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

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236 APPENDIX D. READ-COPY UPDATE IMPLEMENTATIONSidlewaitackidle waitzerowaitack waitmbcompleted++CPU 2 ackCPU 0 ackCPU 1 ackCPU 3 ackOld counters zeroCPU 3 mbCPU 0 mbCPU 2 mbCPU 1 mbcompleted++Either counter mightbe incremented hereFigure D.63: Preemptable RCU State Machine2. A given CPU advances its callback lists just beforeacknowledging the counter increment.3. The blue oval represents the fact that memoryreordering might cause different CPUs tosee the increment at different times. Thismeans that a given CPU might believe thatsome other CPU has jumped the gun, usingthe new value of the counter before the counterwas actually incremented. In fact, in theory,a given CPU might see the next increment ofthe rcu_ctrlblk.completed counter as earlyas the last preceding memory barrier. (Notewell that this sentence is very imprecise. If youintend to do correctness proofs involving memorybarriers, please see Appendix D.4.3.3.4. Because rcu_read_lock() does not containany memory barriers, the corresponding RCUread-side critical sections might be reorderedby the CPU to follow the rcu_read_unlock().Therefore, the memory barriers are required toensure that the actions of the RCU read-sidecritical sections have in fact completed.5. As we will see, the fact that different CPUs cansee the counter flip happening at different timesmeans that a single trip through the state machineis not sufficient for a grace period: multipletrips are required.Figure D.64: Preemptable RCU State MachineTimeline1 void rcu_check_callbacks(int cpu, int user)2 {3 unsigned long flags;4 struct rcu_data *rdp = RCU_DATA_CPU(cpu);56 rcu_check_mb(cpu);7 if (rcu_ctrlblk.completed == rdp->completed)8 rcu_try_flip();9 spin_lock_irqsave(&rdp->lock, flags);10 RCU_TRACE_RDP(rcupreempt_trace_check_callbacks, rdp);11 __rcu_advance_callbacks(rdp);12 spin_unlock_irqrestore(&rdp->lock, flags);13 }Figure D.65: rcu check callbacks() ImplementationGrace-Period State Machine WalkthroughThis section walks through the C code that implementsthe RCU grace-period state machine, which isinvoked from the scheduling-clock interrupt, whichinvokes rcu_check_callbacks() with irqs (andthus also preemption) disabled. This function is implementedas shown in Figure D.65. Line 4 selectsthe rcu_data structure corresponding to the currentCPU, and line 6 checks to see if this CPU needsto execute a memory barrier to advance the statemachine out of the rcu_try_flip_waitmb_statestate. Line 7 checks to see if this CPU is alreadyaware of the current grace-period stage number,and line 8 attempts to advance the state machineif so. Lines 9 and 12 hold the rcu_data’s
D.4. PREEMPTABLE RCU 2371 static void rcu_check_mb(int cpu)2 {3 if (per_cpu(rcu_mb_flag, cpu) == rcu_mb_needed) {4 smp_mb();5 per_cpu(rcu_mb_flag, cpu) = rcu_mb_done;6 }7 }Figure D.66: rcu check mb() Implementation1 static void rcu_try_flip(void)2 {3 unsigned long flags;45 RCU_TRACE_ME(rcupreempt_trace_try_flip_1);6 if (!spin_trylock_irqsave(&rcu_ctrlblk.fliplock, flags)) {7 RCU_TRACE_ME(rcupreempt_trace_try_flip_e1);8 return;9 }10 switch (rcu_try_flip_state) {11 case rcu_try_flip_idle_state:12 if (rcu_try_flip_idle())13 rcu_try_flip_state = rcu_try_flip_waitack_state;14 break;15 case rcu_try_flip_waitack_state:16 if (rcu_try_flip_waitack())17 rcu_try_flip_state = rcu_try_flip_waitzero_state;18 break;19 case rcu_try_flip_waitzero_state:20 if (rcu_try_flip_waitzero())21 rcu_try_flip_state = rcu_try_flip_waitmb_state;22 break;23 case rcu_try_flip_waitmb_state:24 if (rcu_try_flip_waitmb())25 rcu_try_flip_state = rcu_try_flip_idle_state;26 }27 spin_unlock_irqrestore(&rcu_ctrlblk.fliplock, flags);28 }1 static int rcu_try_flip_idle(void)2 {3 int cpu;45 RCU_TRACE_ME(rcupreempt_trace_try_flip_i1);6 if (!rcu_pending(smp_processor_id())) {7 RCU_TRACE_ME(rcupreempt_trace_try_flip_ie1);8 return 0;9 }10 RCU_TRACE_ME(rcupreempt_trace_try_flip_g1);11 rcu_ctrlblk.completed++;12 smp_mb();13 for_each_cpu_mask(cpu, rcu_cpu_online_map)14 per_cpu(rcu_flip_flag, cpu) = rcu_flipped;15 return 1;16 }Figure D.68: rcu try flip idle() Implementation1 static int rcu_try_flip_waitack(void)2 {3 int cpu;45 RCU_TRACE_ME(rcupreempt_trace_try_flip_a1);6 for_each_cpu_mask(cpu, rcu_cpu_online_map)7 if (per_cpu(rcu_flip_flag, cpu) != rcu_flip_seen) {8 RCU_TRACE_ME(rcupreempt_trace_try_flip_ae1);9 return 0;10 }11 smp_mb();12 RCU_TRACE_ME(rcupreempt_trace_try_flip_a2);13 return 1;14 }Figure D.69: rcu try flip waitack() ImplementationFigure D.67: rcu try flip() Implementationlock, and line 11 advances callbacks if appropriate.Line 10 updates RCU tracing statistics, if enabledvia CONFIG_RCU_TRACE.Thercu_check_mb() function executes a memorybarrier as needed as shown in Figure D.66. Line 3checks to see if this CPU needs to execute a memorybarrier, and, if so, line 4 executes one and line 5informs the state machine. Note that this memorybarrierensuresthatanyCPUthatseesthenewvalueof rcu_mb_flag will also see the memory operationsexecuted by this CPU in any prior RCU read-sidecritical section.Thercu_try_flip()functionimplementsthetoplevel of the RCU grace-period state machine, asshowninFigureD.67. Line6attemptstoacquiretheglobal RCU state-machine lock, and returns if unsuccessful.Lines; 5 and 7 accumulate RCU-tracingstatistics (again, if CONFIG_RCU_TRACE is enabled).Lines 10 through 26 execute the state machine, eachinvoking a function specific to that state. Each suchfunction returns 1 if the state needs to be advancedand0otherwise. Inprinciple, thenextstatecouldbeexecuted immediately, but in practice we choose notto do so in order to reduce latency. Finally, line 27releases the global RCU state-machine lock that wasacquired by line 6.The rcu_try_flip_idle() function is calledwhen the RCU grace-period state machine is idle,and is thus responsible for getting it started whenneeded. Its code is shown in Figure D.68. Line 6checks to see if there is any RCU grace-period workpendingforthisCPU,andifnot, line8leaves, tellingthe top-level state machine to remain in the idlestate. If instead there is work to do, line 11 incrementsthe grace-period stage counter, line 12 doesa memory barrier to ensure that CPUs see the newcounter before they see the request to acknowledgeit, and lines 13 and 14 set all of the online CPUs’rcu_flip_flag. Finally, line 15 tells the top-levelstate machine to advance to the next state.Thercu_try_flip_waitack()function,showninFigure D.69, checks to see if all online CPUs haveacknowledged the counter flip (AKA ”increment”,but called ”flip” because the bottom bit, which rcu_read_lock() uses to index the rcu_flipctr array,does flip). If they have, it tells the top-level graceperiodstate machine to move to the next state.Line 6 cycles through all of the online CPUs, and
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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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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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