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

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258 APPENDIX E. FORMAL VERIFICATIONrcu_try_flip_waitmb_needed(), as well asportions of rcu_try_flip_waitack() andrcu_try_flip_waitmb(), all from the 2.6.25-rc4 kernel. The following grace_period() Promelaprocess models these functions as they would beinvoked during a single pass through preemptableRCU’s grace-period processing.1 proctype grace_period()2 {3 byte curr;4 byte snap;56 atomic {7 printf("MDLN = %d\n", MAX_DYNTICK_LOOP_NOHZ);8 snap = dynticks_progress_counter;9 }10 do11 :: 1 ->12 atomic {13 curr = dynticks_progress_counter;14 if15 :: (curr == snap) && ((curr & 1) == 0) ->16 break;17 :: (curr - snap) > 2 || (snap & 1) == 0 ->18 break;19 :: 1 -> skip;20 fi;21 }22 od;23 snap = dynticks_progress_counter;24 do25 :: 1 ->26 atomic {27 curr = dynticks_progress_counter;28 if29 :: (curr == snap) && ((curr & 1) == 0) ->30 break;31 :: (curr != snap) ->32 break;33 :: 1 -> skip;34 fi;35 }36 od;37 }Lines 6-9 print out the loop limit (but only intothe .trail file in case of error) and models a lineof code from rcu_try_flip_idle() and its call todyntick_save_progress_counter(), which takes asnapshotofthecurrentCPU’sdynticks_progress_counter variable. These two lines are executedatomically to reduce state space.Lines 10-22 model the relevant code in rcu_try_flip_waitack() and its call torcu_try_flip_waitack_needed(). This loop is modeling thegrace-period state machine waiting for a counter-flipacknowledgementfromeachCPU,butonlythatpartthat interacts with dynticks-idle CPUs.Line 23 models a line from rcu_try_flip_waitzero() and its call to dyntick_save_progress_counter(), again taking a snapshot ofthe CPU’s dynticks_progress_counter variable.Finally, lines 24-36 model the relevant code inrcu_try_flip_waitack() and its call to rcu_try_flip_waitack_needed(). This loop is modeling thegrace-period state-machine waiting for each CPU toexecute a memory barrier, but again only that partthat interacts with dynticks-idle CPUs.Quick Quiz E.12: Wait a minute! In theLinux kernel, both dynticks_progress_counterand rcu_dyntick_snapshot are per-CPU variables.<strong>So</strong> why are they instead being modeled as singleglobal variables?The resulting model (dyntickRCU-base.spin),when run with the runspin.sh script, generates 691states and passes without errors, which is not at allsurprising given that it completely lacks the assertionsthatcouldfindfailures.Thenextsectionthereforeadds safety assertions.E.7.2.2 Validating SafetyA safe RCU implementation must never permit agrace period to complete before the completion ofany RCU readers that started before the start of thegrace period. This is modeled by a grace_period_state variable that can take on three states as follows:1 #define GP_IDLE 02 #define GP_WAITING 13 #define GP_DONE 24 byte grace_period_state = GP_DONE;The grace_period() process sets this variable asit progresses through the grace-period phases, asshown below:1 proctype grace_period()2 {3 byte curr;4 byte snap;56 grace_period_state = GP_IDLE;7 atomic {8 printf("MDLN = %d\n", MAX_DYNTICK_LOOP_NOHZ);9 snap = dynticks_progress_counter;10 grace_period_state = GP_WAITING;11 }12 do13 :: 1 ->14 atomic {15 curr = dynticks_progress_counter;16 if17 :: (curr == snap) && ((curr & 1) == 0) ->18 break;19 :: (curr - snap) > 2 || (snap & 1) == 0 ->20 break;21 :: 1 -> skip;22 fi;23 }24 od;25 grace_period_state = GP_DONE;26 grace_period_state = GP_IDLE;27 atomic {28 snap = dynticks_progress_counter;29 grace_period_state = GP_WAITING;30 }31 do32 :: 1 ->33 atomic {34 curr = dynticks_progress_counter;35 if
E.7. PROMELA PARABLE: DYNTICKS AND PREEMPTABLE RCU 25936 :: (curr == snap) && ((curr & 1) == 0) ->37 break;38 :: (curr != snap) ->39 break;40 :: 1 -> skip;41 fi;42 }43 od;44 grace_period_state = GP_DONE;45 }Lines 6, 10, 25, 26, 29, and 44 update this variable(combining atomically with algorithmic operationswhere feasible) to allow the dyntick_nohz() processto verify the basic RCU safety property. Theform of this verification is to assert that the valueof the grace_period_state variable cannot jumpfrom GP_IDLE to GP_DONE during a time period overwhich RCU readers could plausibly persist.Quick Quiz E.13: Giventhereareapairofbackto-backchanges to grace_period_state on lines 25and 26, how can we be sure that line 25’s changeswon’t be lost?The dyntick_nohz() Promela process implementsthis verification as shown below:1 proctype dyntick_nohz()2 {3 byte tmp;4 byte i = 0;5 bit old_gp_idle;67 do8 :: i >= MAX_DYNTICK_LOOP_NOHZ -> break;9 :: i < MAX_DYNTICK_LOOP_NOHZ ->10 tmp = dynticks_progress_counter;11 atomic {12 dynticks_progress_counter = tmp + 1;13 old_gp_idle = (grace_period_state == GP_IDLE);14 assert((dynticks_progress_counter & 1) == 1);15 }16 atomic {17 tmp = dynticks_progress_counter;18 assert(!old_gp_idle ||19 grace_period_state != GP_DONE);20 }21 atomic {22 dynticks_progress_counter = tmp + 1;23 assert((dynticks_progress_counter & 1) == 0);24 }25 i++;26 od;27 }Line 13 sets a new old_gp_idle flag if the valueof the grace_period_state variable is GP_IDLE atthe beginning of task execution, and the assertionat lines 18 and 19 fire if the grace_period_statevariable has advanced to GP_DONE during task execution,which would be illegal given that a singleRCU read-side critical section could span the entireintervening time period.Theresultingmodel(dyntickRCU-base-s.spin),when run with the runspin.sh script, generates 964statesandpasseswithouterrors, whichisreassuring.That said, although safety is critically important, itis also quite important to avoid indefinitely stallinggrace periods. The next section therefore covers verifyingliveness.E.7.2.3 Validating LivenessAlthough liveness can be difficult to prove, there isa simple trick that applies here. The first step is tomake dyntick_nohz() indicate that it is done via adyntick_nohz_done variable, as shown on line 27 ofthe following:1 proctype dyntick_nohz()2 {3 byte tmp;4 byte i = 0;5 bit old_gp_idle;67 do8 :: i >= MAX_DYNTICK_LOOP_NOHZ -> break;9 :: i < MAX_DYNTICK_LOOP_NOHZ ->10 tmp = dynticks_progress_counter;11 atomic {12 dynticks_progress_counter = tmp + 1;13 old_gp_idle = (grace_period_state == GP_IDLE);14 assert((dynticks_progress_counter & 1) == 1);15 }16 atomic {17 tmp = dynticks_progress_counter;18 assert(!old_gp_idle ||19 grace_period_state != GP_DONE);20 }21 atomic {22 dynticks_progress_counter = tmp + 1;23 assert((dynticks_progress_counter & 1) == 0);24 }25 i++;26 od;27 dyntick_nohz_done = 1;28 }With this variable in place, we can add assertionsto grace_period() to check for unnecessary blockageas follows: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 shouldexit = 0;11 snap = dynticks_progress_counter;12 grace_period_state = GP_WAITING;13 }14 do15 :: 1 ->16 atomic {17 assert(!shouldexit);18 shouldexit = dyntick_nohz_done;19 curr = dynticks_progress_counter;20 if21 :: (curr == snap) && ((curr & 1) == 0) ->22 break;23 :: (curr - snap) > 2 || (snap & 1) == 0 ->24 break;25 :: else -> skip;26 fi;27 }28 od;29 grace_period_state = GP_DONE;30 grace_period_state = GP_IDLE;31 atomic {
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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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82 CHAPTER 8. DEFERRED PROCESSINGNo
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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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308 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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