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

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208 APPENDIX D. READ-COPY UPDATE IMPLEMENTATIONS1 static void2 __call_rcu(struct rcu_head *head,3 void (*func)(struct rcu_head *rcu),4 struct rcu_state *rsp)5 {6 unsigned long flags;7 struct rcu_data *rdp;89 head->func = func;10 head->next = NULL;11 smp_mb();12 local_irq_save(flags);13 rdp = rsp->rda[smp_processor_id()];14 rcu_process_gp_end(rsp, rdp);15 check_for_new_grace_period(rsp, rdp);16 *rdp->nxttail[RCU_NEXT_TAIL] = head;17 rdp->nxttail[RCU_NEXT_TAIL] = &head->next;18 if (ACCESS_ONCE(rsp->completed) ==19 ACCESS_ONCE(rsp->gpnum)) {20 unsigned long nestflag;21 struct rcu_node *rnp_root = rcu_get_root(rsp);2223 spin_lock_irqsave(&rnp_root->lock, nestflag);24 rcu_start_gp(rsp, nestflag);25 }26 if (unlikely(++rdp->qlen > qhimark)) {27 rdp->blimit = LONG_MAX;28 force_quiescent_state(rsp, 0);29 } else if ((long)(ACCESS_ONCE(rsp->jiffies_force_qs) -30 jiffies) < 0 ||31 (rdp->n_rcu_pending_force_qs -32 rdp->n_rcu_pending) < 0)33 force_quiescent_state(rsp, 1);34 local_irq_restore(flags);35 }3637 void call_rcu(struct rcu_head *head,38 void (*func)(struct rcu_head *rcu))39 {40 __call_rcu(head, func, &rcu_state);41 }4243 void call_rcu_bh(struct rcu_head *head,44 void (*func)(struct rcu_head *rcu))45 {46 __call_rcu(head, func, &rcu_bh_state);47 }Figure D.22: call rcu() Code1 static int __rcu_pending(struct rcu_state *rsp,2 struct rcu_data *rdp)3 {4 rdp->n_rcu_pending++;56 check_cpu_stall(rsp, rdp);7 if (rdp->qs_pending)8 return 1;9 if (cpu_has_callbacks_ready_to_invoke(rdp))10 return 1;11 if (cpu_needs_another_gp(rsp, rdp))12 return 1;13 if (ACCESS_ONCE(rsp->completed) != rdp->completed)14 return 1;15 if (ACCESS_ONCE(rsp->gpnum) != rdp->gpnum)16 return 1;17 if (ACCESS_ONCE(rsp->completed) !=18 ACCESS_ONCE(rsp->gpnum) &&19 ((long)(ACCESS_ONCE(rsp->jiffies_force_qs) -20 jiffies) < 0 ||21 (rdp->n_rcu_pending_force_qs -22 rdp->n_rcu_pending) < 0))23 return 1;24 return 0;25 }2627 int rcu_pending(int cpu)28 {29 return __rcu_pending(&rcu_state,30 &per_cpu(rcu_data, cpu)) ||31 __rcu_pending(&rcu_bh_state,32 &per_cpu(rcu_bh_data, cpu));33 }3435 void rcu_check_callbacks(int cpu, int user)36 {37 if (user ||38 (idle_cpu(cpu) && !in_softirq() &&39 hardirq_count()
D.3. HIERARCHICAL RCU CODE WALKTHROUGH 209D.3.2.3 rcu check callbacks()Figure D.23 shows the code that is called fromthe scheduling-clock interrupt handler once perjiffy from each CPU. The rcu_pending() function(which is a wrapper for __rcu_pending()) is invoked,and if it returns non-zero, then rcu_check_callbacks() is invoked. (Note that there is somethought being given to merging rcu_pending() intorcu_check_callbacks().)Startingwith__rcu_pending(), line4countsthiscall to rcu_pending() for use in deciding when toforce quiescent states. Line 6 invokes check_cpu_stall() in order to report on CPUs that are spinningin the kernel, or perhaps that have hardwareproblems, if CONFIG_RCU_CPU_STALL_DETECTOR isselected. Lines 7-23 perform a series of checks, returningnon-zero if RCU needs the current CPU todo something. Line 7 checks to see if the currentCPU owes RCU a quiescent state for the currentgrace period, line 9 invokes cpu_has_callbacks_ready_to_invoke() to see if the current CPU hascallbacks whose grace period has ended, thus beingready to invoke, line 11 invokes cpu_needs_another_gp() to see if the current CPU has callbacksthat need another RCUgrace periodtoelapse,line 13 checks to see if the current grace periodhas ended, line 15 checks to see if a new graceperiod has started, and, finally, lines 17-22 checkto see if it is time to attempt to force holdoutCPUs to pass through a quiescent state. This lattercheck breaks down as follows: (1) lines 17-18check to see if there is a grace period in progress,and, if so, lines 19-22 check to see if sufficient jiffies(lines 19-20) or calls to rcu_pending() (lines 21-22) have elapsed that force_quiescent_state()should be invoked. <strong>If</strong> none of the checks in the seriestriggers, then line 24 returns zero, indicatingthat rcu_check_callbacks() need not be invoked.Lines 27-33 show rcu_pending(), which simplyinvokes __rcu_pending() twice, once for “rcu” andagain for “rcu bh”.Quick Quiz D.29: Given that rcu_pending()is always called twice on lines 29-32 of Figure D.23,shouldn’t there be some way to combine the checksof the two structures?Lines35-48showrcu_check_callbacks(), whichchecks to see if the scheduling-clock interrupt interruptedan extended quiescent state, and theninitiates RCU’s softirq processing (rcu_process_callbacks()). Lines 37-41 perform this checkfor “rcu”, while lines 42-43 perform the check for“rcu bh”.Lines 37-39 check to see if the scheduling clock in-1 static void2 __rcu_process_callbacks(struct rcu_state *rsp,3 struct rcu_data *rdp)4 {5 unsigned long flags;67 if ((long)(ACCESS_ONCE(rsp->jiffies_force_qs) -8 jiffies) < 0 ||9 (rdp->n_rcu_pending_force_qs -10 rdp->n_rcu_pending) < 0)11 force_quiescent_state(rsp, 1);12 rcu_process_gp_end(rsp, rdp);13 rcu_check_quiescent_state(rsp, rdp);14 if (cpu_needs_another_gp(rsp, rdp)) {15 spin_lock_irqsave(&rcu_get_root(rsp)->lock, flags);16 rcu_start_gp(rsp, flags);17 }18 rcu_do_batch(rdp);19 }2021 static void22 rcu_process_callbacks(struct softirq_action *unused)23 {24 smp_mb();25 __rcu_process_callbacks(&rcu_state,26 &__get_cpu_var(rcu_data));27 __rcu_process_callbacks(&rcu_bh_state,28 &__get_cpu_var(rcu_bh_data));29 smp_mb();30 }Figure D.24: rcu process callbacks() Codeterrupt came from user-mode execution (line 37) ordirectly from the idle loop (line 38’s idle_cpu() invocation)with no intervening levels of interrupt (theremainder of line 38 and all of line 39). <strong>If</strong> this checksucceeds, so that the scheduling clock interrupt didcomefromanextendedquiescentstate, thenbecauseany quiescent state for “rcu” is also a quiescent statefor “rcu bh”, lines 40 and 41 report the quiescentstate for both flavors of RCU.Similarly for “rcu bh”, line 42 checks to see ifthe scheduling-clock interrupt came from a region ofcode with softirqs enabled, and, if so line 43 reportsthe quiescent state for “rcu bh” only.Quick Quiz D.30: Shouldn’t line 42 of FigureD.23 also check for in_hardirq()?In either case, line 45 invokes an RCU softirq,which will result in rcu_process_callbacks() beingcalled on this CPU at some future time (likewhen interrupts are re-enabled after exiting thescheduler-clock interrupt).D.3.2.4 rcu process callbacks()Figure D.24 shows the code for rcu_process_callbacks(), which is a wrapper around __rcu_process_callbacks(). These functions are invokedas a result of a call to raise_softirq(RCU_SOFTIRQ), forexample, line47ofFigureD.23, whichis normally done if there is reason to believe that theRCU core needs this CPU to do something.
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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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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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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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258 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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