Merge branch 'qgroup' of git://git.jan-o-sch.net/btrfs-unstable into for-linus

[mirror_ubuntu-bionic-kernel.git] / fs / btrfs / transaction.c

/*
 * Copyright (C) 2007 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/fs.h>
#include <linux/slab.h>
#include <linux/sched.h>
#include <linux/writeback.h>
#include <linux/pagemap.h>
#include <linux/blkdev.h>
#include "ctree.h"
#include "disk-io.h"
#include "transaction.h"
#include "locking.h"
#include "tree-log.h"
#include "inode-map.h"
#include "volumes.h"

#define BTRFS_ROOT_TRANS_TAG 0

void put_transaction(struct btrfs_transaction *transaction)
{
        WARN_ON(atomic_read(&transaction->use_count) == 0);
        if (atomic_dec_and_test(&transaction->use_count)) {
                BUG_ON(!list_empty(&transaction->list));
                WARN_ON(transaction->delayed_refs.root.rb_node);
                memset(transaction, 0, sizeof(*transaction));
                kmem_cache_free(btrfs_transaction_cachep, transaction);
        }
}

static noinline void switch_commit_root(struct btrfs_root *root)
{
        free_extent_buffer(root->commit_root);
        root->commit_root = btrfs_root_node(root);
}

/*
 * either allocate a new transaction or hop into the existing one
 */
static noinline int join_transaction(struct btrfs_root *root, int nofail)
{
        struct btrfs_transaction *cur_trans;
        struct btrfs_fs_info *fs_info = root->fs_info;

        spin_lock(&fs_info->trans_lock);
loop:
        /* The file system has been taken offline. No new transactions. */
        if (fs_info->fs_state & BTRFS_SUPER_FLAG_ERROR) {
                spin_unlock(&fs_info->trans_lock);
                return -EROFS;
        }

        if (fs_info->trans_no_join) {
                if (!nofail) {
                        spin_unlock(&fs_info->trans_lock);
                        return -EBUSY;
                }
        }

        cur_trans = fs_info->running_transaction;
        if (cur_trans) {
                if (cur_trans->aborted) {
                        spin_unlock(&fs_info->trans_lock);
                        return cur_trans->aborted;
                }
                atomic_inc(&cur_trans->use_count);
                atomic_inc(&cur_trans->num_writers);
                cur_trans->num_joined++;
                spin_unlock(&fs_info->trans_lock);
                return 0;
        }
        spin_unlock(&fs_info->trans_lock);

        cur_trans = kmem_cache_alloc(btrfs_transaction_cachep, GFP_NOFS);
        if (!cur_trans)
                return -ENOMEM;

        spin_lock(&fs_info->trans_lock);
        if (fs_info->running_transaction) {
                /*
                 * someone started a transaction after we unlocked.  Make sure
                 * to redo the trans_no_join checks above
                 */
                kmem_cache_free(btrfs_transaction_cachep, cur_trans);
                cur_trans = fs_info->running_transaction;
                goto loop;
        } else if (fs_info->fs_state & BTRFS_SUPER_FLAG_ERROR) {
                spin_unlock(&fs_info->trans_lock);
                kmem_cache_free(btrfs_transaction_cachep, cur_trans);
                return -EROFS;
        }

        atomic_set(&cur_trans->num_writers, 1);
        cur_trans->num_joined = 0;
        init_waitqueue_head(&cur_trans->writer_wait);
        init_waitqueue_head(&cur_trans->commit_wait);
        cur_trans->in_commit = 0;
        cur_trans->blocked = 0;
        /*
         * One for this trans handle, one so it will live on until we
         * commit the transaction.
         */
        atomic_set(&cur_trans->use_count, 2);
        cur_trans->commit_done = 0;
        cur_trans->start_time = get_seconds();

        cur_trans->delayed_refs.root = RB_ROOT;
        cur_trans->delayed_refs.num_entries = 0;
        cur_trans->delayed_refs.num_heads_ready = 0;
        cur_trans->delayed_refs.num_heads = 0;
        cur_trans->delayed_refs.flushing = 0;
        cur_trans->delayed_refs.run_delayed_start = 0;

        /*
         * although the tree mod log is per file system and not per transaction,
         * the log must never go across transaction boundaries.
         */
        smp_mb();
        if (!list_empty(&fs_info->tree_mod_seq_list)) {
                printk(KERN_ERR "btrfs: tree_mod_seq_list not empty when "
                        "creating a fresh transaction\n");
                WARN_ON(1);
        }
        if (!RB_EMPTY_ROOT(&fs_info->tree_mod_log)) {
                printk(KERN_ERR "btrfs: tree_mod_log rb tree not empty when "
                        "creating a fresh transaction\n");
                WARN_ON(1);
        }
        atomic_set(&fs_info->tree_mod_seq, 0);

        spin_lock_init(&cur_trans->commit_lock);
        spin_lock_init(&cur_trans->delayed_refs.lock);

        INIT_LIST_HEAD(&cur_trans->pending_snapshots);
        list_add_tail(&cur_trans->list, &fs_info->trans_list);
        extent_io_tree_init(&cur_trans->dirty_pages,
                             fs_info->btree_inode->i_mapping);
        fs_info->generation++;
        cur_trans->transid = fs_info->generation;
        fs_info->running_transaction = cur_trans;
        cur_trans->aborted = 0;
        spin_unlock(&fs_info->trans_lock);

        return 0;
}

/*
 * this does all the record keeping required to make sure that a reference
 * counted root is properly recorded in a given transaction.  This is required
 * to make sure the old root from before we joined the transaction is deleted
 * when the transaction commits
 */
static int record_root_in_trans(struct btrfs_trans_handle *trans,
                               struct btrfs_root *root)
{
        if (root->ref_cows && root->last_trans < trans->transid) {
                WARN_ON(root == root->fs_info->extent_root);
                WARN_ON(root->commit_root != root->node);

                /*
                 * see below for in_trans_setup usage rules
                 * we have the reloc mutex held now, so there
                 * is only one writer in this function
                 */
                root->in_trans_setup = 1;

                /* make sure readers find in_trans_setup before
                 * they find our root->last_trans update
                 */
                smp_wmb();

                spin_lock(&root->fs_info->fs_roots_radix_lock);
                if (root->last_trans == trans->transid) {
                        spin_unlock(&root->fs_info->fs_roots_radix_lock);
                        return 0;
                }
                radix_tree_tag_set(&root->fs_info->fs_roots_radix,
                           (unsigned long)root->root_key.objectid,
                           BTRFS_ROOT_TRANS_TAG);
                spin_unlock(&root->fs_info->fs_roots_radix_lock);
                root->last_trans = trans->transid;

                /* this is pretty tricky.  We don't want to
                 * take the relocation lock in btrfs_record_root_in_trans
                 * unless we're really doing the first setup for this root in
                 * this transaction.
                 *
                 * Normally we'd use root->last_trans as a flag to decide
                 * if we want to take the expensive mutex.
                 *
                 * But, we have to set root->last_trans before we
                 * init the relocation root, otherwise, we trip over warnings
                 * in ctree.c.  The solution used here is to flag ourselves
                 * with root->in_trans_setup.  When this is 1, we're still
                 * fixing up the reloc trees and everyone must wait.
                 *
                 * When this is zero, they can trust root->last_trans and fly
                 * through btrfs_record_root_in_trans without having to take the
                 * lock.  smp_wmb() makes sure that all the writes above are
                 * done before we pop in the zero below
                 */
                btrfs_init_reloc_root(trans, root);
                smp_wmb();
                root->in_trans_setup = 0;
        }
        return 0;
}


int btrfs_record_root_in_trans(struct btrfs_trans_handle *trans,
                               struct btrfs_root *root)
{
        if (!root->ref_cows)
                return 0;

        /*
         * see record_root_in_trans for comments about in_trans_setup usage
         * and barriers
         */
        smp_rmb();
        if (root->last_trans == trans->transid &&
            !root->in_trans_setup)
                return 0;

        mutex_lock(&root->fs_info->reloc_mutex);
        record_root_in_trans(trans, root);
        mutex_unlock(&root->fs_info->reloc_mutex);

        return 0;
}

/* wait for commit against the current transaction to become unblocked
 * when this is done, it is safe to start a new transaction, but the current
 * transaction might not be fully on disk.
 */
static void wait_current_trans(struct btrfs_root *root)
{
        struct btrfs_transaction *cur_trans;

        spin_lock(&root->fs_info->trans_lock);
        cur_trans = root->fs_info->running_transaction;
        if (cur_trans && cur_trans->blocked) {
                atomic_inc(&cur_trans->use_count);
                spin_unlock(&root->fs_info->trans_lock);

                wait_event(root->fs_info->transaction_wait,
                           !cur_trans->blocked);
                put_transaction(cur_trans);
        } else {
                spin_unlock(&root->fs_info->trans_lock);
        }
}

enum btrfs_trans_type {
        TRANS_START,
        TRANS_JOIN,
        TRANS_USERSPACE,
        TRANS_JOIN_NOLOCK,
};

static int may_wait_transaction(struct btrfs_root *root, int type)
{
        if (root->fs_info->log_root_recovering)
                return 0;

        if (type == TRANS_USERSPACE)
                return 1;

        if (type == TRANS_START &&
            !atomic_read(&root->fs_info->open_ioctl_trans))
                return 1;

        return 0;
}

static struct btrfs_trans_handle *start_transaction(struct btrfs_root *root,
                                                    u64 num_items, int type)
{
        struct btrfs_trans_handle *h;
        struct btrfs_transaction *cur_trans;
        u64 num_bytes = 0;
        int ret;
        u64 qgroup_reserved = 0;

        if (root->fs_info->fs_state & BTRFS_SUPER_FLAG_ERROR)
                return ERR_PTR(-EROFS);

        if (current->journal_info) {
                WARN_ON(type != TRANS_JOIN && type != TRANS_JOIN_NOLOCK);
                h = current->journal_info;
                h->use_count++;
                h->orig_rsv = h->block_rsv;
                h->block_rsv = NULL;
                goto got_it;
        }

        /*
         * Do the reservation before we join the transaction so we can do all
         * the appropriate flushing if need be.
         */
        if (num_items > 0 && root != root->fs_info->chunk_root) {
                if (root->fs_info->quota_enabled &&
                    is_fstree(root->root_key.objectid)) {
                        qgroup_reserved = num_items * root->leafsize;
                        ret = btrfs_qgroup_reserve(root, qgroup_reserved);
                        if (ret)
                                return ERR_PTR(ret);
                }

                num_bytes = btrfs_calc_trans_metadata_size(root, num_items);
                ret = btrfs_block_rsv_add(root,
                                          &root->fs_info->trans_block_rsv,
                                          num_bytes);
                if (ret)
                        return ERR_PTR(ret);
        }
again:
        h = kmem_cache_alloc(btrfs_trans_handle_cachep, GFP_NOFS);
        if (!h)
                return ERR_PTR(-ENOMEM);

        if (may_wait_transaction(root, type))
                wait_current_trans(root);

        do {
                ret = join_transaction(root, type == TRANS_JOIN_NOLOCK);
                if (ret == -EBUSY)
                        wait_current_trans(root);
        } while (ret == -EBUSY);

        if (ret < 0) {
                kmem_cache_free(btrfs_trans_handle_cachep, h);
                return ERR_PTR(ret);
        }

        cur_trans = root->fs_info->running_transaction;

        h->transid = cur_trans->transid;
        h->transaction = cur_trans;
        h->blocks_used = 0;
        h->bytes_reserved = 0;
        h->root = root;
        h->delayed_ref_updates = 0;
        h->use_count = 1;
        h->adding_csums = 0;
        h->block_rsv = NULL;
        h->orig_rsv = NULL;
        h->aborted = 0;
        h->qgroup_reserved = qgroup_reserved;
        h->delayed_ref_elem.seq = 0;
        INIT_LIST_HEAD(&h->qgroup_ref_list);

        smp_mb();
        if (cur_trans->blocked && may_wait_transaction(root, type)) {
                btrfs_commit_transaction(h, root);
                goto again;
        }

        if (num_bytes) {
                trace_btrfs_space_reservation(root->fs_info, "transaction",
                                              h->transid, num_bytes, 1);
                h->block_rsv = &root->fs_info->trans_block_rsv;
                h->bytes_reserved = num_bytes;
        }

got_it:
        btrfs_record_root_in_trans(h, root);

        if (!current->journal_info && type != TRANS_USERSPACE)
                current->journal_info = h;
        return h;
}

struct btrfs_trans_handle *btrfs_start_transaction(struct btrfs_root *root,
                                                   int num_items)
{
        return start_transaction(root, num_items, TRANS_START);
}
struct btrfs_trans_handle *btrfs_join_transaction(struct btrfs_root *root)
{
        return start_transaction(root, 0, TRANS_JOIN);
}

struct btrfs_trans_handle *btrfs_join_transaction_nolock(struct btrfs_root *root)
{
        return start_transaction(root, 0, TRANS_JOIN_NOLOCK);
}

struct btrfs_trans_handle *btrfs_start_ioctl_transaction(struct btrfs_root *root)
{
        return start_transaction(root, 0, TRANS_USERSPACE);
}

/* wait for a transaction commit to be fully complete */
static noinline void wait_for_commit(struct btrfs_root *root,
                                    struct btrfs_transaction *commit)
{
        wait_event(commit->commit_wait, commit->commit_done);
}

int btrfs_wait_for_commit(struct btrfs_root *root, u64 transid)
{
        struct btrfs_transaction *cur_trans = NULL, *t;
        int ret;

        ret = 0;
        if (transid) {
                if (transid <= root->fs_info->last_trans_committed)
                        goto out;

                /* find specified transaction */
                spin_lock(&root->fs_info->trans_lock);
                list_for_each_entry(t, &root->fs_info->trans_list, list) {
                        if (t->transid == transid) {
                                cur_trans = t;
                                atomic_inc(&cur_trans->use_count);
                                break;
                        }
                        if (t->transid > transid)
                                break;
                }
                spin_unlock(&root->fs_info->trans_lock);
                ret = -EINVAL;
                if (!cur_trans)
                        goto out;  /* bad transid */
        } else {
                /* find newest transaction that is committing | committed */
                spin_lock(&root->fs_info->trans_lock);
                list_for_each_entry_reverse(t, &root->fs_info->trans_list,
                                            list) {
                        if (t->in_commit) {
                                if (t->commit_done)
                                        break;
                                cur_trans = t;
                                atomic_inc(&cur_trans->use_count);
                                break;
                        }
                }
                spin_unlock(&root->fs_info->trans_lock);
                if (!cur_trans)
                        goto out;  /* nothing committing|committed */
        }

        wait_for_commit(root, cur_trans);

        put_transaction(cur_trans);
        ret = 0;
out:
        return ret;
}

void btrfs_throttle(struct btrfs_root *root)
{
        if (!atomic_read(&root->fs_info->open_ioctl_trans))
                wait_current_trans(root);
}

static int should_end_transaction(struct btrfs_trans_handle *trans,
                                  struct btrfs_root *root)
{
        int ret;

        ret = btrfs_block_rsv_check(root, &root->fs_info->global_block_rsv, 5);
        return ret ? 1 : 0;
}

int btrfs_should_end_transaction(struct btrfs_trans_handle *trans,
                                 struct btrfs_root *root)
{
        struct btrfs_transaction *cur_trans = trans->transaction;
        int updates;
        int err;

        smp_mb();
        if (cur_trans->blocked || cur_trans->delayed_refs.flushing)
                return 1;

        updates = trans->delayed_ref_updates;
        trans->delayed_ref_updates = 0;
        if (updates) {
                err = btrfs_run_delayed_refs(trans, root, updates);
                if (err) /* Error code will also eval true */
                        return err;
        }

        return should_end_transaction(trans, root);
}

static int __btrfs_end_transaction(struct btrfs_trans_handle *trans,
                          struct btrfs_root *root, int throttle, int lock)
{
        struct btrfs_transaction *cur_trans = trans->transaction;
        struct btrfs_fs_info *info = root->fs_info;
        int count = 0;
        int err = 0;

        if (--trans->use_count) {
                trans->block_rsv = trans->orig_rsv;
                return 0;
        }

        /*
         * do the qgroup accounting as early as possible
         */
        err = btrfs_delayed_refs_qgroup_accounting(trans, info);

        btrfs_trans_release_metadata(trans, root);
        trans->block_rsv = NULL;
        /*
         * the same root has to be passed to start_transaction and
         * end_transaction. Subvolume quota depends on this.
         */
        WARN_ON(trans->root != root);

        if (trans->qgroup_reserved) {
                btrfs_qgroup_free(root, trans->qgroup_reserved);
                trans->qgroup_reserved = 0;
        }

        while (count < 2) {
                unsigned long cur = trans->delayed_ref_updates;
                trans->delayed_ref_updates = 0;
                if (cur &&
                    trans->transaction->delayed_refs.num_heads_ready > 64) {
                        trans->delayed_ref_updates = 0;
                        btrfs_run_delayed_refs(trans, root, cur);
                } else {
                        break;
                }
                count++;
        }
        btrfs_trans_release_metadata(trans, root);
        trans->block_rsv = NULL;

        if (lock && !atomic_read(&root->fs_info->open_ioctl_trans) &&
            should_end_transaction(trans, root)) {
                trans->transaction->blocked = 1;
                smp_wmb();
        }

        if (lock && cur_trans->blocked && !cur_trans->in_commit) {
                if (throttle) {
                        /*
                         * We may race with somebody else here so end up having
                         * to call end_transaction on ourselves again, so inc
                         * our use_count.
                         */
                        trans->use_count++;
                        return btrfs_commit_transaction(trans, root);
                } else {
                        wake_up_process(info->transaction_kthread);
                }
        }

        WARN_ON(cur_trans != info->running_transaction);
        WARN_ON(atomic_read(&cur_trans->num_writers) < 1);
        atomic_dec(&cur_trans->num_writers);

        smp_mb();
        if (waitqueue_active(&cur_trans->writer_wait))
                wake_up(&cur_trans->writer_wait);
        put_transaction(cur_trans);

        if (current->journal_info == trans)
                current->journal_info = NULL;

        if (throttle)
                btrfs_run_delayed_iputs(root);

        if (trans->aborted ||
            root->fs_info->fs_state & BTRFS_SUPER_FLAG_ERROR) {
                err = -EIO;
        }
        assert_qgroups_uptodate(trans);

        memset(trans, 0, sizeof(*trans));
        kmem_cache_free(btrfs_trans_handle_cachep, trans);
        return err;
}

int btrfs_end_transaction(struct btrfs_trans_handle *trans,
                          struct btrfs_root *root)
{
        int ret;

        ret = __btrfs_end_transaction(trans, root, 0, 1);
        if (ret)
                return ret;
        return 0;
}

int btrfs_end_transaction_throttle(struct btrfs_trans_handle *trans,
                                   struct btrfs_root *root)
{
        int ret;

        ret = __btrfs_end_transaction(trans, root, 1, 1);
        if (ret)
                return ret;
        return 0;
}

int btrfs_end_transaction_nolock(struct btrfs_trans_handle *trans,
                                 struct btrfs_root *root)
{
        int ret;

        ret = __btrfs_end_transaction(trans, root, 0, 0);
        if (ret)
                return ret;
        return 0;
}

int btrfs_end_transaction_dmeta(struct btrfs_trans_handle *trans,
                                struct btrfs_root *root)
{
        return __btrfs_end_transaction(trans, root, 1, 1);
}

/*
 * when btree blocks are allocated, they have some corresponding bits set for
 * them in one of two extent_io trees.  This is used to make sure all of
 * those extents are sent to disk but does not wait on them
 */
int btrfs_write_marked_extents(struct btrfs_root *root,
                               struct extent_io_tree *dirty_pages, int mark)
{
        int err = 0;
        int werr = 0;
        struct address_space *mapping = root->fs_info->btree_inode->i_mapping;
        u64 start = 0;
        u64 end;

        while (!find_first_extent_bit(dirty_pages, start, &start, &end,
                                      mark)) {
                convert_extent_bit(dirty_pages, start, end, EXTENT_NEED_WAIT, mark,
                                   GFP_NOFS);
                err = filemap_fdatawrite_range(mapping, start, end);
                if (err)
                        werr = err;
                cond_resched();
                start = end + 1;
        }
        if (err)
                werr = err;
        return werr;
}

/*
 * when btree blocks are allocated, they have some corresponding bits set for
 * them in one of two extent_io trees.  This is used to make sure all of
 * those extents are on disk for transaction or log commit.  We wait
 * on all the pages and clear them from the dirty pages state tree
 */
int btrfs_wait_marked_extents(struct btrfs_root *root,
                              struct extent_io_tree *dirty_pages, int mark)
{
        int err = 0;
        int werr = 0;
        struct address_space *mapping = root->fs_info->btree_inode->i_mapping;
        u64 start = 0;
        u64 end;

        while (!find_first_extent_bit(dirty_pages, start, &start, &end,
                                      EXTENT_NEED_WAIT)) {
                clear_extent_bits(dirty_pages, start, end, EXTENT_NEED_WAIT, GFP_NOFS);
                err = filemap_fdatawait_range(mapping, start, end);
                if (err)
                        werr = err;
                cond_resched();
                start = end + 1;
        }
        if (err)
                werr = err;
        return werr;
}

/*
 * when btree blocks are allocated, they have some corresponding bits set for
 * them in one of two extent_io trees.  This is used to make sure all of
 * those extents are on disk for transaction or log commit
 */
int btrfs_write_and_wait_marked_extents(struct btrfs_root *root,
                                struct extent_io_tree *dirty_pages, int mark)
{
        int ret;
        int ret2;

        ret = btrfs_write_marked_extents(root, dirty_pages, mark);
        ret2 = btrfs_wait_marked_extents(root, dirty_pages, mark);

        if (ret)
                return ret;
        if (ret2)
                return ret2;
        return 0;
}

int btrfs_write_and_wait_transaction(struct btrfs_trans_handle *trans,
                                     struct btrfs_root *root)
{
        if (!trans || !trans->transaction) {
                struct inode *btree_inode;
                btree_inode = root->fs_info->btree_inode;
                return filemap_write_and_wait(btree_inode->i_mapping);
        }
        return btrfs_write_and_wait_marked_extents(root,
                                           &trans->transaction->dirty_pages,
                                           EXTENT_DIRTY);
}

/*
 * this is used to update the root pointer in the tree of tree roots.
 *
 * But, in the case of the extent allocation tree, updating the root
 * pointer may allocate blocks which may change the root of the extent
 * allocation tree.
 *
 * So, this loops and repeats and makes sure the cowonly root didn't
 * change while the root pointer was being updated in the metadata.
 */
static int update_cowonly_root(struct btrfs_trans_handle *trans,
                               struct btrfs_root *root)
{
        int ret;
        u64 old_root_bytenr;
        u64 old_root_used;
        struct btrfs_root *tree_root = root->fs_info->tree_root;

        old_root_used = btrfs_root_used(&root->root_item);
        btrfs_write_dirty_block_groups(trans, root);

        while (1) {
                old_root_bytenr = btrfs_root_bytenr(&root->root_item);
                if (old_root_bytenr == root->node->start &&
                    old_root_used == btrfs_root_used(&root->root_item))
                        break;

                btrfs_set_root_node(&root->root_item, root->node);
                ret = btrfs_update_root(trans, tree_root,
                                        &root->root_key,
                                        &root->root_item);
                if (ret)
                        return ret;

                old_root_used = btrfs_root_used(&root->root_item);
                ret = btrfs_write_dirty_block_groups(trans, root);
                if (ret)
                        return ret;
        }

        if (root != root->fs_info->extent_root)
                switch_commit_root(root);

        return 0;
}

/*
 * update all the cowonly tree roots on disk
 *
 * The error handling in this function may not be obvious. Any of the
 * failures will cause the file system to go offline. We still need
 * to clean up the delayed refs.
 */
static noinline int commit_cowonly_roots(struct btrfs_trans_handle *trans,
                                         struct btrfs_root *root)
{
        struct btrfs_fs_info *fs_info = root->fs_info;
        struct list_head *next;
        struct extent_buffer *eb;
        int ret;

        ret = btrfs_run_delayed_refs(trans, root, (unsigned long)-1);
        if (ret)
                return ret;

        eb = btrfs_lock_root_node(fs_info->tree_root);
        ret = btrfs_cow_block(trans, fs_info->tree_root, eb, NULL,
                              0, &eb);
        btrfs_tree_unlock(eb);
        free_extent_buffer(eb);

        if (ret)
                return ret;

        ret = btrfs_run_delayed_refs(trans, root, (unsigned long)-1);
        if (ret)
                return ret;

        ret = btrfs_run_dev_stats(trans, root->fs_info);
        BUG_ON(ret);

        ret = btrfs_run_qgroups(trans, root->fs_info);
        BUG_ON(ret);

        /* run_qgroups might have added some more refs */
        ret = btrfs_run_delayed_refs(trans, root, (unsigned long)-1);
        BUG_ON(ret);

        while (!list_empty(&fs_info->dirty_cowonly_roots)) {
                next = fs_info->dirty_cowonly_roots.next;
                list_del_init(next);
                root = list_entry(next, struct btrfs_root, dirty_list);

                ret = update_cowonly_root(trans, root);
                if (ret)
                        return ret;
        }

        down_write(&fs_info->extent_commit_sem);
        switch_commit_root(fs_info->extent_root);
        up_write(&fs_info->extent_commit_sem);

        return 0;
}

/*
 * dead roots are old snapshots that need to be deleted.  This allocates
 * a dirty root struct and adds it into the list of dead roots that need to
 * be deleted
 */
int btrfs_add_dead_root(struct btrfs_root *root)
{
        spin_lock(&root->fs_info->trans_lock);
        list_add(&root->root_list, &root->fs_info->dead_roots);
        spin_unlock(&root->fs_info->trans_lock);
        return 0;
}

/*
 * update all the cowonly tree roots on disk
 */
static noinline int commit_fs_roots(struct btrfs_trans_handle *trans,
                                    struct btrfs_root *root)
{
        struct btrfs_root *gang[8];
        struct btrfs_fs_info *fs_info = root->fs_info;
        int i;
        int ret;
        int err = 0;

        spin_lock(&fs_info->fs_roots_radix_lock);
        while (1) {
                ret = radix_tree_gang_lookup_tag(&fs_info->fs_roots_radix,
                                                 (void **)gang, 0,
                                                 ARRAY_SIZE(gang),
                                                 BTRFS_ROOT_TRANS_TAG);
                if (ret == 0)
                        break;
                for (i = 0; i < ret; i++) {
                        root = gang[i];
                        radix_tree_tag_clear(&fs_info->fs_roots_radix,
                                        (unsigned long)root->root_key.objectid,
                                        BTRFS_ROOT_TRANS_TAG);
                        spin_unlock(&fs_info->fs_roots_radix_lock);

                        btrfs_free_log(trans, root);
                        btrfs_update_reloc_root(trans, root);
                        btrfs_orphan_commit_root(trans, root);

                        btrfs_save_ino_cache(root, trans);

                        /* see comments in should_cow_block() */
                        root->force_cow = 0;
                        smp_wmb();

                        if (root->commit_root != root->node) {
                                mutex_lock(&root->fs_commit_mutex);
                                switch_commit_root(root);
                                btrfs_unpin_free_ino(root);
                                mutex_unlock(&root->fs_commit_mutex);

                                btrfs_set_root_node(&root->root_item,
                                                    root->node);
                        }

                        err = btrfs_update_root(trans, fs_info->tree_root,
                                                &root->root_key,
                                                &root->root_item);
                        spin_lock(&fs_info->fs_roots_radix_lock);
                        if (err)
                                break;
                }
        }
        spin_unlock(&fs_info->fs_roots_radix_lock);
        return err;
}

/*
 * defrag a given btree.  If cacheonly == 1, this won't read from the disk,
 * otherwise every leaf in the btree is read and defragged.
 */
int btrfs_defrag_root(struct btrfs_root *root, int cacheonly)
{
        struct btrfs_fs_info *info = root->fs_info;
        struct btrfs_trans_handle *trans;
        int ret;
        unsigned long nr;

        if (xchg(&root->defrag_running, 1))
                return 0;

        while (1) {
                trans = btrfs_start_transaction(root, 0);
                if (IS_ERR(trans))
                        return PTR_ERR(trans);

                ret = btrfs_defrag_leaves(trans, root, cacheonly);

                nr = trans->blocks_used;
                btrfs_end_transaction(trans, root);
                btrfs_btree_balance_dirty(info->tree_root, nr);
                cond_resched();

                if (btrfs_fs_closing(root->fs_info) || ret != -EAGAIN)
                        break;
        }
        root->defrag_running = 0;
        return ret;
}

/*
 * new snapshots need to be created at a very specific time in the
 * transaction commit.  This does the actual creation
 */
static noinline int create_pending_snapshot(struct btrfs_trans_handle *trans,
                                   struct btrfs_fs_info *fs_info,
                                   struct btrfs_pending_snapshot *pending)
{
        struct btrfs_key key;
        struct btrfs_root_item *new_root_item;
        struct btrfs_root *tree_root = fs_info->tree_root;
        struct btrfs_root *root = pending->root;
        struct btrfs_root *parent_root;
        struct btrfs_block_rsv *rsv;
        struct inode *parent_inode;
        struct dentry *parent;
        struct dentry *dentry;
        struct extent_buffer *tmp;
        struct extent_buffer *old;
        int ret;
        u64 to_reserve = 0;
        u64 index = 0;
        u64 objectid;
        u64 root_flags;

        rsv = trans->block_rsv;

        new_root_item = kmalloc(sizeof(*new_root_item), GFP_NOFS);
        if (!new_root_item) {
                ret = pending->error = -ENOMEM;
                goto fail;
        }

        ret = btrfs_find_free_objectid(tree_root, &objectid);
        if (ret) {
                pending->error = ret;
                goto fail;
        }

        btrfs_reloc_pre_snapshot(trans, pending, &to_reserve);

        if (to_reserve > 0) {
                ret = btrfs_block_rsv_add_noflush(root, &pending->block_rsv,
                                                  to_reserve);
                if (ret) {
                        pending->error = ret;
                        goto fail;
                }
        }

        ret = btrfs_qgroup_inherit(trans, fs_info, root->root_key.objectid,
                                   objectid, pending->inherit);
        kfree(pending->inherit);
        if (ret) {
                pending->error = ret;
                goto fail;
        }

        key.objectid = objectid;
        key.offset = (u64)-1;
        key.type = BTRFS_ROOT_ITEM_KEY;

        trans->block_rsv = &pending->block_rsv;

        dentry = pending->dentry;
        parent = dget_parent(dentry);
        parent_inode = parent->d_inode;
        parent_root = BTRFS_I(parent_inode)->root;
        record_root_in_trans(trans, parent_root);

        /*
         * insert the directory item
         */
        ret = btrfs_set_inode_index(parent_inode, &index);
        BUG_ON(ret); /* -ENOMEM */
        ret = btrfs_insert_dir_item(trans, parent_root,
                                dentry->d_name.name, dentry->d_name.len,
                                parent_inode, &key,
                                BTRFS_FT_DIR, index);
        if (ret == -EEXIST) {
                pending->error = -EEXIST;
                dput(parent);
                goto fail;
        } else if (ret) {
                goto abort_trans_dput;
        }

        btrfs_i_size_write(parent_inode, parent_inode->i_size +
                                         dentry->d_name.len * 2);
        ret = btrfs_update_inode(trans, parent_root, parent_inode);
        if (ret)
                goto abort_trans_dput;

        /*
         * pull in the delayed directory update
         * and the delayed inode item
         * otherwise we corrupt the FS during
         * snapshot
         */
        ret = btrfs_run_delayed_items(trans, root);
        if (ret) { /* Transaction aborted */
                dput(parent);
                goto fail;
        }

        record_root_in_trans(trans, root);
        btrfs_set_root_last_snapshot(&root->root_item, trans->transid);
        memcpy(new_root_item, &root->root_item, sizeof(*new_root_item));
        btrfs_check_and_init_root_item(new_root_item);

        root_flags = btrfs_root_flags(new_root_item);
        if (pending->readonly)
                root_flags |= BTRFS_ROOT_SUBVOL_RDONLY;
        else
                root_flags &= ~BTRFS_ROOT_SUBVOL_RDONLY;
        btrfs_set_root_flags(new_root_item, root_flags);

        old = btrfs_lock_root_node(root);
        ret = btrfs_cow_block(trans, root, old, NULL, 0, &old);
        if (ret) {
                btrfs_tree_unlock(old);
                free_extent_buffer(old);
                goto abort_trans_dput;
        }

        btrfs_set_lock_blocking(old);

        ret = btrfs_copy_root(trans, root, old, &tmp, objectid);
        /* clean up in any case */
        btrfs_tree_unlock(old);
        free_extent_buffer(old);
        if (ret)
                goto abort_trans_dput;

        /* see comments in should_cow_block() */
        root->force_cow = 1;
        smp_wmb();

        btrfs_set_root_node(new_root_item, tmp);
        /* record when the snapshot was created in key.offset */
        key.offset = trans->transid;
        ret = btrfs_insert_root(trans, tree_root, &key, new_root_item);
        btrfs_tree_unlock(tmp);
        free_extent_buffer(tmp);
        if (ret)
                goto abort_trans_dput;

        /*
         * insert root back/forward references
         */
        ret = btrfs_add_root_ref(trans, tree_root, objectid,
                                 parent_root->root_key.objectid,
                                 btrfs_ino(parent_inode), index,
                                 dentry->d_name.name, dentry->d_name.len);
        dput(parent);
        if (ret)
                goto fail;

        key.offset = (u64)-1;
        pending->snap = btrfs_read_fs_root_no_name(root->fs_info, &key);
        if (IS_ERR(pending->snap)) {
                ret = PTR_ERR(pending->snap);
                goto abort_trans;
        }

        ret = btrfs_reloc_post_snapshot(trans, pending);
        if (ret)
                goto abort_trans;
        ret = 0;
fail:
        kfree(new_root_item);
        trans->block_rsv = rsv;
        btrfs_block_rsv_release(root, &pending->block_rsv, (u64)-1);
        return ret;

abort_trans_dput:
        dput(parent);
abort_trans:
        btrfs_abort_transaction(trans, root, ret);
        goto fail;
}

/*
 * create all the snapshots we've scheduled for creation
 */
static noinline int create_pending_snapshots(struct btrfs_trans_handle *trans,
                                             struct btrfs_fs_info *fs_info)
{
        struct btrfs_pending_snapshot *pending;
        struct list_head *head = &trans->transaction->pending_snapshots;

        list_for_each_entry(pending, head, list)
                create_pending_snapshot(trans, fs_info, pending);
        return 0;
}

static void update_super_roots(struct btrfs_root *root)
{
        struct btrfs_root_item *root_item;
        struct btrfs_super_block *super;

        super = root->fs_info->super_copy;

        root_item = &root->fs_info->chunk_root->root_item;
        super->chunk_root = root_item->bytenr;
        super->chunk_root_generation = root_item->generation;
        super->chunk_root_level = root_item->level;

        root_item = &root->fs_info->tree_root->root_item;
        super->root = root_item->bytenr;
        super->generation = root_item->generation;
        super->root_level = root_item->level;
        if (btrfs_test_opt(root, SPACE_CACHE))
                super->cache_generation = root_item->generation;
}

int btrfs_transaction_in_commit(struct btrfs_fs_info *info)
{
        int ret = 0;
        spin_lock(&info->trans_lock);
        if (info->running_transaction)
                ret = info->running_transaction->in_commit;
        spin_unlock(&info->trans_lock);
        return ret;
}

int btrfs_transaction_blocked(struct btrfs_fs_info *info)
{
        int ret = 0;
        spin_lock(&info->trans_lock);
        if (info->running_transaction)
                ret = info->running_transaction->blocked;
        spin_unlock(&info->trans_lock);
        return ret;
}

/*
 * wait for the current transaction commit to start and block subsequent
 * transaction joins
 */
static void wait_current_trans_commit_start(struct btrfs_root *root,
                                            struct btrfs_transaction *trans)
{
        wait_event(root->fs_info->transaction_blocked_wait, trans->in_commit);
}

/*
 * wait for the current transaction to start and then become unblocked.
 * caller holds ref.
 */
static void wait_current_trans_commit_start_and_unblock(struct btrfs_root *root,
                                         struct btrfs_transaction *trans)
{
        wait_event(root->fs_info->transaction_wait,
                   trans->commit_done || (trans->in_commit && !trans->blocked));
}

/*
 * commit transactions asynchronously. once btrfs_commit_transaction_async
 * returns, any subsequent transaction will not be allowed to join.
 */
struct btrfs_async_commit {
        struct btrfs_trans_handle *newtrans;
        struct btrfs_root *root;
        struct delayed_work work;
};

static void do_async_commit(struct work_struct *work)
{
        struct btrfs_async_commit *ac =
                container_of(work, struct btrfs_async_commit, work.work);

        btrfs_commit_transaction(ac->newtrans, ac->root);
        kfree(ac);
}

int btrfs_commit_transaction_async(struct btrfs_trans_handle *trans,
                                   struct btrfs_root *root,
                                   int wait_for_unblock)
{
        struct btrfs_async_commit *ac;
        struct btrfs_transaction *cur_trans;

        ac = kmalloc(sizeof(*ac), GFP_NOFS);
        if (!ac)
                return -ENOMEM;

        INIT_DELAYED_WORK(&ac->work, do_async_commit);
        ac->root = root;
        ac->newtrans = btrfs_join_transaction(root);
        if (IS_ERR(ac->newtrans)) {
                int err = PTR_ERR(ac->newtrans);
                kfree(ac);
                return err;
        }

        /* take transaction reference */
        cur_trans = trans->transaction;
        atomic_inc(&cur_trans->use_count);

        btrfs_end_transaction(trans, root);
        schedule_delayed_work(&ac->work, 0);

        /* wait for transaction to start and unblock */
        if (wait_for_unblock)
                wait_current_trans_commit_start_and_unblock(root, cur_trans);
        else
                wait_current_trans_commit_start(root, cur_trans);

        if (current->journal_info == trans)
                current->journal_info = NULL;

        put_transaction(cur_trans);
        return 0;
}


static void cleanup_transaction(struct btrfs_trans_handle *trans,
                                struct btrfs_root *root, int err)
{
        struct btrfs_transaction *cur_trans = trans->transaction;

        WARN_ON(trans->use_count > 1);

        btrfs_abort_transaction(trans, root, err);

        spin_lock(&root->fs_info->trans_lock);
        list_del_init(&cur_trans->list);
        if (cur_trans == root->fs_info->running_transaction) {
                root->fs_info->running_transaction = NULL;
                root->fs_info->trans_no_join = 0;
        }
        spin_unlock(&root->fs_info->trans_lock);

        btrfs_cleanup_one_transaction(trans->transaction, root);

        put_transaction(cur_trans);
        put_transaction(cur_trans);

        trace_btrfs_transaction_commit(root);

        btrfs_scrub_continue(root);

        if (current->journal_info == trans)
                current->journal_info = NULL;

        kmem_cache_free(btrfs_trans_handle_cachep, trans);
}

/*
 * btrfs_transaction state sequence:
 *    in_commit = 0, blocked = 0  (initial)
 *    in_commit = 1, blocked = 1
 *    blocked = 0
 *    commit_done = 1
 */
int btrfs_commit_transaction(struct btrfs_trans_handle *trans,
                             struct btrfs_root *root)
{
        unsigned long joined = 0;
        struct btrfs_transaction *cur_trans = trans->transaction;
        struct btrfs_transaction *prev_trans = NULL;
        DEFINE_WAIT(wait);
        int ret = -EIO;
        int should_grow = 0;
        unsigned long now = get_seconds();
        int flush_on_commit = btrfs_test_opt(root, FLUSHONCOMMIT);

        btrfs_run_ordered_operations(root, 0);

        if (cur_trans->aborted)
                goto cleanup_transaction;

        /* make a pass through all the delayed refs we have so far
         * any runnings procs may add more while we are here
         */
        ret = btrfs_run_delayed_refs(trans, root, 0);
        if (ret)
                goto cleanup_transaction;

        btrfs_trans_release_metadata(trans, root);
        trans->block_rsv = NULL;

        cur_trans = trans->transaction;

        /*
         * set the flushing flag so procs in this transaction have to
         * start sending their work down.
         */
        cur_trans->delayed_refs.flushing = 1;

        ret = btrfs_run_delayed_refs(trans, root, 0);
        if (ret)
                goto cleanup_transaction;

        spin_lock(&cur_trans->commit_lock);
        if (cur_trans->in_commit) {
                spin_unlock(&cur_trans->commit_lock);
                atomic_inc(&cur_trans->use_count);
                ret = btrfs_end_transaction(trans, root);

                wait_for_commit(root, cur_trans);

                put_transaction(cur_trans);

                return ret;
        }

        trans->transaction->in_commit = 1;
        trans->transaction->blocked = 1;
        spin_unlock(&cur_trans->commit_lock);
        wake_up(&root->fs_info->transaction_blocked_wait);

        spin_lock(&root->fs_info->trans_lock);
        if (cur_trans->list.prev != &root->fs_info->trans_list) {
                prev_trans = list_entry(cur_trans->list.prev,
                                        struct btrfs_transaction, list);
                if (!prev_trans->commit_done) {
                        atomic_inc(&prev_trans->use_count);
                        spin_unlock(&root->fs_info->trans_lock);

                        wait_for_commit(root, prev_trans);

                        put_transaction(prev_trans);
                } else {
                        spin_unlock(&root->fs_info->trans_lock);
                }
        } else {
                spin_unlock(&root->fs_info->trans_lock);
        }

        if (!btrfs_test_opt(root, SSD) &&
            (now < cur_trans->start_time || now - cur_trans->start_time < 1))
                should_grow = 1;

        do {
                int snap_pending = 0;

                joined = cur_trans->num_joined;
                if (!list_empty(&trans->transaction->pending_snapshots))
                        snap_pending = 1;

                WARN_ON(cur_trans != trans->transaction);

                if (flush_on_commit || snap_pending) {
                        btrfs_start_delalloc_inodes(root, 1);
                        btrfs_wait_ordered_extents(root, 0, 1);
                }

                ret = btrfs_run_delayed_items(trans, root);
                if (ret)
                        goto cleanup_transaction;

                /*
                 * running the delayed items may have added new refs. account
                 * them now so that they hinder processing of more delayed refs
                 * as little as possible.
                 */
                btrfs_delayed_refs_qgroup_accounting(trans, root->fs_info);

                /*
                 * rename don't use btrfs_join_transaction, so, once we
                 * set the transaction to blocked above, we aren't going
                 * to get any new ordered operations.  We can safely run
                 * it here and no for sure that nothing new will be added
                 * to the list
                 */
                btrfs_run_ordered_operations(root, 1);

                prepare_to_wait(&cur_trans->writer_wait, &wait,
                                TASK_UNINTERRUPTIBLE);

                if (atomic_read(&cur_trans->num_writers) > 1)
                        schedule_timeout(MAX_SCHEDULE_TIMEOUT);
                else if (should_grow)
                        schedule_timeout(1);

                finish_wait(&cur_trans->writer_wait, &wait);
        } while (atomic_read(&cur_trans->num_writers) > 1 ||
                 (should_grow && cur_trans->num_joined != joined));

        /*
         * Ok now we need to make sure to block out any other joins while we
         * commit the transaction.  We could have started a join before setting
         * no_join so make sure to wait for num_writers to == 1 again.
         */
        spin_lock(&root->fs_info->trans_lock);
        root->fs_info->trans_no_join = 1;
        spin_unlock(&root->fs_info->trans_lock);
        wait_event(cur_trans->writer_wait,
                   atomic_read(&cur_trans->num_writers) == 1);

        /*
         * the reloc mutex makes sure that we stop
         * the balancing code from coming in and moving
         * extents around in the middle of the commit
         */
        mutex_lock(&root->fs_info->reloc_mutex);

        ret = btrfs_run_delayed_items(trans, root);
        if (ret) {
                mutex_unlock(&root->fs_info->reloc_mutex);
                goto cleanup_transaction;
        }

        ret = create_pending_snapshots(trans, root->fs_info);
        if (ret) {
                mutex_unlock(&root->fs_info->reloc_mutex);
                goto cleanup_transaction;
        }

        ret = btrfs_run_delayed_refs(trans, root, (unsigned long)-1);
        if (ret) {
                mutex_unlock(&root->fs_info->reloc_mutex);
                goto cleanup_transaction;
        }

        /*
         * make sure none of the code above managed to slip in a
         * delayed item
         */
        btrfs_assert_delayed_root_empty(root);

        WARN_ON(cur_trans != trans->transaction);

        btrfs_scrub_pause(root);
        /* btrfs_commit_tree_roots is responsible for getting the
         * various roots consistent with each other.  Every pointer
         * in the tree of tree roots has to point to the most up to date
         * root for every subvolume and other tree.  So, we have to keep
         * the tree logging code from jumping in and changing any
         * of the trees.
         *
         * At this point in the commit, there can't be any tree-log
         * writers, but a little lower down we drop the trans mutex
         * and let new people in.  By holding the tree_log_mutex
         * from now until after the super is written, we avoid races
         * with the tree-log code.
         */
        mutex_lock(&root->fs_info->tree_log_mutex);

        ret = commit_fs_roots(trans, root);
        if (ret) {
                mutex_unlock(&root->fs_info->tree_log_mutex);
                mutex_unlock(&root->fs_info->reloc_mutex);
                goto cleanup_transaction;
        }

        /* commit_fs_roots gets rid of all the tree log roots, it is now
         * safe to free the root of tree log roots
         */
        btrfs_free_log_root_tree(trans, root->fs_info);

        ret = commit_cowonly_roots(trans, root);
        if (ret) {
                mutex_unlock(&root->fs_info->tree_log_mutex);
                mutex_unlock(&root->fs_info->reloc_mutex);
                goto cleanup_transaction;
        }

        btrfs_prepare_extent_commit(trans, root);

        cur_trans = root->fs_info->running_transaction;

        btrfs_set_root_node(&root->fs_info->tree_root->root_item,
                            root->fs_info->tree_root->node);
        switch_commit_root(root->fs_info->tree_root);

        btrfs_set_root_node(&root->fs_info->chunk_root->root_item,
                            root->fs_info->chunk_root->node);
        switch_commit_root(root->fs_info->chunk_root);

        assert_qgroups_uptodate(trans);
        update_super_roots(root);

        if (!root->fs_info->log_root_recovering) {
                btrfs_set_super_log_root(root->fs_info->super_copy, 0);
                btrfs_set_super_log_root_level(root->fs_info->super_copy, 0);
        }

        memcpy(root->fs_info->super_for_commit, root->fs_info->super_copy,
               sizeof(*root->fs_info->super_copy));

        trans->transaction->blocked = 0;
        spin_lock(&root->fs_info->trans_lock);
        root->fs_info->running_transaction = NULL;
        root->fs_info->trans_no_join = 0;
        spin_unlock(&root->fs_info->trans_lock);
        mutex_unlock(&root->fs_info->reloc_mutex);

        wake_up(&root->fs_info->transaction_wait);

        ret = btrfs_write_and_wait_transaction(trans, root);
        if (ret) {
                btrfs_error(root->fs_info, ret,
                            "Error while writing out transaction.");
                mutex_unlock(&root->fs_info->tree_log_mutex);
                goto cleanup_transaction;
        }

        ret = write_ctree_super(trans, root, 0);
        if (ret) {
                mutex_unlock(&root->fs_info->tree_log_mutex);
                goto cleanup_transaction;
        }

        /*
         * the super is written, we can safely allow the tree-loggers
         * to go about their business
         */
        mutex_unlock(&root->fs_info->tree_log_mutex);

        btrfs_finish_extent_commit(trans, root);

        cur_trans->commit_done = 1;

        root->fs_info->last_trans_committed = cur_trans->transid;

        wake_up(&cur_trans->commit_wait);

        spin_lock(&root->fs_info->trans_lock);
        list_del_init(&cur_trans->list);
        spin_unlock(&root->fs_info->trans_lock);

        put_transaction(cur_trans);
        put_transaction(cur_trans);

        trace_btrfs_transaction_commit(root);

        btrfs_scrub_continue(root);

        if (current->journal_info == trans)
                current->journal_info = NULL;

        kmem_cache_free(btrfs_trans_handle_cachep, trans);

        if (current != root->fs_info->transaction_kthread)
                btrfs_run_delayed_iputs(root);

        return ret;

cleanup_transaction:
        btrfs_trans_release_metadata(trans, root);
        trans->block_rsv = NULL;
        btrfs_printk(root->fs_info, "Skipping commit of aborted transaction.\n");
//      WARN_ON(1);
        if (current->journal_info == trans)
                current->journal_info = NULL;
        cleanup_transaction(trans, root, ret);

        return ret;
}

/*
 * interface function to delete all the snapshots we have scheduled for deletion
 */
int btrfs_clean_old_snapshots(struct btrfs_root *root)
{
        LIST_HEAD(list);
        struct btrfs_fs_info *fs_info = root->fs_info;

        spin_lock(&fs_info->trans_lock);
        list_splice_init(&fs_info->dead_roots, &list);
        spin_unlock(&fs_info->trans_lock);

        while (!list_empty(&list)) {
                int ret;

                root = list_entry(list.next, struct btrfs_root, root_list);
                list_del(&root->root_list);

                btrfs_kill_all_delayed_nodes(root);

                if (btrfs_header_backref_rev(root->node) <
                    BTRFS_MIXED_BACKREF_REV)
                        ret = btrfs_drop_snapshot(root, NULL, 0, 0);
                else
                        ret =btrfs_drop_snapshot(root, NULL, 1, 0);
                BUG_ON(ret < 0);
        }
        return 0;
}