btrfs: fix lockdep warning when creating free space tree

A lock dependency loop exists between the root tree lock, the extent tree
lock, and the free space tree lock.

The root tree lock depends on the free space tree lock because
btrfs_create_tree holds the new tree's lock while adding it to the root
tree.

The extent tree lock depends on the root tree lock because during
umount, we write out space cache v1, which writes inodes in the root
tree, which results in holding the root tree lock while doing a lookup
in the extent tree.

Finally, the free space tree depends on the extent tree because
populate_free_space_tree holds a locked path in the extent tree and then
does a lookup in the free space tree to add the new item.

The simplest of the three to break is the one during tree creation: we
unlock the leaf before inserting the tree node into the root tree, which
fixes the lockdep warning.

  [30.480136] ======================================================
  [30.480830] WARNING: possible circular locking dependency detected
  [30.481457] 5.9.0-rc8+ #76 Not tainted
  [30.481897] ------------------------------------------------------
  [30.482500] mount/520 is trying to acquire lock:
  [30.483064] ffff9babebe03908 (btrfs-free-space-00){++++}-{3:3}, at: __btrfs_tree_read_lock+0x39/0x180
  [30.484054]
	      but task is already holding lock:
  [30.484637] ffff9babebe24468 (btrfs-extent-01#2){++++}-{3:3}, at: __btrfs_tree_read_lock+0x39/0x180
  [30.485581]
	      which lock already depends on the new lock.

  [30.486397]
	      the existing dependency chain (in reverse order) is:
  [30.487205]
	      -> #2 (btrfs-extent-01#2){++++}-{3:3}:
  [30.487825]        down_read_nested+0x43/0x150
  [30.488306]        __btrfs_tree_read_lock+0x39/0x180
  [30.488868]        __btrfs_read_lock_root_node+0x3a/0x50
  [30.489477]        btrfs_search_slot+0x464/0x9b0
  [30.490009]        check_committed_ref+0x59/0x1d0
  [30.490603]        btrfs_cross_ref_exist+0x65/0xb0
  [30.491108]        run_delalloc_nocow+0x405/0x930
  [30.491651]        btrfs_run_delalloc_range+0x60/0x6b0
  [30.492203]        writepage_delalloc+0xd4/0x150
  [30.492688]        __extent_writepage+0x18d/0x3a0
  [30.493199]        extent_write_cache_pages+0x2af/0x450
  [30.493743]        extent_writepages+0x34/0x70
  [30.494231]        do_writepages+0x31/0xd0
  [30.494642]        __filemap_fdatawrite_range+0xad/0xe0
  [30.495194]        btrfs_fdatawrite_range+0x1b/0x50
  [30.495677]        __btrfs_write_out_cache+0x40d/0x460
  [30.496227]        btrfs_write_out_cache+0x8b/0x110
  [30.496716]        btrfs_start_dirty_block_groups+0x211/0x4e0
  [30.497317]        btrfs_commit_transaction+0xc0/0xba0
  [30.497861]        sync_filesystem+0x71/0x90
  [30.498303]        btrfs_remount+0x81/0x433
  [30.498767]        reconfigure_super+0x9f/0x210
  [30.499261]        path_mount+0x9d1/0xa30
  [30.499722]        do_mount+0x55/0x70
  [30.500158]        __x64_sys_mount+0xc4/0xe0
  [30.500616]        do_syscall_64+0x33/0x40
  [30.501091]        entry_SYSCALL_64_after_hwframe+0x44/0xa9
  [30.501629]
	      -> #1 (btrfs-root-00){++++}-{3:3}:
  [30.502241]        down_read_nested+0x43/0x150
  [30.502727]        __btrfs_tree_read_lock+0x39/0x180
  [30.503291]        __btrfs_read_lock_root_node+0x3a/0x50
  [30.503903]        btrfs_search_slot+0x464/0x9b0
  [30.504405]        btrfs_insert_empty_items+0x60/0xa0
  [30.504973]        btrfs_insert_item+0x60/0xd0
  [30.505412]        btrfs_create_tree+0x1b6/0x210
  [30.505913]        btrfs_create_free_space_tree+0x54/0x110
  [30.506460]        btrfs_mount_rw+0x15d/0x20f
  [30.506937]        btrfs_remount+0x356/0x433
  [30.507369]        reconfigure_super+0x9f/0x210
  [30.507868]        path_mount+0x9d1/0xa30
  [30.508264]        do_mount+0x55/0x70
  [30.508668]        __x64_sys_mount+0xc4/0xe0
  [30.509186]        do_syscall_64+0x33/0x40
  [30.509652]        entry_SYSCALL_64_after_hwframe+0x44/0xa9
  [30.510271]
	      -> #0 (btrfs-free-space-00){++++}-{3:3}:
  [30.510972]        __lock_acquire+0x11ad/0x1b60
  [30.511432]        lock_acquire+0xa2/0x360
  [30.511917]        down_read_nested+0x43/0x150
  [30.512383]        __btrfs_tree_read_lock+0x39/0x180
  [30.512947]        __btrfs_read_lock_root_node+0x3a/0x50
  [30.513455]        btrfs_search_slot+0x464/0x9b0
  [30.513947]        search_free_space_info+0x45/0x90
  [30.514465]        __add_to_free_space_tree+0x92/0x39d
  [30.515010]        btrfs_create_free_space_tree.cold.22+0x1ee/0x45d
  [30.515639]        btrfs_mount_rw+0x15d/0x20f
  [30.516142]        btrfs_remount+0x356/0x433
  [30.516538]        reconfigure_super+0x9f/0x210
  [30.517065]        path_mount+0x9d1/0xa30
  [30.517438]        do_mount+0x55/0x70
  [30.517824]        __x64_sys_mount+0xc4/0xe0
  [30.518293]        do_syscall_64+0x33/0x40
  [30.518776]        entry_SYSCALL_64_after_hwframe+0x44/0xa9
  [30.519335]
	      other info that might help us debug this:

  [30.520210] Chain exists of:
		btrfs-free-space-00 --> btrfs-root-00 --> btrfs-extent-01#2

  [30.521407]  Possible unsafe locking scenario:

  [30.522037]        CPU0                    CPU1
  [30.522456]        ----                    ----
  [30.522941]   lock(btrfs-extent-01#2);
  [30.523311]                                lock(btrfs-root-00);
  [30.523952]                                lock(btrfs-extent-01#2);
  [30.524620]   lock(btrfs-free-space-00);
  [30.525068]
	       *** DEADLOCK ***

  [30.525669] 5 locks held by mount/520:
  [30.526116]  #0: ffff9babebc520e0 (&type->s_umount_key#37){+.+.}-{3:3}, at: path_mount+0x7ef/0xa30
  [30.527056]  #1: ffff9babebc52640 (sb_internal#2){.+.+}-{0:0}, at: start_transaction+0x3d5/0x5c0
  [30.527960]  #2: ffff9babeae8f2e8 (&cache->free_space_lock#2){+.+.}-{3:3}, at: btrfs_create_free_space_tree.cold.22+0x101/0x45d
  [30.529118]  #3: ffff9babebe24468 (btrfs-extent-01#2){++++}-{3:3}, at: __btrfs_tree_read_lock+0x39/0x180
  [30.530113]  #4: ffff9babebd52eb8 (btrfs-extent-00){++++}-{3:3}, at: btrfs_try_tree_read_lock+0x16/0x100
  [30.531124]
	      stack backtrace:
  [30.531528] CPU: 0 PID: 520 Comm: mount Not tainted 5.9.0-rc8+ #76
  [30.532166] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.11.1-4.module_el8.1.0+248+298dec18 04/01/2014
  [30.533215] Call Trace:
  [30.533452]  dump_stack+0x8d/0xc0
  [30.533797]  check_noncircular+0x13c/0x150
  [30.534233]  __lock_acquire+0x11ad/0x1b60
  [30.534667]  lock_acquire+0xa2/0x360
  [30.535063]  ? __btrfs_tree_read_lock+0x39/0x180
  [30.535525]  down_read_nested+0x43/0x150
  [30.535939]  ? __btrfs_tree_read_lock+0x39/0x180
  [30.536400]  __btrfs_tree_read_lock+0x39/0x180
  [30.536862]  __btrfs_read_lock_root_node+0x3a/0x50
  [30.537304]  btrfs_search_slot+0x464/0x9b0
  [30.537713]  ? trace_hardirqs_on+0x1c/0xf0
  [30.538148]  search_free_space_info+0x45/0x90
  [30.538572]  __add_to_free_space_tree+0x92/0x39d
  [30.539071]  ? printk+0x48/0x4a
  [30.539367]  btrfs_create_free_space_tree.cold.22+0x1ee/0x45d
  [30.539972]  btrfs_mount_rw+0x15d/0x20f
  [30.540350]  btrfs_remount+0x356/0x433
  [30.540773]  ? shrink_dcache_sb+0xd9/0x100
  [30.541203]  reconfigure_super+0x9f/0x210
  [30.541642]  path_mount+0x9d1/0xa30
  [30.542040]  do_mount+0x55/0x70
  [30.542366]  __x64_sys_mount+0xc4/0xe0
  [30.542822]  do_syscall_64+0x33/0x40
  [30.543197]  entry_SYSCALL_64_after_hwframe+0x44/0xa9
  [30.543691] RIP: 0033:0x7f109f7ab93a
  [30.546042] RSP: 002b:00007ffc47c4f858 EFLAGS: 00000246 ORIG_RAX: 00000000000000a5
  [30.546770] RAX: ffffffffffffffda RBX: 00007f109f8cf264 RCX: 00007f109f7ab93a
  [30.547485] RDX: 0000557e6fc10770 RSI: 0000557e6fc19cf0 RDI: 0000557e6fc19cd0
  [30.548185] RBP: 0000557e6fc10520 R08: 0000557e6fc18e30 R09: 0000557e6fc18cb0
  [30.548911] R10: 0000000000200020 R11: 0000000000000246 R12: 0000000000000000
  [30.549606] R13: 0000557e6fc19cd0 R14: 0000557e6fc10770 R15: 0000557e6fc10520

Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: Boris Burkov <boris@bur.io>
Signed-off-by: David Sterba <dsterba@suse.com>

1 files changed, 5 insertions, 4 deletions

diff --git a/fs/btrfs/disk-io.c b/fs/btrfs/disk-io.c
index 2dcc5e52140c..46dd9e0b077e 100644
--- a/fs/btrfs/disk-io.c
+++ b/fs/btrfs/disk-io.c
@@ -1142,7 +1142,7 @@ struct btrfs_root *btrfs_create_tree(struct btrfs_trans_handle *trans,
 	if (IS_ERR(leaf)) {
 		ret = PTR_ERR(leaf);
 		leaf = NULL;
-		goto fail;
+		goto fail_unlock;
 	}
 
 	root->node = leaf;
@@ -1166,6 +1166,8 @@ struct btrfs_root *btrfs_create_tree(struct btrfs_trans_handle *trans,
 		export_guid(root->root_item.uuid, &guid_null);
 	btrfs_set_root_drop_level(&root->root_item, 0);
 
+	btrfs_tree_unlock(leaf);
+
 	key.objectid = objectid;
 	key.type = BTRFS_ROOT_ITEM_KEY;
 	key.offset = 0;
@@ -1173,13 +1175,12 @@ struct btrfs_root *btrfs_create_tree(struct btrfs_trans_handle *trans,
 	if (ret)
 		goto fail;
 
-	btrfs_tree_unlock(leaf);
-
 	return root;
 
-fail:
+fail_unlock:
 	if (leaf)
 		btrfs_tree_unlock(leaf);
+fail:
 	btrfs_put_root(root);
 
 	return ERR_PTR(ret);