locking.c

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/*
 * Copyright (C) 2008 Oracle.  All rights reserved.
 *
 * This program is free software; you can redistribute it and/or
 * modify it under the terms of the GNU General Public
 * License v2 as published by the Free Software Foundation.
 *
 * 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, write to the
 * Free Software Foundation, Inc., 59 Temple Place - Suite 330,
 * Boston, MA 021110-1307, USA.
 */
#include <linux/sched.h>
#include <linux/pagemap.h>
#include <linux/spinlock.h>
#include <linux/page-flags.h>
#include <asm/bug.h>
#include "ctree.h"
#include "extent_io.h"
#include "locking.h"

void btrfs_assert_tree_read_locked(struct extent_buffer *eb);

/*
 * if we currently have a spinning reader or writer lock
 * (indicated by the rw flag) this will bump the count
 * of blocking holders and drop the spinlock.
 */
void btrfs_set_lock_blocking_rw(struct extent_buffer *eb, int rw)
{
    if (eb->lock_nested) {
        read_lock(&eb->lock);
        if (eb->lock_nested && current->pid == eb->lock_owner) {
            read_unlock(&eb->lock);
            return;
        }
        read_unlock(&eb->lock);
    }
    if (rw == BTRFS_WRITE_LOCK) {
        if (atomic_read(&eb->blocking_writers) == 0) {
            WARN_ON(atomic_read(&eb->spinning_writers) != 1);
            atomic_dec(&eb->spinning_writers);
            btrfs_assert_tree_locked(eb);
            atomic_inc(&eb->blocking_writers);
            write_unlock(&eb->lock);
        }
    } else if (rw == BTRFS_READ_LOCK) {
        btrfs_assert_tree_read_locked(eb);
        atomic_inc(&eb->blocking_readers);
        WARN_ON(atomic_read(&eb->spinning_readers) == 0);
        atomic_dec(&eb->spinning_readers);
        read_unlock(&eb->lock);
    }
    return;
}

/*
 * if we currently have a blocking lock, take the spinlock
 * and drop our blocking count
 */
void btrfs_clear_lock_blocking_rw(struct extent_buffer *eb, int rw)
{
    if (eb->lock_nested) {
        read_lock(&eb->lock);
        if (eb->lock_nested && current->pid == eb->lock_owner) {
            read_unlock(&eb->lock);
            return;
        }
        read_unlock(&eb->lock);
    }
    if (rw == BTRFS_WRITE_LOCK_BLOCKING) {
        BUG_ON(atomic_read(&eb->blocking_writers) != 1);
        write_lock(&eb->lock);
        WARN_ON(atomic_read(&eb->spinning_writers));
        atomic_inc(&eb->spinning_writers);
        if (atomic_dec_and_test(&eb->blocking_writers) &&
            waitqueue_active(&eb->write_lock_wq))
            wake_up(&eb->write_lock_wq);
    } else if (rw == BTRFS_READ_LOCK_BLOCKING) {
        BUG_ON(atomic_read(&eb->blocking_readers) == 0);
        read_lock(&eb->lock);
        atomic_inc(&eb->spinning_readers);
        if (atomic_dec_and_test(&eb->blocking_readers) &&
            waitqueue_active(&eb->read_lock_wq))
            wake_up(&eb->read_lock_wq);
    }
    return;
}

/*
 * take a spinning read lock.  This will wait for any blocking
 * writers
 */
void btrfs_tree_read_lock(struct extent_buffer *eb)
{
again:
    read_lock(&eb->lock);
    if (atomic_read(&eb->blocking_writers) &&
        current->pid == eb->lock_owner) {
        /*
         * This extent is already write-locked by our thread. We allow
         * an additional read lock to be added because it's for the same
         * thread. btrfs_find_all_roots() depends on this as it may be
         * called on a partly (write-)locked tree.
         */
        BUG_ON(eb->lock_nested);
        eb->lock_nested = 1;
        read_unlock(&eb->lock);
        return;
    }
    read_unlock(&eb->lock);
    wait_event(eb->write_lock_wq, atomic_read(&eb->blocking_writers) == 0);
    read_lock(&eb->lock);
    if (atomic_read(&eb->blocking_writers)) {
        read_unlock(&eb->lock);
        goto again;
    }
    atomic_inc(&eb->read_locks);
    atomic_inc(&eb->spinning_readers);
}

/*
 * returns 1 if we get the read lock and 0 if we don't
 * this won't wait for blocking writers
 */
int btrfs_try_tree_read_lock(struct extent_buffer *eb)
{
    if (atomic_read(&eb->blocking_writers))
        return 0;

    read_lock(&eb->lock);
    if (atomic_read(&eb->blocking_writers)) {
        read_unlock(&eb->lock);
        return 0;
    }
    atomic_inc(&eb->read_locks);
    atomic_inc(&eb->spinning_readers);
    return 1;
}

/*
 * returns 1 if we get the read lock and 0 if we don't
 * this won't wait for blocking writers or readers
 */
int btrfs_try_tree_write_lock(struct extent_buffer *eb)
{
    if (atomic_read(&eb->blocking_writers) ||
        atomic_read(&eb->blocking_readers))
        return 0;
    write_lock(&eb->lock);
    if (atomic_read(&eb->blocking_writers) ||
        atomic_read(&eb->blocking_readers)) {
        write_unlock(&eb->lock);
        return 0;
    }
    atomic_inc(&eb->write_locks);
    atomic_inc(&eb->spinning_writers);
    eb->lock_owner = current->pid;
    return 1;
}

/*
 * drop a spinning read lock
 */
void btrfs_tree_read_unlock(struct extent_buffer *eb)
{
    if (eb->lock_nested) {
        read_lock(&eb->lock);
        if (eb->lock_nested && current->pid == eb->lock_owner) {
            eb->lock_nested = 0;
            read_unlock(&eb->lock);
            return;
        }
        read_unlock(&eb->lock);
    }
    btrfs_assert_tree_read_locked(eb);
    WARN_ON(atomic_read(&eb->spinning_readers) == 0);
    atomic_dec(&eb->spinning_readers);
    atomic_dec(&eb->read_locks);
    read_unlock(&eb->lock);
}

/*
 * drop a blocking read lock
 */
void btrfs_tree_read_unlock_blocking(struct extent_buffer *eb)
{
    if (eb->lock_nested) {
        read_lock(&eb->lock);
        if (eb->lock_nested && current->pid == eb->lock_owner) {
            eb->lock_nested = 0;
            read_unlock(&eb->lock);
            return;
        }
        read_unlock(&eb->lock);
    }
    btrfs_assert_tree_read_locked(eb);
    WARN_ON(atomic_read(&eb->blocking_readers) == 0);
    if (atomic_dec_and_test(&eb->blocking_readers) &&
        waitqueue_active(&eb->read_lock_wq))
        wake_up(&eb->read_lock_wq);
    atomic_dec(&eb->read_locks);
}

/*
 * take a spinning write lock.  This will wait for both
 * blocking readers or writers
 */
void btrfs_tree_lock(struct extent_buffer *eb)
{
again:
    wait_event(eb->read_lock_wq, atomic_read(&eb->blocking_readers) == 0);
    wait_event(eb->write_lock_wq, atomic_read(&eb->blocking_writers) == 0);
    write_lock(&eb->lock);
    if (atomic_read(&eb->blocking_readers)) {
        write_unlock(&eb->lock);
        wait_event(eb->read_lock_wq,
               atomic_read(&eb->blocking_readers) == 0);
        goto again;
    }
    if (atomic_read(&eb->blocking_writers)) {
        write_unlock(&eb->lock);
        wait_event(eb->write_lock_wq,
               atomic_read(&eb->blocking_writers) == 0);
        goto again;
    }
    WARN_ON(atomic_read(&eb->spinning_writers));
    atomic_inc(&eb->spinning_writers);
    atomic_inc(&eb->write_locks);
    eb->lock_owner = current->pid;
}

/*
 * drop a spinning or a blocking write lock.
 */
void btrfs_tree_unlock(struct extent_buffer *eb)
{
    int blockers = atomic_read(&eb->blocking_writers);

    BUG_ON(blockers > 1);

    btrfs_assert_tree_locked(eb);
    atomic_dec(&eb->write_locks);

    if (blockers) {
        WARN_ON(atomic_read(&eb->spinning_writers));
        atomic_dec(&eb->blocking_writers);
        smp_mb();
        if (waitqueue_active(&eb->write_lock_wq))
            wake_up(&eb->write_lock_wq);
    } else {
        WARN_ON(atomic_read(&eb->spinning_writers) != 1);
        atomic_dec(&eb->spinning_writers);
        write_unlock(&eb->lock);
    }
}

void btrfs_assert_tree_locked(struct extent_buffer *eb)
{
    BUG_ON(!atomic_read(&eb->write_locks));
}

void btrfs_assert_tree_read_locked(struct extent_buffer *eb)
{
    BUG_ON(!atomic_read(&eb->read_locks));
}