/*
 * RCU-based infrastructure for lightweight reader-writer locking
 *
 * This program is free software; you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation; either version 2 of the License, or
 * (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program; if not, you can access it online at
 * http://www.gnu.org/licenses/gpl-2.0.html.
 *
 * Copyright (c) 2015, Red Hat, Inc.
 *
 * Author: Oleg Nesterov <oleg@redhat.com>
 */

#include <linux/rcu_sync.h>
#include <linux/sched.h>

#ifdef CONFIG_PROVE_RCU
#define __INIT_HELD(func)       .held = func,
#else
#define __INIT_HELD(func)
#endif

static const struct {
        void (*sync)(void);
        void (*call)(struct rcu_head *, void (*)(struct rcu_head *));
        void (*wait)(void);
#ifdef CONFIG_PROVE_RCU
        int  (*held)(void);
#endif
} gp_ops[] = {
        [RCU_SYNC] = {
                .sync = synchronize_rcu,
                .call = call_rcu,
                .wait = rcu_barrier,
                __INIT_HELD(rcu_read_lock_held)
        },
        [RCU_SCHED_SYNC] = {
                .sync = synchronize_sched,
                .call = call_rcu_sched,
                .wait = rcu_barrier_sched,
                __INIT_HELD(rcu_read_lock_sched_held)
        },
        [RCU_BH_SYNC] = {
                .sync = synchronize_rcu_bh,
                .call = call_rcu_bh,
                .wait = rcu_barrier_bh,
                __INIT_HELD(rcu_read_lock_bh_held)
        },
};

enum { GP_IDLE = 0, GP_PENDING, GP_PASSED };
enum { CB_IDLE = 0, CB_PENDING, CB_REPLAY };

#define rss_lock        gp_wait.lock

#ifdef CONFIG_PROVE_RCU
void rcu_sync_lockdep_assert(struct rcu_sync *rsp)
{
        RCU_LOCKDEP_WARN(!gp_ops[rsp->gp_type].held(),
                         "suspicious rcu_sync_is_idle() usage");
}

EXPORT_SYMBOL_GPL(rcu_sync_lockdep_assert);
#endif

/**
 * rcu_sync_init() - Initialize an rcu_sync structure
 * @rsp: Pointer to rcu_sync structure to be initialized
 * @type: Flavor of RCU with which to synchronize rcu_sync structure
 */
void rcu_sync_init(struct rcu_sync *rsp, enum rcu_sync_type type)
{
        memset(rsp, 0, sizeof(*rsp));
        init_waitqueue_head(&rsp->gp_wait);
        rsp->gp_type = type;
}

/**
 * rcu_sync_enter_start - Force readers onto slow path for multiple updates
 * @rsp: Pointer to rcu_sync structure to use for synchronization
 *
 * Must be called after rcu_sync_init() and before first use.
 *
 * Ensures rcu_sync_is_idle() returns false and rcu_sync_{enter,exit}()
 * pairs turn into NO-OPs.
 */
void rcu_sync_enter_start(struct rcu_sync *rsp)
{
        rsp->gp_count++;
        rsp->gp_state = GP_PASSED;
}

/**
 * rcu_sync_enter() - Force readers onto slowpath
 * @rsp: Pointer to rcu_sync structure to use for synchronization
 *
 * This function is used by updaters who need readers to make use of
 * a slowpath during the update.  After this function returns, all
 * subsequent calls to rcu_sync_is_idle() will return false, which
 * tells readers to stay off their fastpaths.  A later call to
 * rcu_sync_exit() re-enables reader slowpaths.
 *
 * When called in isolation, rcu_sync_enter() must wait for a grace
 * period, however, closely spaced calls to rcu_sync_enter() can
 * optimize away the grace-period wait via a state machine implemented
 * by rcu_sync_enter(), rcu_sync_exit(), and rcu_sync_func().
 */
void rcu_sync_enter(struct rcu_sync *rsp)
{
        bool need_wait, need_sync;

        spin_lock_irq(&rsp->rss_lock);
        need_wait = rsp->gp_count++;
        need_sync = rsp->gp_state == GP_IDLE;
        if (need_sync)
                rsp->gp_state = GP_PENDING;
        spin_unlock_irq(&rsp->rss_lock);

        BUG_ON(need_wait && need_sync);

        if (need_sync) {
                gp_ops[rsp->gp_type].sync();
                rsp->gp_state = GP_PASSED;
                wake_up_all(&rsp->gp_wait);
        } else if (need_wait) {
                wait_event(rsp->gp_wait, rsp->gp_state == GP_PASSED);
        } else {
                /*
                 * Possible when there's a pending CB from a rcu_sync_exit().
                 * Nobody has yet been allowed the 'fast' path and thus we can
                 * avoid doing any sync(). The callback will get 'dropped'.
                 */
                BUG_ON(rsp->gp_state != GP_PASSED);
        }
}

/**
 * rcu_sync_func() - Callback function managing reader access to fastpath
 * @rhp: Pointer to rcu_head in rcu_sync structure to use for synchronization
 *
 * This function is passed to one of the call_rcu() functions by
 * rcu_sync_exit(), so that it is invoked after a grace period following the
 * that invocation of rcu_sync_exit().  It takes action based on events that
 * have taken place in the meantime, so that closely spaced rcu_sync_enter()
 * and rcu_sync_exit() pairs need not wait for a grace period.
 *
 * If another rcu_sync_enter() is invoked before the grace period
 * ended, reset state to allow the next rcu_sync_exit() to let the
 * readers back onto their fastpaths (after a grace period).  If both
 * another rcu_sync_enter() and its matching rcu_sync_exit() are invoked
 * before the grace period ended, re-invoke call_rcu() on behalf of that
 * rcu_sync_exit().  Otherwise, set all state back to idle so that readers
 * can again use their fastpaths.
 */
static void rcu_sync_func(struct rcu_head *rhp)
{
        struct rcu_sync *rsp = container_of(rhp, struct rcu_sync, cb_head);
        unsigned long flags;

        BUG_ON(rsp->gp_state != GP_PASSED);
        BUG_ON(rsp->cb_state == CB_IDLE);

        spin_lock_irqsave(&rsp->rss_lock, flags);
        if (rsp->gp_count) {
                /*
                 * A new rcu_sync_begin() has happened; drop the callback.
                 */
                rsp->cb_state = CB_IDLE;
        } else if (rsp->cb_state == CB_REPLAY) {
                /*
                 * A new rcu_sync_exit() has happened; requeue the callback
                 * to catch a later GP.
                 */
                rsp->cb_state = CB_PENDING;
                gp_ops[rsp->gp_type].call(&rsp->cb_head, rcu_sync_func);
        } else {
                /*
                 * We're at least a GP after rcu_sync_exit(); eveybody will now
                 * have observed the write side critical section. Let 'em rip!.
                 */
                rsp->cb_state = CB_IDLE;
                rsp->gp_state = GP_IDLE;
        }
        spin_unlock_irqrestore(&rsp->rss_lock, flags);
}

/**
 * rcu_sync_exit() - Allow readers back onto fast patch after grace period
 * @rsp: Pointer to rcu_sync structure to use for synchronization
 *
 * This function is used by updaters who have completed, and can therefore
 * now allow readers to make use of their fastpaths after a grace period
 * has elapsed.  After this grace period has completed, all subsequent
 * calls to rcu_sync_is_idle() will return true, which tells readers that
 * they can once again use their fastpaths.
 */
void rcu_sync_exit(struct rcu_sync *rsp)
{
        spin_lock_irq(&rsp->rss_lock);
        if (!--rsp->gp_count) {
                if (rsp->cb_state == CB_IDLE) {
                        rsp->cb_state = CB_PENDING;
                        gp_ops[rsp->gp_type].call(&rsp->cb_head, rcu_sync_func);
                } else if (rsp->cb_state == CB_PENDING) {
                        rsp->cb_state = CB_REPLAY;
                }
        }
        spin_unlock_irq(&rsp->rss_lock);
}

/**
 * rcu_sync_dtor() - Clean up an rcu_sync structure
 * @rsp: Pointer to rcu_sync structure to be cleaned up
 */
void rcu_sync_dtor(struct rcu_sync *rsp)
{
        int cb_state;

        BUG_ON(rsp->gp_count);

        spin_lock_irq(&rsp->rss_lock);
        if (rsp->cb_state == CB_REPLAY)
                rsp->cb_state = CB_PENDING;
        cb_state = rsp->cb_state;
        spin_unlock_irq(&rsp->rss_lock);

        if (cb_state != CB_IDLE) {
                gp_ops[rsp->gp_type].wait();
                BUG_ON(rsp->cb_state != CB_IDLE);
        }
}