/* -*- mode: C++; c-basic-offset: 4; indent-tabs-mode: nil -*- */
// vim: ft=cpp:expandtab:ts=8:sw=4:softtabstop=4:
#ident "$Id$"
/*======
This file is part of PerconaFT.


Copyright (c) 2006, 2015, Percona and/or its affiliates. All rights reserved.

    PerconaFT is free software: you can redistribute it and/or modify
    it under the terms of the GNU General Public License, version 2,
    as published by the Free Software Foundation.

    PerconaFT 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 PerconaFT.  If not, see <http://www.gnu.org/licenses/>.

----------------------------------------

    PerconaFT is free software: you can redistribute it and/or modify
    it under the terms of the GNU Affero General Public License, version 3,
    as published by the Free Software Foundation.

    PerconaFT 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 Affero General Public License for more details.

    You should have received a copy of the GNU Affero General Public License
    along with PerconaFT.  If not, see <http://www.gnu.org/licenses/>.
======= */

#ident "Copyright (c) 2006, 2015, Percona and/or its affiliates. All rights reserved."

#pragma once

// fair mutex
struct fmutex {
    pthread_mutex_t mutex;
    int mutex_held;
    int num_want_mutex;
    struct queue_item *wait_head;
    struct queue_item *wait_tail;
};

// item on the queue
struct queue_item {
    pthread_cond_t *cond;
    struct queue_item *next;
};

static void enq_item(struct fmutex *fm, struct queue_item *const item) {
    assert(item->next == NULL);
    if (fm->wait_tail != NULL) {
        fm->wait_tail->next = item;
    } else {
        assert(fm->wait_head == NULL);
        fm->wait_head = item;
    }
    fm->wait_tail = item;
}

static pthread_cond_t *deq_item(struct fmutex *fm) {
    assert(fm->wait_head != NULL);
    assert(fm->wait_tail != NULL);
    struct queue_item *item = fm->wait_head;
    fm->wait_head = fm->wait_head->next;
    if (fm->wait_tail == item) {
        fm->wait_tail = NULL;
    }
    return item->cond;
}

void fmutex_create(struct fmutex *fm) {
    pthread_mutex_init(&fm->mutex, NULL);
    fm->mutex_held = 0;
    fm->num_want_mutex = 0;
    fm->wait_head = NULL;
    fm->wait_tail = NULL;
}

void fmutex_destroy(struct fmutex *fm) {
    pthread_mutex_destroy(&fm->mutex);
}

// Prerequisite: Holds m_mutex.
void fmutex_lock(struct fmutex *fm) {
    pthread_mutex_lock(&fm->mutex);

    if (fm->mutex_held == 0 || fm->num_want_mutex == 0) {
        // No one holds the lock.  Grant the write lock.
        fm->mutex_held = 1;
        return;
    }

    pthread_cond_t cond;
    pthread_cond_init(&cond, NULL);
    struct queue_item item = { .cond = &cond, .next = NULL };
    enq_item(fm, &item);

    // Wait for our turn.
    ++fm->num_want_mutex;
    pthread_cond_wait(&cond, &fm->mutex);
    pthread_cond_destroy(&cond);

    // Now it's our turn.
    assert(fm->num_want_mutex > 0);
    assert(fm->mutex_held == 0);

    // Not waiting anymore; grab the lock.
    --fm->num_want_mutex;
    fm->mutex_held = 1;

    pthread_mutex_unlock();
}

void fmutex_mutex_unlock(struct fmutex *fm) {
    pthread_mutex_lock();

    fm->mutex_held = 0;
    if (fm->wait_head == NULL) {
        assert(fm->num_want_mutex == 0);
        return;
    }
    assert(fm->num_want_mutex > 0);

    // Grant lock to the next waiter
    pthread_cond_t *cond = deq_item(fm);
    pthread_cond_signal(cond);

    pthread_mutex_unlock();
}

int fmutex_users(struct fmutex *fm) const {
    return fm->mutex_held + fm->num_want_mutex;
}

int fmutex_blocked_users(struct fmutex *fm) const {
    return fm->num_want_mutex;
}
