btrfs: comment the rest of implicit barriers before waitqueue_active

[linux-2.6-block.git] / fs / btrfs / locking.c

/*
 * 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"

static 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)
{
	/*
	 * no lock is required.  The lock owner may change if
	 * we have a read lock, but it won't change to or away
	 * from us.  If we have the write lock, we are the owner
	 * and it'll never change.
	 */
	if (eb->lock_nested && current->pid == eb->lock_owner)
		return;
	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)
{
	/*
	 * no lock is required.  The lock owner may change if
	 * we have a read lock, but it won't change to or away
	 * from us.  If we have the write lock, we are the owner
	 * and it'll never change.
	 */
	if (eb->lock_nested && current->pid == eb->lock_owner)
		return;

	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);
		/*
		 * atomic_dec_and_test implies a barrier for waitqueue_active
		 */
		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);
		/*
		 * atomic_dec_and_test implies a barrier for waitqueue_active
		 */
		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:
	BUG_ON(!atomic_read(&eb->blocking_writers) &&
	       current->pid == eb->lock_owner);

	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;
	}
	if (atomic_read(&eb->blocking_writers)) {
		read_unlock(&eb->lock);
		wait_event(eb->write_lock_wq,
			   atomic_read(&eb->blocking_writers) == 0);
		goto again;
	}
	atomic_inc(&eb->read_locks);
	atomic_inc(&eb->spinning_readers);
}

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

	if (!read_trylock(&eb->lock))
		return 0;

	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 we're nested, we have the write lock.  No new locking
	 * is needed as long as we are the lock owner.
	 * The write unlock will do a barrier for us, and the lock_nested
	 * field only matters to the lock owner.
	 */
	if (eb->lock_nested && current->pid == eb->lock_owner) {
		eb->lock_nested = 0;
		return;
	}
	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 we're nested, we have the write lock.  No new locking
	 * is needed as long as we are the lock owner.
	 * The write unlock will do a barrier for us, and the lock_nested
	 * field only matters to the lock owner.
	 */
	if (eb->lock_nested && current->pid == eb->lock_owner) {
		eb->lock_nested = 0;
		return;
	}
	btrfs_assert_tree_read_locked(eb);
	WARN_ON(atomic_read(&eb->blocking_readers) == 0);
	/*
	 * atomic_dec_and_test implies a barrier for waitqueue_active
	 */
	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)
{
	WARN_ON(eb->lock_owner == current->pid);
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);
	eb->lock_owner = 0;
	atomic_dec(&eb->write_locks);

	if (blockers) {
		WARN_ON(atomic_read(&eb->spinning_writers));
		atomic_dec(&eb->blocking_writers);
		/*
		 * Make sure counter is updated before we wake up waiters.
		 */
		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));
}

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