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

214 APPENDIX D. READ-COPY UPDATE IMPLEMENTATIONS1 static void2 rcu_init_percpu_data(int cpu, struct rcu_state *rsp)3 {4 unsigned long flags;5 int i;6 long lastcomp;7 unsigned long mask;8 struct rcu_data *rdp = rsp->rda[cpu];9 struct rcu_node *rnp = rcu_get_root(rsp);1011 spin_lock_irqsave(&rnp->lock, flags);12 lastcomp = rsp->completed;13 rdp->completed = lastcomp;14 rdp->gpnum = lastcomp;15 rdp->passed_quiesc = 0;16 rdp->qs_pending = 1;17 rdp->beenonline = 1;18 rdp->passed_quiesc_completed = lastcomp - 1;19 rdp->grpmask = 1UL mynode->grplo);20 rdp->nxtlist = NULL;21 for (i = 0; i < RCU_NEXT_SIZE; i++)22 rdp->nxttail[i] = &rdp->nxtlist;23 rdp->qlen = 0;24 rdp->blimit = blimit;25 #ifdef CONFIG_NO_HZ26 rdp->dynticks = &per_cpu(rcu_dynticks, cpu);27 #endif /* #ifdef CONFIG_NO_HZ */28 rdp->cpu = cpu;29 spin_unlock(&rnp->lock);30 spin_lock(&rsp->onofflock);31 rnp = rdp->mynode;32 mask = rdp->grpmask;33 do {34 spin_lock(&rnp->lock);35 rnp->qsmaskinit |= mask;36 mask = rnp->grpmask;37 spin_unlock(&rnp->lock);38 rnp = rnp->parent;39 } while (rnp != NULL && !(rnp->qsmaskinit & mask));40 spin_unlock(&rsp->onofflock);41 cpu_quiet(cpu, rsp, rdp, lastcomp);42 local_irq_restore(flags);43 }Figure D.30: rcu init percpu data() CodeLine 8 gets a pointer to this CPU’s rcu_datastructure, based on the specified rcu_state structure,and places this pointer into the local variablerdp. Line 9 gets a pointer to the root rcu_node structure for the specified rcu_state structure,placing it in local variable rnp.Lines 11-29 initialize the fields of the rcu_datastructure under the protection of the root rcu_nodestructure’s lock in order to ensure consistent values.Line 17 is important for tracing, due to thefact that many Linux distributions set NR_CPUS toa very large number, which could result in excessiveoutput when tracing rcu_data structures. The->beenonline field is used to solve this problem,as it will be set to the value one on any rcu_datastructurecorrespondingtoaCPUthathaseverbeenonline, and set to zero for all other rcu_data structures.This allows the tracing code to easily ignoreirrelevant CPUs.Lines 30-40 propagate the onlining CPU’s bit upthe rcu_node hierarchy, proceeding until either theroot rcu_node is reached or until the correspondingbit is already set, whichever comes first. This bitsettingis done under the protection of ->onofflockin order to exclude initialization of a new grace period,and, in addition, each rcu_node structure isinitialized under the protection of its lock. Line 41then invokes cpu_quiet() to signal RCU that thisCPU has been in an extended quiescent state, andfinally, line 42 re-enables irqs.Quick Quiz D.36: Why call cpu_quiet() online 41 of Figure D.30, given that we are excludinggrace periods with various locks, and given that anyearlier grace periods would not have been waiting onthis previously-offlined CPU?It is important to note that rcu_init_percpu_data() is invoked not only at boot time, but alsoevery time that a given CPU is brought online.D.3.4.2 rcu online cpu()Figure D.31 shows the code for rcu_online_cpu(),which informs RCU that the specified CPU is comingonline.When dynticks (CONFIG_NO_HZ) is enabled, line 6obtains a reference to the specified CPU’s rcu_dynticks structure, which is shared between the“rcu” and “rcu bh” implementations of RCU. Line 7sets the ->dynticks_nesting field to the value one,reflecting the fact that a newly onlined CPU is notin dynticks-idle mode (recall that the ->dynticks_nesting field tracks the number of reasons thatthe corresponding CPU needs to be tracked forRCU read-side critical sections, in this case because