RCU (Read-Copy-Update) 기초 — Quiescent State·Grace Period

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struct config *cfg;
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void writer(void) {
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    struct config *old, *new;
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    new = kmalloc(sizeof(*new), GFP_KERNEL);
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    *new = *current_cfg();
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    new->max_threads = 16;
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    old = rcu_dereference_protected(cfg, lockdep_is_held(&cfg_mtx));
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    rcu_assign_pointer(cfg, new);
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    synchronize_rcu();      /* 모든 reader가 끝날 때까지 wait */
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    kfree(old);
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}
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void reader(void) {
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    struct config *c;
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    rcu_read_lock();
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    c = rcu_dereference(cfg);
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    process(c);
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    rcu_read_unlock();
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}

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#include <urcu.h>
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struct config *cfg;
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void *reader_thread(void *arg) {
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    rcu_register_thread();
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    for (;;) {
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        rcu_read_lock();
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        struct config *c = rcu_dereference(cfg);
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        process(c);
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        rcu_read_unlock();
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    }
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    rcu_unregister_thread();
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}
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void update_config(struct config *new_cfg) {
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    struct config *old = rcu_xchg_pointer(&cfg, new_cfg);
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    synchronize_rcu();
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    free(old);
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}
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int main(void) {
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    rcu_init();
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    /* threads ... */
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}

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struct foo {
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    struct rcu_head rcu;
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    int data;
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};
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static void foo_free(struct rcu_head *r) {
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    struct foo *f = container_of(r, struct foo, rcu);
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    kfree(f);
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}
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void writer(void) {
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    struct foo *old = rcu_dereference_protected(g, ...);
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    rcu_assign_pointer(g, new);
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    call_rcu(&old->rcu, foo_free);    /* sleep 없이 grace period 예약 */
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}

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#include <linux/rculist.h>
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struct entry {
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    struct list_head list;
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    int key;
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};
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LIST_HEAD(g_list);
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void add_entry(struct entry *e) {
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    spin_lock(&list_lock);
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    list_add_rcu(&e->list, &g_list);
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    spin_unlock(&list_lock);
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}
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void remove_entry(struct entry *e) {
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    spin_lock(&list_lock);
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    list_del_rcu(&e->list);
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    spin_unlock(&list_lock);
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    synchronize_rcu();
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    kfree(e);
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}
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void scan(void) {
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    struct entry *e;
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    rcu_read_lock();
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    list_for_each_entry_rcu(e, &g_list, list) {
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        process(e);
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    }
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    rcu_read_unlock();
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}

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/* per-CPU counter — RCU 변종 */
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DEFINE_PER_CPU(unsigned long, hits);
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void hit(void) {
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    this_cpu_inc(hits);
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}
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unsigned long total(void) {
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    unsigned long s = 0;
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    for_each_possible_cpu(c) s += per_cpu(hits, c);
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    return s;
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}

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writer 비용 비교
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spinlock writer         150 ns
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rwlock writer           수 µs (모든 reader가 끝나야)
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RCU writer + grace     수 ms (grace period 대기)
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RCU writer + call_rcu  150 ns (callback 예약)

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struct config *c = rcu_dereference(cfg);   /* unlock 밖 — UB */

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rcu_read_lock();
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msleep(10);    /* preempt 가능 → grace period 추정 깨짐 */
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rcu_read_unlock();

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list_add_rcu(...);   /* spinlock 없음 — writer끼리 race */

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for (i = 0; i < N; i++) {
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    new = ...;
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    rcu_assign_pointer(p, new);
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    synchronize_rcu();     /* 매 iteration ms 대기 */
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}

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/* URCU thread가 rcu_register_thread 안 부름 */
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rcu_read_lock();    /* 등록 안 된 thread → assert fail 또는 silent corruption */