From: Josef Bacik josef@toxicpanda.com
[ Upstream commit 49d11bead7d596e031fbd34051d8765587cd645b ]
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 Signed-off-by: Sasha Levin sashal@kernel.org --- fs/btrfs/backref.c | 13 ++++- fs/btrfs/disk-io.c | 139 ++++++++++++++++++++++++++++++++------------- fs/btrfs/disk-io.h | 3 + 3 files changed, 114 insertions(+), 41 deletions(-)
diff --git a/fs/btrfs/backref.c b/fs/btrfs/backref.c index ea1c28ccb44ff..b948df7a929eb 100644 --- a/fs/btrfs/backref.c +++ b/fs/btrfs/backref.c @@ -544,7 +544,18 @@ static int resolve_indirect_ref(struct btrfs_fs_info *fs_info, int level = ref->level; struct btrfs_key search_key = ref->key_for_search;
- root = btrfs_get_fs_root(fs_info, ref->root_id, false); + /* + * If we're search_commit_root we could possibly be holding locks on + * other tree nodes. This happens when qgroups does backref walks when + * adding new delayed refs. To deal with this we need to look in cache + * for the root, and if we don't find it then we need to search the + * tree_root's commit root, thus the btrfs_get_fs_root_commit_root usage + * here. + */ + if (path->search_commit_root) + root = btrfs_get_fs_root_commit_root(fs_info, path, ref->root_id); + else + root = btrfs_get_fs_root(fs_info, ref->root_id, false); if (IS_ERR(root)) { ret = PTR_ERR(root); goto out_free; diff --git a/fs/btrfs/disk-io.c b/fs/btrfs/disk-io.c index 7882c07645014..2a0a1c032a72c 100644 --- a/fs/btrfs/disk-io.c +++ b/fs/btrfs/disk-io.c @@ -1338,32 +1338,26 @@ int btrfs_add_log_tree(struct btrfs_trans_handle *trans, return 0; }
-struct btrfs_root *btrfs_read_tree_root(struct btrfs_root *tree_root, - struct btrfs_key *key) +static struct btrfs_root *read_tree_root_path(struct btrfs_root *tree_root, + struct btrfs_path *path, + struct btrfs_key *key) { struct btrfs_root *root; struct btrfs_fs_info *fs_info = tree_root->fs_info; - struct btrfs_path *path; u64 generation; int ret; int level;
- path = btrfs_alloc_path(); - if (!path) - return ERR_PTR(-ENOMEM); - root = btrfs_alloc_root(fs_info, key->objectid, GFP_NOFS); - if (!root) { - ret = -ENOMEM; - goto alloc_fail; - } + if (!root) + return ERR_PTR(-ENOMEM);
ret = btrfs_find_root(tree_root, key, path, &root->root_item, &root->root_key); if (ret) { if (ret > 0) ret = -ENOENT; - goto find_fail; + goto fail; }
generation = btrfs_root_generation(&root->root_item); @@ -1374,21 +1368,31 @@ struct btrfs_root *btrfs_read_tree_root(struct btrfs_root *tree_root, if (IS_ERR(root->node)) { ret = PTR_ERR(root->node); root->node = NULL; - goto find_fail; + goto fail; } else if (!btrfs_buffer_uptodate(root->node, generation, 0)) { ret = -EIO; - goto find_fail; + goto fail; } root->commit_root = btrfs_root_node(root); -out: - btrfs_free_path(path); return root; - -find_fail: +fail: btrfs_put_root(root); -alloc_fail: - root = ERR_PTR(ret); - goto out; + return ERR_PTR(ret); +} + +struct btrfs_root *btrfs_read_tree_root(struct btrfs_root *tree_root, + struct btrfs_key *key) +{ + struct btrfs_root *root; + struct btrfs_path *path; + + path = btrfs_alloc_path(); + if (!path) + return ERR_PTR(-ENOMEM); + root = read_tree_root_path(tree_root, path, key); + btrfs_free_path(path); + + return root; }
/* @@ -1476,6 +1480,31 @@ static struct btrfs_root *btrfs_lookup_fs_root(struct btrfs_fs_info *fs_info, return root; }
+static struct btrfs_root *btrfs_get_global_root(struct btrfs_fs_info *fs_info, + u64 objectid) +{ + if (objectid == BTRFS_ROOT_TREE_OBJECTID) + return btrfs_grab_root(fs_info->tree_root); + if (objectid == BTRFS_EXTENT_TREE_OBJECTID) + return btrfs_grab_root(fs_info->extent_root); + if (objectid == BTRFS_CHUNK_TREE_OBJECTID) + return btrfs_grab_root(fs_info->chunk_root); + if (objectid == BTRFS_DEV_TREE_OBJECTID) + return btrfs_grab_root(fs_info->dev_root); + if (objectid == BTRFS_CSUM_TREE_OBJECTID) + return btrfs_grab_root(fs_info->csum_root); + if (objectid == BTRFS_QUOTA_TREE_OBJECTID) + return btrfs_grab_root(fs_info->quota_root) ? + fs_info->quota_root : ERR_PTR(-ENOENT); + if (objectid == BTRFS_UUID_TREE_OBJECTID) + return btrfs_grab_root(fs_info->uuid_root) ? + fs_info->uuid_root : ERR_PTR(-ENOENT); + if (objectid == BTRFS_FREE_SPACE_TREE_OBJECTID) + return btrfs_grab_root(fs_info->free_space_root) ? + fs_info->free_space_root : ERR_PTR(-ENOENT); + return NULL; +} + int btrfs_insert_fs_root(struct btrfs_fs_info *fs_info, struct btrfs_root *root) { @@ -1573,25 +1602,9 @@ static struct btrfs_root *btrfs_get_root_ref(struct btrfs_fs_info *fs_info, struct btrfs_key key; int ret;
- if (objectid == BTRFS_ROOT_TREE_OBJECTID) - return btrfs_grab_root(fs_info->tree_root); - if (objectid == BTRFS_EXTENT_TREE_OBJECTID) - return btrfs_grab_root(fs_info->extent_root); - if (objectid == BTRFS_CHUNK_TREE_OBJECTID) - return btrfs_grab_root(fs_info->chunk_root); - if (objectid == BTRFS_DEV_TREE_OBJECTID) - return btrfs_grab_root(fs_info->dev_root); - if (objectid == BTRFS_CSUM_TREE_OBJECTID) - return btrfs_grab_root(fs_info->csum_root); - if (objectid == BTRFS_QUOTA_TREE_OBJECTID) - return btrfs_grab_root(fs_info->quota_root) ? - fs_info->quota_root : ERR_PTR(-ENOENT); - if (objectid == BTRFS_UUID_TREE_OBJECTID) - return btrfs_grab_root(fs_info->uuid_root) ? - fs_info->uuid_root : ERR_PTR(-ENOENT); - if (objectid == BTRFS_FREE_SPACE_TREE_OBJECTID) - return btrfs_grab_root(fs_info->free_space_root) ? - fs_info->free_space_root : ERR_PTR(-ENOENT); + root = btrfs_get_global_root(fs_info, objectid); + if (root) + return root; again: root = btrfs_lookup_fs_root(fs_info, objectid); if (root) { @@ -1676,6 +1689,52 @@ struct btrfs_root *btrfs_get_new_fs_root(struct btrfs_fs_info *fs_info, return btrfs_get_root_ref(fs_info, objectid, anon_dev, true); }
+/* + * btrfs_get_fs_root_commit_root - return a root for the given objectid + * @fs_info: the fs_info + * @objectid: the objectid we need to lookup + * + * This is exclusively used for backref walking, and exists specifically because + * of how qgroups does lookups. Qgroups will do a backref lookup at delayed ref + * creation time, which means we may have to read the tree_root in order to look + * up a fs root that is not in memory. If the root is not in memory we will + * read the tree root commit root and look up the fs root from there. This is a + * temporary root, it will not be inserted into the radix tree as it doesn't + * have the most uptodate information, it'll simply be discarded once the + * backref code is finished using the root. + */ +struct btrfs_root *btrfs_get_fs_root_commit_root(struct btrfs_fs_info *fs_info, + struct btrfs_path *path, + u64 objectid) +{ + struct btrfs_root *root; + struct btrfs_key key; + + ASSERT(path->search_commit_root && path->skip_locking); + + /* + * This can return -ENOENT if we ask for a root that doesn't exist, but + * since this is called via the backref walking code we won't be looking + * up a root that doesn't exist, unless there's corruption. So if root + * != NULL just return it. + */ + root = btrfs_get_global_root(fs_info, objectid); + if (root) + return root; + + root = btrfs_lookup_fs_root(fs_info, objectid); + if (root) + return root; + + key.objectid = objectid; + key.type = BTRFS_ROOT_ITEM_KEY; + key.offset = (u64)-1; + root = read_tree_root_path(fs_info->tree_root, path, &key); + btrfs_release_path(path); + + return root; +} + /* * called by the kthread helper functions to finally call the bio end_io * functions. This is where read checksum verification actually happens diff --git a/fs/btrfs/disk-io.h b/fs/btrfs/disk-io.h index 00dc39d47ed34..2e6da9ae8462f 100644 --- a/fs/btrfs/disk-io.h +++ b/fs/btrfs/disk-io.h @@ -69,6 +69,9 @@ struct btrfs_root *btrfs_get_fs_root(struct btrfs_fs_info *fs_info, u64 objectid, bool check_ref); struct btrfs_root *btrfs_get_new_fs_root(struct btrfs_fs_info *fs_info, u64 objectid, dev_t anon_dev); +struct btrfs_root *btrfs_get_fs_root_commit_root(struct btrfs_fs_info *fs_info, + struct btrfs_path *path, + u64 objectid);
void btrfs_free_fs_info(struct btrfs_fs_info *fs_info); int btrfs_cleanup_fs_roots(struct btrfs_fs_info *fs_info);