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

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186 APPENDIX D. READ-COPY UPDATE IMPLEMENTATIONSCPU 0 CPU 1 CPU 2 CPU 31 i0=srcu_read_lock(&s1) i3=srcu_read_lock(&s2)2 synchronize_srcu(&s1) enter3 i2=srcu_read_lock(&s1)4 srcu_read_unlock(&s1,i0)5 synchronize_srcu(&s1) exit6 srcu_read_unlock(&s1,i2)Table D.1: SRCU Update and Read-Side Critical Sectionsand therefore must not enter the SRCU read-sidecritical section, so we return -EINVAL instead. Onthe other hand, if we are not yet cleaning up, weproceed into the SRCU read-side critical section.Thecleanup() function first sets thenomoresrcuvariable on line 19, but then must wait for allcurrently executing RCU read-side critical sectionsto complete via the synchronize_rcu() primitiveon line 20. Once the cleanup() function reachesline 21, all calls to readside() that could possiblyhave seen nomorersrcu equal to zero must have alreadyreached line 11, and therefore already musthave entered their SRCU read-side critical section.All future calls to readside() will exit via line 8,and will thus refrain from entering the read-side criticalsection.Therefore, once cleanup() completes its call tosynchronize_srcu() on line 21, all SRCU read-sidecritical sections will have completed, and no newones will be able to start. It is therefore safe online 22 to call cleanup_srcu_struct() to clean up.D.1.3 ImplementationThis section describes SRCU’s data structures, initializationand cleanup primitives, read-side primitives,and update-side primitives.D.1.3.1 Data StructuresSRCU’s data structures are shown in Figure D.4,and are depicted schematically in Figure D.5. Thecompleted field is a count of the number of graceperiods since the struct srcu was initialized, andas shown in the diagram, its low-order bit is usedto index the struct srcu_struct_array. The per_cpu_reffieldpointstothearray, andthemutexfieldis used to permit but one synchronize_srcu() at atime to proceed.D.1.3.2 Initialization ImplementationSRCU’s initialization function, init_srcu_struct(), is shown in Figure D.6. This functionsimply initializes the fields in the struct1 struct srcu_struct_array {2 int c[2];3 };4 struct srcu_struct {5 int completed;6 struct srcu_struct_array *per_cpu_ref;7 struct mutex mutex;8 };Figure D.4: SRCU Data Structuresstruct srcu_structcompletedper_cpu_refmutexstruct srcu_struct_arrayCPU #0123Low−Order Bit}GP ctr LSB0 1# ## ## ## #Figure D.5: SRCU Data-Structure Diagramsrcu_struct, returning zero if initializationsucceeds or -ENOMEM otherwise.1 int init_srcu_struct(struct srcu_struct *sp)2 {3 sp->completed = 0;4 mutex_init(&sp->mutex);5 sp->per_cpu_ref =6 alloc_percpu(struct srcu_struct_array);7 return (sp->per_cpu_ref ? 0 : -ENOMEM);8 }Figure D.6: SRCU InitializationSRCU’s cleanup functions are shown in FigureD.7. The main cleanup function, cleanup_srcu_struct()isshownonlines19-29ofthisfigure,however, it immediately invokes srcu_readers_active(), shown on lines 13-17 of this figure, toverify that there are no readers currently using thisstruct srcu_struct.
D.1. SLEEPABLE RCU IMPLEMENTATION 187The srcu_readers_active() function simplyreturns the sum of srcu_readers_active_idx()on both possible indexes, while srcu_readers_active_idx(), as shown on lines 1-11, sums up theper-CPU counters corresponding to the specified index,returning the result.If the value returned from srcu_readers_active() is non-zero, then cleanup_srcu_struct() issues a warning on line 24 and simplyreturns on lines 25 and 26, declining to destroy astruct srcu_struct that is still in use. Such awarning always indicates a bug, and given that thebug has been reported, it is better to allow thesystem to continue with a modest memory leakthan to introduce possible memory corruption.Otherwise, cleanup_srcu_struct() frees the arrayof per-CPU counters and NULLs the pointer onlines 27 and 28.1 int srcu_readers_active_idx(struct srcu_struct *sp,2 int idx)3 {4 int cpu;5 int sum;67 sum = 0;8 for_each_possible_cpu(cpu)9 sum += per_cpu_ptr(sp->per_cpu_ref, cpu)->c[idx];10 return sum;11 }1213 int srcu_readers_active(struct srcu_struct *sp)14 {15 return srcu_readers_active_idx(sp, 0) +16 srcu_readers_active_idx(sp, 1);17 }1819 void cleanup_srcu_struct(struct srcu_struct *sp)20 {21 int sum;2223 sum = srcu_readers_active(sp);24 WARN_ON(sum);25 if (sum != 0)26 return;27 free_percpu(sp->per_cpu_ref);28 sp->per_cpu_ref = NULL;29 }Figure D.7: SRCU CleanupD.1.3.3 Read-Side ImplementationThecodeimplementingsrcu_read_lock()isshownin Figure D.8. This function has been carefully constructedto avoid the need for memory barriers andatomic instructions.Lines 4 and 11 disable and re-enable preemption,in order to force the sequence of code to executeunpreempted on a single CPU. Line 6 picks up thebottombitofthegrace-periodcounter, whichwillbeused to select which rank of per-CPU counters is tobeusedforthisSRCUread-sidecritical section. Thebarrier()callonline7isadirectivetothecompilerthat ensures that the index is fetched but once, 2 sothattheindexusedonline9isthesameonereturnedon line 12. Lines 8-9 increment the selected counterfor the current CPU. 3 Line 10 forces subsequent executionto occur after lines 8-9, in order to prevent tomisordering of any code in a non-CONFIG_PREEMPTbuild, but only from the perspective of an interveninginterrupt handler. However, in a CONFIG_PREEMPT kernel, the required barrier() call is embeddedin the preempt_enable() on line 11, so thesrcu_barrier() is a no-op in that case. Finally,line 12 returns the index so that it may be passed into the corresponding srcu_read_unlock().1 int srcu_read_lock(struct srcu_struct *sp)2 {3 int idx;45 preempt_disable();6 idx = sp->completed & 0x1;7 barrier();8 per_cpu_ptr(sp->per_cpu_ref,9 smp_processor_id())->c[idx]++;10 srcu_barrier();11 preempt_enable();12 return idx;13 }Figure D.8: SRCU Read-Side AcquisitionThe code for srcu_read_unlock() is shown inFigure D.9. Again, lines 3 and 7 disable andre-enable preemption so that the whole code sequenceexecutes unpreempted on a single CPU. InCONFIG_PREEMPT kernels, the preempt_disable()on line 3 contains a barrier() primitive, otherwise,the barrier() is supplied by line 4. Again, this directiveforces the subsequent code to execute afterthe critical section from the perspective of interveninginterrupt handlers. Lines 5 and 6 decrement thecounter for this CPU, but with the same index aswas used by the corresponding srcu_read_lock().The key point is that the a given CPU’s counterscan be observed by other CPUs only in cooperationwith that CPU’s interrupt handlers. These interrupthandlers are responsible for ensuring that anyneeded memory barriers are executed prior to observingthe counters.2 Please note that, despite the name, barrier() has absolutelyno effect on the CPU’s ability to reorder execution ofboth code and of memory accesses.3 It is important to note that the smp_processor_id()primitive has long-term meaning only if preemption is disabled.In absence of preemption disabling, a potential preemptionimmediately following execution of this primitivecould cause the subsequent code to execute on some otherCPU.
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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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20 CHAPTER 3. TOOLS OF THE TRADE1 p
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24 CHAPTER 3. TOOLS OF THE TRADE1.1
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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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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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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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244 APPENDIX E. FORMAL VERIFICATION
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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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