/*
 *	BIRD Library -- Read-Copy-Update Basic Operations
 *
 *	(c) 2021 Maria Matejka <mq@jmq.cz>
 *	(c) 2021 CZ.NIC z.s.p.o.
 *
 *	Can be freely distributed and used under the terms of the GNU GPL.
 *	Note: all the relevant patents shall be expired.
 *
 *	Using the Supplementary Material for User-Level Implementations of Read-Copy-Update
 *	by Matthieu Desnoyers, Paul E. McKenney, Alan S. Stern, Michel R. Dagenais and Jonathan Walpole
 *	obtained from https://www.efficios.com/pub/rcu/urcu-supp-accepted.pdf
 */

#include "lib/rcu.h"
#include "lib/io-loop.h"
#include "lib/locking.h"

_Atomic uint rcu_gp_ctl = RCU_NEST_CNT;
_Thread_local struct rcu_thread *this_rcu_thread = NULL;
_Thread_local uint rcu_blocked;

static list rcu_thread_list;

static struct rcu_thread main_rcu_thread;

static DOMAIN(resource) rcu_domain;

static int
rcu_gp_ongoing(_Atomic uint *ctl)
{
  uint val = atomic_load(ctl);
  return (val & RCU_NEST_CNT) && ((val ^ atomic_load_explicit(&rcu_gp_ctl, memory_order_acquire)) & RCU_GP_PHASE);
}

static void
update_counter_and_wait(void)
{
  atomic_fetch_xor(&rcu_gp_ctl, RCU_GP_PHASE);
  struct rcu_thread *rc;
  WALK_LIST(rc, rcu_thread_list)
    while (rcu_gp_ongoing(&rc->ctl))
      birdloop_yield();
}

void
synchronize_rcu(void)
{
  if (!rcu_blocked && last_locked)
    bug("Forbidden to synchronize RCU unless an appropriate lock is taken");

  LOCK_DOMAIN(resource, rcu_domain);
  update_counter_and_wait();
  update_counter_and_wait();
  UNLOCK_DOMAIN(resource, rcu_domain);
}

void
rcu_thread_start(struct rcu_thread *rc)
{
  LOCK_DOMAIN(resource, rcu_domain);
  add_tail(&rcu_thread_list, &rc->n);
  this_rcu_thread = rc;
  UNLOCK_DOMAIN(resource, rcu_domain);
}

void
rcu_thread_stop(struct rcu_thread *rc)
{
  LOCK_DOMAIN(resource, rcu_domain);
  this_rcu_thread = NULL;
  rem_node(&rc->n);
  UNLOCK_DOMAIN(resource, rcu_domain);
}

void
rcu_init(void)
{
  rcu_domain = DOMAIN_NEW(resource);
  DOMAIN_SETUP(resource, rcu_domain, "Read-Copy-Update", NULL);
  init_list(&rcu_thread_list);
  rcu_thread_start(&main_rcu_thread);
}