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SubjectRe: [PATCH, RFC, tip/core/rcu] v3 scalable classic RCU implementation
I just had a fast review. so my comments is nothing but cleanup.

Thanks, Lai.

Paul E. McKenney wrote:
> Hello!

> +rcu_start_gp(struct rcu_state *rsp, unsigned long iflg)
> + __releases(rsp->rda[smp_processor_id()]->lock)
> +{
> + unsigned long flags = iflg;
> + struct rcu_data *rdp = rsp->rda[smp_processor_id()];
> + struct rcu_node *rnp = rcu_get_root(rsp);
> + struct rcu_node *rnp_cur;
> + struct rcu_node *rnp_end;
> +
> + if (!cpu_needs_another_gp(rsp, rdp)) {
>
> /*
> - * Accessing nohz_cpu_mask before incrementing rcp->cur needs a
> - * Barrier Otherwise it can cause tickless idle CPUs to be
> - * included in rcp->cpumask, which will extend graceperiods
> - * unnecessarily.
> + * Either there is no need to detect any more grace periods
> + * at the moment, or we are already in the process of
> + * detecting one. Either way, we should not start a new
> + * RCU grace period, so drop the lock and return.
> */
> - smp_mb();
> - cpus_andnot(rcp->cpumask, cpu_online_map, nohz_cpu_mask);
> + spin_unlock_irqrestore(&rnp->lock, flags);
> + return;
> + }
> +
> + /* Advance to a new grace period and initialize state. */
> +
> + rsp->gpnum++;
> + rsp->signaled = RCU_SIGNAL_INIT;
> + rsp->jiffies_force_qs = jiffies + RCU_JIFFIES_TILL_FORCE_QS;
> + record_gp_stall_check_time();
> + dyntick_save_completed(rsp, rsp->completed - 1);
> + note_new_gpnum(rsp, rdp);
> +
> + /*
> + * Because we are first, we know that all our callbacks will
> + * be covered by this upcoming grace period, even the ones
> + * that were registered arbitrarily recently.
> + */
> +
> + rcu_next_callbacks_are_ready(rdp);
> + rdp->nxttail[RCU_WAIT_TAIL] = rdp->nxttail[RCU_NEXT_TAIL];
>
> - rcp->signaled = 0;
> + /* Special-case the common single-level case. */
> +
> + if (NUM_RCU_NODES == 1) {
> + rnp->qsmask = rnp->qsmaskinit;

I tried a mask like qsmaskinit before. The system came to deadlock
when I did on/offline cpus.
I didn't find out the whys for I bethought of these two problem:

problem 1:
----race condition 1:
<cpu_down>
synchronize_rcu <called from offline handler in other subsystem>
rcu_offline_cpu

-----race condition 2:
rcu_online_cpu
synchronize_rcu <called from online handler in other subsystem>
<cpu_up>
in these two condition, synchronize_rcu isblocked for ever for
synchronize_rcu have to wait a cpu in rnp->qsmask, but this
cpu don't run.



problem 2:
we need call rcu_offline_cpu() in these two cases in rcu_cpu_notify()
since qsmaskinit had changed by rcu_online_cpu()

case CPU_UP_CANCELED:
case CPU_UP_CANCELED_FROZEN:


> +static void
> +cpu_quiet(int cpu, struct rcu_state *rsp, struct rcu_data *rdp, long *lastcomp)
> {
> unsigned long flags;
> + long mask;

long mask -> unsigned long mask


> +static void __rcu_offline_cpu(int cpu, struct rcu_state *rsp)
> {
> - if (list) {
> - local_irq_disable();
> - this_rdp->batch = batch;
> - *this_rdp->nxttail[2] = list;
> - this_rdp->nxttail[2] = tail;
> - local_irq_enable();
> + int i;
> + unsigned long flags;
> + long mask;

long mask -> unsigned long mask


> + * Queue an RCU callback for invocation after a grace period.
> + */
> +void call_rcu(struct rcu_head *head, void (*func)(struct rcu_head *rcu))
> +{
> + unsigned long flags;
> +
> + head->func = func;
> + head->next = NULL;
> + local_irq_save(flags);
> + __call_rcu(head, &rcu_state, &__get_cpu_var(rcu_data));
> + local_irq_restore(flags);
> +}

struct rcu_state has a field: struct rcu_data *rda[NR_CPUS]
so we can move these lines around __call_rcu into __call_rcu.

__call_rcu(struct rcu_head *head, struct rcu_state *rsp)
{
local_irq_save(flags);
struct rcu_data *rdp = rsp->rda[smp_processor_id()];
.....
local_irq_save(flags);
}

> +static void
> +rcu_init_percpu_data(int cpu, struct rcu_state *rsp)
> {
> - if (user ||
> - (idle_cpu(cpu) && !in_softirq() &&
> - hardirq_count() <= (1 << HARDIRQ_SHIFT))) {
> -
> - /*
> - * Get here if this CPU took its interrupt from user
> - * mode or from the idle loop, and if this is not a
> - * nested interrupt. In this case, the CPU is in
> - * a quiescent state, so count it.
> - *
> - * Also do a memory barrier. This is needed to handle
> - * the case where writes from a preempt-disable section
> - * of code get reordered into schedule() by this CPU's
> - * write buffer. The memory barrier makes sure that
> - * the rcu_qsctr_inc() and rcu_bh_qsctr_inc() are see
> - * by other CPUs to happen after any such write.
> - */
> + unsigned long flags;
> + int i;
> + long mask;

long mask -> unsigned long mask


> +
> +/*
> + * Helper function for rcu_init() that initializes one rcu_state structure.
> + */
> +static void __init rcu_init_one(struct rcu_state *rsp)
> +{
> + int i;
> + int j;
> + struct rcu_node *rnp;
> +
> + /* Initialize the level-tracking arrays. */
> +
> + for (i = 1; i < NUM_RCU_LVLS; i++) {
> + rsp->level[i] = rsp->level[i - 1] + rsp->levelcnt[i - 1];
> + }
> + rcu_init_levelspread(rsp);
> +
> + /* Initialize the elements themselves, starting from the leaves. */
> +
> + for (i = NUM_RCU_LVLS - 1; i >= 0; i--) {
> + rnp = rsp->level[i];
> + for (j = 0; j < rsp->levelcnt[i]; j++, rnp++) {
> + spin_lock_init(&rnp->lock);
> + rnp->qsmask = 0;
> + rnp->grplo = j * rsp->levelspread[i];
> + rnp->grphi = (j + 1) * rsp->levelspread[i] - 1;
> + if (rnp->grphi >= rsp->levelcnt[i + 1])
> + rnp->grphi = rsp->levelcnt[i + 1] - 1;
> + rnp->qsmaskinit = 0;

if no other reason, I will init fields with the order as they are declared.

> + if (i != NUM_RCU_LVLS - 1)
> + rnp->grplo = rnp->grphi = 0;
> + if (i == 0) {
> + rnp->grpnum = 0;
> + rnp->parent = NULL;
> + } else {
> + rnp->grpnum = j % rsp->levelspread[i - 1];
> + rnp->parent = rsp->level[i - 1] +
> + j / rsp->levelspread[i - 1];
> + }
> + rnp->level = i;
> + }
> + }
> +}
> +
> +/*
> + * Helper macro for rcu_init(). To be used nowhere else!

rcu_init -> __rcu_init

> + * Assigns leaf node pointers into each CPU's rcu_data structure.
> + */
> +#define RCU_DATA_PTR_INIT(rsp, rcu_data) \
> +do { \
> + rnp = (rsp)->level[NUM_RCU_LVLS - 1]; \
> + j = 0; \
> + for_each_possible_cpu(i) { \
> + if (i > rnp[j].grphi) \
> + j++; \
> + per_cpu(rcu_data, i).mynode = &rnp[j]; \
> + (rsp)->rda[i] = &per_cpu(rcu_data, i); \
> + } \
> +} while (0)
> +
> static struct notifier_block __cpuinitdata rcu_nb = {
> .notifier_call = rcu_cpu_notify,
> };
>



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