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

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264 APPENDIX E. FORMAL VERIFICATION41 fi;42 :: else -> skip;43 fi;44 atomic {45 i++;46 in_dyntick_nmi = 0;47 }48 od;49 dyntick_nmi_done = 1;50 }Of course, the fact that we have NMIs requiresadjustments in the other components. For example,the EXECUTE_MAINLINE() macro now needs to payattention to the NMI handler (in_dyntick_nmi) aswell as the interrupt handler (in_dyntick_irq) bychecking the dyntick_nmi_done variable as follows:1 #define EXECUTE_MAINLINE(label, stmt) \2 label: skip; \3 atomic { \4 if \5 :: in_dyntick_irq || \6 in_dyntick_nmi -> goto label; \7 :: else -> stmt; \8 fi; \9 } \We will also need to introduce an EXECUTE_IRQ()macrothatchecksin_dyntick_nmiinordertoallowdyntick_irq() to exclude dyntick_nmi():1 #define EXECUTE_IRQ(label, stmt) \2 label: skip; \3 atomic { \4 if \5 :: in_dyntick_nmi -> goto label; \6 :: else -> stmt; \7 fi; \8 } \It is further necessary to convert dyntick_irq()to EXECUTE_IRQ() as follows:1 proctype dyntick_irq()2 {3 byte tmp;4 byte i = 0;5 byte j = 0;6 bit old_gp_idle;7 bit outermost;89 do10 :: i >= MAX_DYNTICK_LOOP_IRQ &&11 j >= MAX_DYNTICK_LOOP_IRQ -> break;12 :: i < MAX_DYNTICK_LOOP_IRQ ->13 atomic {14 outermost = (in_dyntick_irq == 0);15 in_dyntick_irq = 1;16 }17 stmt1: skip;18 atomic {19 if20 :: in_dyntick_nmi -> goto stmt1;21 :: !in_dyntick_nmi && rcu_update_flag ->22 goto stmt1_then;23 :: else -> goto stmt1_else;24 fi;25 }26 stmt1_then: skip;27 EXECUTE_IRQ(stmt1_1, tmp = rcu_update_flag)28 EXECUTE_IRQ(stmt1_2, rcu_update_flag = tmp + 1)29 stmt1_else: skip;30 stmt2: skip; atomic {31 if32 :: in_dyntick_nmi -> goto stmt2;33 :: !in_dyntick_nmi &&34 !in_interrupt &&35 (dynticks_progress_counter & 1) == 0 ->36 goto stmt2_then;37 :: else -> goto stmt2_else;38 fi;39 }40 stmt2_then: skip;41 EXECUTE_IRQ(stmt2_1, tmp = dynticks_progress_counter)42 EXECUTE_IRQ(stmt2_2,43 dynticks_progress_counter = tmp + 1)44 EXECUTE_IRQ(stmt2_3, tmp = rcu_update_flag)45 EXECUTE_IRQ(stmt2_4, rcu_update_flag = tmp + 1)46 stmt2_else: skip;47 EXECUTE_IRQ(stmt3, tmp = in_interrupt)48 EXECUTE_IRQ(stmt4, in_interrupt = tmp + 1)49 stmt5: skip;50 atomic {51 if52 :: in_dyntick_nmi -> goto stmt4;53 :: !in_dyntick_nmi && outermost ->54 old_gp_idle = (grace_period_state == GP_IDLE);55 :: else -> skip;56 fi;57 }58 i++;59 :: j 60 stmt6: skip;61 atomic {62 if63 :: in_dyntick_nmi -> goto stmt6;64 :: !in_dyntick_nmi && j + 1 == i ->65 assert(!old_gp_idle ||66 grace_period_state != GP_DONE);67 :: else -> skip;68 fi;69 }70 EXECUTE_IRQ(stmt7, tmp = in_interrupt);71 EXECUTE_IRQ(stmt8, in_interrupt = tmp - 1);7273 stmt9: skip;74 atomic {75 if76 :: in_dyntick_nmi -> goto stmt9;77 :: !in_dyntick_nmi && rcu_update_flag != 0 ->78 goto stmt9_then;79 :: else -> goto stmt9_else;80 fi;81 }82 stmt9_then: skip;83 EXECUTE_IRQ(stmt9_1, tmp = rcu_update_flag)84 EXECUTE_IRQ(stmt9_2, rcu_update_flag = tmp - 1)85 stmt9_3: skip;86 atomic {87 if88 :: in_dyntick_nmi -> goto stmt9_3;89 :: !in_dyntick_nmi && rcu_update_flag == 0 ->90 goto stmt9_3_then;91 :: else -> goto stmt9_3_else;92 fi;93 }94 stmt9_3_then: skip;95 EXECUTE_IRQ(stmt9_3_1,96 tmp = dynticks_progress_counter)97 EXECUTE_IRQ(stmt9_3_2,98 dynticks_progress_counter = tmp + 1)99 stmt9_3_else:100 stmt9_else: skip;101 atomic {102 j++;103 in_dyntick_irq = (i != j);104 }105 od;106 dyntick_irq_done = 1;107 }Note that we have open-coded the “if” statements(for example, lines 17-29). In addition, statements
E.7. PROMELA PARABLE: DYNTICKS AND PREEMPTABLE RCU 265thatprocessstrictlylocalstate(suchasline58)neednot exclude dyntick_nmi().Finally, grace_period() requires only a fewchanges:1 proctype grace_period()2 {3 byte curr;4 byte snap;5 bit shouldexit;67 grace_period_state = GP_IDLE;8 atomic {9 printf("MDLN = %d\n", MAX_DYNTICK_LOOP_NOHZ);10 printf("MDLI = %d\n", MAX_DYNTICK_LOOP_IRQ);11 printf("MDLN = %d\n", MAX_DYNTICK_LOOP_NMI);12 shouldexit = 0;13 snap = dynticks_progress_counter;14 grace_period_state = GP_WAITING;15 }16 do17 :: 1 ->18 atomic {19 assert(!shouldexit);20 shouldexit = dyntick_nohz_done &&21 dyntick_irq_done &&22 dyntick_nmi_done;23 curr = dynticks_progress_counter;24 if25 :: (curr - snap) >= 2 || (curr & 1) == 0 ->26 break;27 :: else -> skip;28 fi;29 }30 od;31 grace_period_state = GP_DONE;32 grace_period_state = GP_IDLE;33 atomic {34 shouldexit = 0;35 snap = dynticks_progress_counter;36 grace_period_state = GP_WAITING;37 }38 do39 :: 1 ->40 atomic {41 assert(!shouldexit);42 shouldexit = dyntick_nohz_done &&43 dyntick_irq_done &&44 dyntick_nmi_done;45 curr = dynticks_progress_counter;46 if47 :: (curr != snap) || ((curr & 1) == 0) ->48 break;49 :: else -> skip;50 fi;51 }52 od;53 grace_period_state = GP_DONE;54 }We have added the printf() for the new MAX_DYNTICK_LOOP_NMI parameter on line 11 and addeddyntick_nmi_done to the shouldexit assignmentson lines 22 and 44.The model (dyntickRCU-irq-nmi-ssl.spin) resultsin a correct verification with several hundredmillion states, passing without errors.Quick Quiz E.18: Does Paul always write hiscode in this painfully incremental manner???E.7.3 Lessons (Re)LearnedThis effort provided some lessons (re)learned:static inline void rcu_enter_nohz(void){+ mb();__get_cpu_var(dynticks_progress_counter)++;- mb();}static inline void rcu_exit_nohz(void){- mb();__get_cpu_var(dynticks_progress_counter)++;+ mb();}Figure E.16: Memory-Barrier Fix Patch- if ((curr - snap) > 2 || (snap & 0x1) == 0)+ if ((curr - snap) > 2 || (curr & 0x1) == 0)Figure E.17: Variable-Name-Typo Fix Patch1. Promela and spin can verifyinterrupt/NMI-handler interactions.2. Documenting code can help locate bugs.In this case, the documentation effort locateda misplaced memory barrier in rcu_enter_nohz() and rcu_exit_nohz(), as shown by thepatch in Figure E.16.3. Validate your code early, often, and upto the point of destruction. This effortlocated one subtle bug in rcu_try_flip_waitack_needed() that would have been quitedifficult to test or debug, as shown by the patchin Figure E.17.4. Always verify your verification code. Theusual way to do this is to insert a deliberate bugand verify that the verification code catches it.Of course, if the verification code fails to catchthis bug, you may also need to verify the bugitself, and so on, recursing infinitely. However,if you find yourself in this position, getting agood night’s sleep can be an extremely effectivedebugging technique.5. Use of atomic instructions can simplifyverification. Unfortunately, use of thecmpxchg atomic instruction would also slowdown the critical irq fastpath, so they are notappropriate in this case.6. The need for complex formal verificationoften indicates a need to re-think yourdesign. In fact the design verified in this sectionturns out to have a much simpler solution,which is presented in the next section.
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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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74 CHAPTER 8. DEFERRED PROCESSINGth
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76 CHAPTER 8. DEFERRED PROCESSING
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78 CHAPTER 8. DEFERRED PROCESSINGfi
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80 CHAPTER 8. DEFERRED PROCESSINGti
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82 CHAPTER 8. DEFERRED PROCESSINGNo
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104 CHAPTER 8. DEFERRED PROCESSINGs
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106 CHAPTER 8. DEFERRED PROCESSINGo
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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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184 APPENDIX D. READ-COPY UPDATE IM
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314 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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