btrfs: add a helper to read the tree_root commit root for backref lookup
I got the following lockdep splat with tree locks converted to rwsem patches on btrfs/104: ====================================================== WARNING: possible circular locking dependency detected 5.9.0+ #102 Not tainted ------------------------------------------------------ btrfs-cleaner/903 is trying to acquire lock: ffff8e7fab6ffe30 (btrfs-root-00){++++}-{3:3}, at: __btrfs_tree_read_lock+0x32/0x170 but task is already holding lock: ffff8e7fab628a88 (&fs_info->commit_root_sem){++++}-{3:3}, at: btrfs_find_all_roots+0x41/0x80 which lock already depends on the new lock. the existing dependency chain (in reverse order) is: -> #3 (&fs_info->commit_root_sem){++++}-{3:3}: down_read+0x40/0x130 caching_thread+0x53/0x5a0 btrfs_work_helper+0xfa/0x520 process_one_work+0x238/0x540 worker_thread+0x55/0x3c0 kthread+0x13a/0x150 ret_from_fork+0x1f/0x30 -> #2 (&caching_ctl->mutex){+.+.}-{3:3}: __mutex_lock+0x7e/0x7b0 btrfs_cache_block_group+0x1e0/0x510 find_free_extent+0xb6e/0x12f0 btrfs_reserve_extent+0xb3/0x1b0 btrfs_alloc_tree_block+0xb1/0x330 alloc_tree_block_no_bg_flush+0x4f/0x60 __btrfs_cow_block+0x11d/0x580 btrfs_cow_block+0x10c/0x220 commit_cowonly_roots+0x47/0x2e0 btrfs_commit_transaction+0x595/0xbd0 sync_filesystem+0x74/0x90 generic_shutdown_super+0x22/0x100 kill_anon_super+0x14/0x30 btrfs_kill_super+0x12/0x20 deactivate_locked_super+0x36/0xa0 cleanup_mnt+0x12d/0x190 task_work_run+0x5c/0xa0 exit_to_user_mode_prepare+0x1df/0x200 syscall_exit_to_user_mode+0x54/0x280 entry_SYSCALL_64_after_hwframe+0x44/0xa9 -> #1 (&space_info->groups_sem){++++}-{3:3}: down_read+0x40/0x130 find_free_extent+0x2ed/0x12f0 btrfs_reserve_extent+0xb3/0x1b0 btrfs_alloc_tree_block+0xb1/0x330 alloc_tree_block_no_bg_flush+0x4f/0x60 __btrfs_cow_block+0x11d/0x580 btrfs_cow_block+0x10c/0x220 commit_cowonly_roots+0x47/0x2e0 btrfs_commit_transaction+0x595/0xbd0 sync_filesystem+0x74/0x90 generic_shutdown_super+0x22/0x100 kill_anon_super+0x14/0x30 btrfs_kill_super+0x12/0x20 deactivate_locked_super+0x36/0xa0 cleanup_mnt+0x12d/0x190 task_work_run+0x5c/0xa0 exit_to_user_mode_prepare+0x1df/0x200 syscall_exit_to_user_mode+0x54/0x280 entry_SYSCALL_64_after_hwframe+0x44/0xa9 -> #0 (btrfs-root-00){++++}-{3:3}: __lock_acquire+0x1167/0x2150 lock_acquire+0xb9/0x3d0 down_read_nested+0x43/0x130 __btrfs_tree_read_lock+0x32/0x170 __btrfs_read_lock_root_node+0x3a/0x50 btrfs_search_slot+0x614/0x9d0 btrfs_find_root+0x35/0x1b0 btrfs_read_tree_root+0x61/0x120 btrfs_get_root_ref+0x14b/0x600 find_parent_nodes+0x3e6/0x1b30 btrfs_find_all_roots_safe+0xb4/0x130 btrfs_find_all_roots+0x60/0x80 btrfs_qgroup_trace_extent_post+0x27/0x40 btrfs_add_delayed_data_ref+0x3fd/0x460 btrfs_free_extent+0x42/0x100 __btrfs_mod_ref+0x1d7/0x2f0 walk_up_proc+0x11c/0x400 walk_up_tree+0xf0/0x180 btrfs_drop_snapshot+0x1c7/0x780 btrfs_clean_one_deleted_snapshot+0xfb/0x110 cleaner_kthread+0xd4/0x140 kthread+0x13a/0x150 ret_from_fork+0x1f/0x30 other info that might help us debug this: Chain exists of: btrfs-root-00 --> &caching_ctl->mutex --> &fs_info->commit_root_sem Possible unsafe locking scenario: CPU0 CPU1 ---- ---- lock(&fs_info->commit_root_sem); lock(&caching_ctl->mutex); lock(&fs_info->commit_root_sem); lock(btrfs-root-00); *** DEADLOCK *** 3 locks held by btrfs-cleaner/903: #0: ffff8e7fab628838 (&fs_info->cleaner_mutex){+.+.}-{3:3}, at: cleaner_kthread+0x6e/0x140 #1: ffff8e7faadac640 (sb_internal){.+.+}-{0:0}, at: start_transaction+0x40b/0x5c0 #2: ffff8e7fab628a88 (&fs_info->commit_root_sem){++++}-{3:3}, at: btrfs_find_all_roots+0x41/0x80 stack backtrace: CPU: 0 PID: 903 Comm: btrfs-cleaner Not tainted 5.9.0+ #102 Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.13.0-2.fc32 04/01/2014 Call Trace: dump_stack+0x8b/0xb0 check_noncircular+0xcf/0xf0 __lock_acquire+0x1167/0x2150 ? __bfs+0x42/0x210 lock_acquire+0xb9/0x3d0 ? __btrfs_tree_read_lock+0x32/0x170 down_read_nested+0x43/0x130 ? __btrfs_tree_read_lock+0x32/0x170 __btrfs_tree_read_lock+0x32/0x170 __btrfs_read_lock_root_node+0x3a/0x50 btrfs_search_slot+0x614/0x9d0 ? find_held_lock+0x2b/0x80 btrfs_find_root+0x35/0x1b0 ? do_raw_spin_unlock+0x4b/0xa0 btrfs_read_tree_root+0x61/0x120 btrfs_get_root_ref+0x14b/0x600 find_parent_nodes+0x3e6/0x1b30 btrfs_find_all_roots_safe+0xb4/0x130 btrfs_find_all_roots+0x60/0x80 btrfs_qgroup_trace_extent_post+0x27/0x40 btrfs_add_delayed_data_ref+0x3fd/0x460 btrfs_free_extent+0x42/0x100 __btrfs_mod_ref+0x1d7/0x2f0 walk_up_proc+0x11c/0x400 walk_up_tree+0xf0/0x180 btrfs_drop_snapshot+0x1c7/0x780 ? btrfs_clean_one_deleted_snapshot+0x73/0x110 btrfs_clean_one_deleted_snapshot+0xfb/0x110 cleaner_kthread+0xd4/0x140 ? btrfs_alloc_root+0x50/0x50 kthread+0x13a/0x150 ? kthread_create_worker_on_cpu+0x40/0x40 ret_from_fork+0x1f/0x30 BTRFS info (device sdb): disk space caching is enabled BTRFS info (device sdb): has skinny extents This happens because qgroups does a backref lookup when we create a delayed ref. From here it may have to look up a root from an indirect ref, which does a normal lookup on the tree_root, which takes the read lock on the tree_root nodes. To fix this we need to add a variant for looking up roots that searches the commit root of the tree_root. Then when we do the backref search using the commit root we are sure to not take any locks on the tree_root nodes. This gets rid of the lockdep splat when running btrfs/104. Reviewed-by: Filipe Manana <fdmanana@suse.com> Signed-off-by: Josef Bacik <josef@toxicpanda.com> Signed-off-by: David Sterba <dsterba@suse.com>
This commit is contained in:
Josef Bacik
David Sterba
parent
5223cc60b4
commit
49d11bead7
@ -544,7 +544,18 @@ static int resolve_indirect_ref(struct btrfs_fs_info *fs_info,
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int level = ref->level;
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struct btrfs_key search_key = ref->key_for_search;
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root = btrfs_get_fs_root(fs_info, ref->root_id, false);
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/*
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* If we're search_commit_root we could possibly be holding locks on
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* other tree nodes. This happens when qgroups does backref walks when
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* adding new delayed refs. To deal with this we need to look in cache
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* for the root, and if we don't find it then we need to search the
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* tree_root's commit root, thus the btrfs_get_fs_root_commit_root usage
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* here.
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*/
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if (path->search_commit_root)
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root = btrfs_get_fs_root_commit_root(fs_info, path, ref->root_id);
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else
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root = btrfs_get_fs_root(fs_info, ref->root_id, false);
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if (IS_ERR(root)) {
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ret = PTR_ERR(root);
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goto out_free;
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@ -1281,32 +1281,26 @@ int btrfs_add_log_tree(struct btrfs_trans_handle *trans,
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return 0;
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}
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struct btrfs_root *btrfs_read_tree_root(struct btrfs_root *tree_root,
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struct btrfs_key *key)
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static struct btrfs_root *read_tree_root_path(struct btrfs_root *tree_root,
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struct btrfs_path *path,
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struct btrfs_key *key)
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{
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struct btrfs_root *root;
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struct btrfs_fs_info *fs_info = tree_root->fs_info;
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struct btrfs_path *path;
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u64 generation;
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int ret;
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int level;
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path = btrfs_alloc_path();
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if (!path)
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return ERR_PTR(-ENOMEM);
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root = btrfs_alloc_root(fs_info, key->objectid, GFP_NOFS);
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if (!root) {
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ret = -ENOMEM;
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goto alloc_fail;
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}
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if (!root)
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return ERR_PTR(-ENOMEM);
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ret = btrfs_find_root(tree_root, key, path,
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&root->root_item, &root->root_key);
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if (ret) {
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if (ret > 0)
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ret = -ENOENT;
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goto find_fail;
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goto fail;
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}
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generation = btrfs_root_generation(&root->root_item);
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@ -1317,21 +1311,31 @@ struct btrfs_root *btrfs_read_tree_root(struct btrfs_root *tree_root,
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if (IS_ERR(root->node)) {
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ret = PTR_ERR(root->node);
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root->node = NULL;
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goto find_fail;
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goto fail;
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} else if (!btrfs_buffer_uptodate(root->node, generation, 0)) {
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ret = -EIO;
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goto find_fail;
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goto fail;
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}
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root->commit_root = btrfs_root_node(root);
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out:
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btrfs_free_path(path);
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return root;
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find_fail:
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fail:
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btrfs_put_root(root);
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alloc_fail:
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root = ERR_PTR(ret);
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goto out;
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return ERR_PTR(ret);
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}
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struct btrfs_root *btrfs_read_tree_root(struct btrfs_root *tree_root,
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struct btrfs_key *key)
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{
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struct btrfs_root *root;
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struct btrfs_path *path;
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path = btrfs_alloc_path();
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if (!path)
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return ERR_PTR(-ENOMEM);
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root = read_tree_root_path(tree_root, path, key);
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btrfs_free_path(path);
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return root;
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}
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/*
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@ -1419,6 +1423,31 @@ static struct btrfs_root *btrfs_lookup_fs_root(struct btrfs_fs_info *fs_info,
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return root;
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}
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static struct btrfs_root *btrfs_get_global_root(struct btrfs_fs_info *fs_info,
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u64 objectid)
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{
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if (objectid == BTRFS_ROOT_TREE_OBJECTID)
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return btrfs_grab_root(fs_info->tree_root);
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if (objectid == BTRFS_EXTENT_TREE_OBJECTID)
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return btrfs_grab_root(fs_info->extent_root);
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if (objectid == BTRFS_CHUNK_TREE_OBJECTID)
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return btrfs_grab_root(fs_info->chunk_root);
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if (objectid == BTRFS_DEV_TREE_OBJECTID)
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return btrfs_grab_root(fs_info->dev_root);
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if (objectid == BTRFS_CSUM_TREE_OBJECTID)
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return btrfs_grab_root(fs_info->csum_root);
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if (objectid == BTRFS_QUOTA_TREE_OBJECTID)
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return btrfs_grab_root(fs_info->quota_root) ?
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fs_info->quota_root : ERR_PTR(-ENOENT);
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if (objectid == BTRFS_UUID_TREE_OBJECTID)
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return btrfs_grab_root(fs_info->uuid_root) ?
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fs_info->uuid_root : ERR_PTR(-ENOENT);
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if (objectid == BTRFS_FREE_SPACE_TREE_OBJECTID)
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return btrfs_grab_root(fs_info->free_space_root) ?
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fs_info->free_space_root : ERR_PTR(-ENOENT);
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return NULL;
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}
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int btrfs_insert_fs_root(struct btrfs_fs_info *fs_info,
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struct btrfs_root *root)
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{
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@ -1518,25 +1547,9 @@ static struct btrfs_root *btrfs_get_root_ref(struct btrfs_fs_info *fs_info,
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struct btrfs_key key;
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int ret;
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if (objectid == BTRFS_ROOT_TREE_OBJECTID)
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return btrfs_grab_root(fs_info->tree_root);
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if (objectid == BTRFS_EXTENT_TREE_OBJECTID)
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return btrfs_grab_root(fs_info->extent_root);
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if (objectid == BTRFS_CHUNK_TREE_OBJECTID)
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return btrfs_grab_root(fs_info->chunk_root);
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if (objectid == BTRFS_DEV_TREE_OBJECTID)
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return btrfs_grab_root(fs_info->dev_root);
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if (objectid == BTRFS_CSUM_TREE_OBJECTID)
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return btrfs_grab_root(fs_info->csum_root);
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if (objectid == BTRFS_QUOTA_TREE_OBJECTID)
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return btrfs_grab_root(fs_info->quota_root) ?
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fs_info->quota_root : ERR_PTR(-ENOENT);
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if (objectid == BTRFS_UUID_TREE_OBJECTID)
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return btrfs_grab_root(fs_info->uuid_root) ?
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fs_info->uuid_root : ERR_PTR(-ENOENT);
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if (objectid == BTRFS_FREE_SPACE_TREE_OBJECTID)
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return btrfs_grab_root(fs_info->free_space_root) ?
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fs_info->free_space_root : ERR_PTR(-ENOENT);
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root = btrfs_get_global_root(fs_info, objectid);
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if (root)
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return root;
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again:
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root = btrfs_lookup_fs_root(fs_info, objectid);
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if (root) {
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@ -1621,6 +1634,52 @@ struct btrfs_root *btrfs_get_new_fs_root(struct btrfs_fs_info *fs_info,
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return btrfs_get_root_ref(fs_info, objectid, anon_dev, true);
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}
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/*
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* btrfs_get_fs_root_commit_root - return a root for the given objectid
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* @fs_info: the fs_info
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* @objectid: the objectid we need to lookup
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*
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* This is exclusively used for backref walking, and exists specifically because
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* of how qgroups does lookups. Qgroups will do a backref lookup at delayed ref
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* creation time, which means we may have to read the tree_root in order to look
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* up a fs root that is not in memory. If the root is not in memory we will
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* read the tree root commit root and look up the fs root from there. This is a
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* temporary root, it will not be inserted into the radix tree as it doesn't
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* have the most uptodate information, it'll simply be discarded once the
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* backref code is finished using the root.
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*/
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struct btrfs_root *btrfs_get_fs_root_commit_root(struct btrfs_fs_info *fs_info,
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struct btrfs_path *path,
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u64 objectid)
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{
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struct btrfs_root *root;
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struct btrfs_key key;
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ASSERT(path->search_commit_root && path->skip_locking);
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/*
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* This can return -ENOENT if we ask for a root that doesn't exist, but
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* since this is called via the backref walking code we won't be looking
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* up a root that doesn't exist, unless there's corruption. So if root
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* != NULL just return it.
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*/
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root = btrfs_get_global_root(fs_info, objectid);
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if (root)
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return root;
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root = btrfs_lookup_fs_root(fs_info, objectid);
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if (root)
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return root;
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key.objectid = objectid;
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key.type = BTRFS_ROOT_ITEM_KEY;
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key.offset = (u64)-1;
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root = read_tree_root_path(fs_info->tree_root, path, &key);
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btrfs_release_path(path);
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return root;
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}
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/*
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* called by the kthread helper functions to finally call the bio end_io
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* functions. This is where read checksum verification actually happens
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@ -69,6 +69,9 @@ struct btrfs_root *btrfs_get_fs_root(struct btrfs_fs_info *fs_info,
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u64 objectid, bool check_ref);
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struct btrfs_root *btrfs_get_new_fs_root(struct btrfs_fs_info *fs_info,
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u64 objectid, dev_t anon_dev);
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struct btrfs_root *btrfs_get_fs_root_commit_root(struct btrfs_fs_info *fs_info,
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struct btrfs_path *path,
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u64 objectid);
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void btrfs_free_fs_info(struct btrfs_fs_info *fs_info);
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int btrfs_cleanup_fs_roots(struct btrfs_fs_info *fs_info);
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