), which updates state to reflect a new graceperiod, as well as check_for_new_grace_period(),which is used by CPUs to detect when other CPUshave started a new grace period.Line 4 of note_new_gpnum() sets the ->qs_pending flag is the current CPU’s rcu_data structureto indicate that RCU needs a quiescent statefrom this CPU, line 5 clears the ->passed_quiescflag to indicate that this CPU has not yet passedthrough such a quiescent state, line 6 copies thegrace-period number from the global rcu_statestructure to this CPU’s rcu_data structure so thatthis CPU will remember that it has already notedthe beginning of this new grace period. Finally,lines 7-8 record the time in jiffies at which this CPUwill attempt to force holdout CPUs to pass throughquiescent states (by invoking force_quiescent_state()onorafter thatfuturetime), assumingthatthe grace period does not end beforehand.Lines 18 and 23 of check_for_new_grace_period() disable and re-enable interrupts, respectively.Line 19 checks to see if there is a new graceperiod that the current CPU has not yet noted, and,if so, line 20 invokes note_new_gpnum() in order tonote the new grace period, and line 21 sets the returnvalue accordingly. Either way, line 24 returnsstatus: non-zero if a new grace period has started,and zero otherwise.Quick Quiz D.42: Why not just expand note_new_gpnum() inline into check_for_new_grace_period() in Figure D.34?D.3.6.2 Noting End of Old Grace PeriodsFigure D.35 shows rcu_process_gp_end(), whichis invoked when a CPU suspects that a grace periodmight have ended (possibly because the CPUin question in fact ended the grace period). If agrace period really has ended, then this function advancesthe current CPU’s RCU callbacks, which are
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Is Parallel Programming Hard, And,
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Contents1 Introduction 11.1 Histori
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CONTENTSv6 Locking 676.1 Staying Al
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CONTENTSviiB Synchronization Primit
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CONTENTSixE.7.1 Introduction to Pre
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PrefaceThe purpose of this book is
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Chapter 1IntroductionParallel progr
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1.2. PARALLEL PROGRAMMING GOALS 3CP
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1.3. ALTERNATIVES TO PARALLEL PROGR
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1.4. WHAT MAKES PARALLEL PROGRAMMIN
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1.5. GUIDE TO THIS BOOK 9other hand
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Chapter 2Hardware and its HabitsMos
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2.1. OVERVIEW 13Therefore, as shown
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16 CHAPTER 2. HARDWARE AND ITS HABI
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18 CHAPTER 2. HARDWARE AND ITS HABI
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20 CHAPTER 3. TOOLS OF THE TRADE1 p
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22 CHAPTER 3. TOOLS OF THE TRADE1 p
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24 CHAPTER 3. TOOLS OF THE TRADE1.1
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26 CHAPTER 3. TOOLS OF THE TRADEQui
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28 CHAPTER 3. TOOLS OF THE TRADE
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30 CHAPTER 4. COUNTING1 atomic_t co
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32 CHAPTER 4. COUNTING4.2.3 Eventua
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34 CHAPTER 4. COUNTINGvanish when t
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36 CHAPTER 4. COUNTINGper-thread va
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38 CHAPTER 4. COUNTING1 unsigned lo
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42 CHAPTER 4. COUNTING1 #define THE
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46 CHAPTER 4. COUNTINGReadsAlgorith
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48 CHAPTER 5. PARTITIONING AND SYNC
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68 CHAPTER 6. LOCKING1 int delete(i
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70 CHAPTER 7. DATA OWNERSHIP
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72 CHAPTER 8. DEFERRED PROCESSINGfo
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108 CHAPTER 8. DEFERRED PROCESSING
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110 CHAPTER 9. APPLYING RCU1 struct
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112 CHAPTER 9. APPLYING RCU
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114 CHAPTER 10. VALIDATION: DEBUGGI
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116 CHAPTER 11. DATA STRUCTURES
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118 CHAPTER 12. ADVANCED SYNCHRONIZ
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138 CHAPTER 13. EASE OF USEFigure 1
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140 CHAPTER 13. EASE OF USE
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142 CHAPTER 14. TIME MANAGEMENT
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144 CHAPTER 15. CONFLICTING VISIONS
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154 APPENDIX A. IMPORTANT QUESTIONS
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156 APPENDIX A. IMPORTANT QUESTIONS
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158 APPENDIX B. SYNCHRONIZATION PRI
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160 APPENDIX B. SYNCHRONIZATION PRI
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162 APPENDIX C. WHY MEMORY BARRIERS
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164 APPENDIX C. WHY MEMORY BARRIERS
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266 APPENDIX E. FORMAL VERIFICATION
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272 APPENDIX F. ANSWERS TO QUICK QU
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330 APPENDIX G. GLOSSARY(2) A physi
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332 APPENDIX G. GLOSSARYnear by. Th
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334 APPENDIX G. GLOSSARY
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336 BIBLIOGRAPHY[But97]USA, March 2
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338 BIBLIOGRAPHY[HMB06][Hol03][HP95
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340 BIBLIOGRAPHY[McK06] Paul E. McK
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342 BIBLIOGRAPHYtor. Software - Pra
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344 BIBLIOGRAPHY[UoC08][VGS08]Berke
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346 APPENDIX H. CREDITSH.4 Original
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