btrfs: fix encoded write i_size corruption with no-holes

We have observed a btrfs filesystem corruption on workloads using
no-holes and encoded writes via send stream v2. The symptom is that a
file appears to be truncated to the end of its last aligned extent, even
though the final unaligned extent and even the file extent and otherwise
correctly updated inode item have been written.

So if we were writing out a 1MiB+X file via 8 128K extents and one
extent of length X, i_size would be set to 1MiB, but the ninth extent,
nbyte, etc. would all appear correct otherwise.

The source of the race is a narrow (one line of code) window in which a
no-holes fs has read in an updated i_size, but has not yet set a shared
disk_i_size variable to write. Therefore, if two ordered extents run in
parallel (par for the course for receive workloads), the following
sequence can play out: (following "threads" a bit loosely, since there
are callbacks involved for endio but extra threads aren't needed to
cause the issue)

  ENC-WR1 (second to last)                                         ENC-WR2 (last)
  -------                                                          -------
  btrfs_do_encoded_write
    set i_size = 1M
    submit bio B1 ending at 1M
  endio B1
  btrfs_inode_safe_disk_i_size_write
    local i_size = 1M
    falls off a cliff for some reason
							      btrfs_do_encoded_write
								set i_size = 1M+X
								submit bio B2 ending at 1M+X
							      endio B2
							      btrfs_inode_safe_disk_i_size_write
								local i_size = 1M+X
								disk_i_size = 1M+X
    disk_i_size = 1M
							      btrfs_delayed_update_inode
    btrfs_delayed_update_inode

And the delayed inode ends up filled with nbytes=1M+X and isize=1M, and
writes respect i_size and present a corrupted file missing its last
extents.

Fix this by holding the inode lock in the no-holes case so that a thread
can't sneak in a write to disk_i_size that gets overwritten with an out
of date i_size.

Fixes: 41a2ee75 ("btrfs: introduce per-inode file extent tree")
CC: stable@vger.kernel.org # 5.10+
Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: Boris Burkov <boris@bur.io>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/file-item.c

@@ -52,13 +52,13 @@ void btrfs_inode_safe_disk_i_size_write(struct btrfs_inode *inode, u64 new_i_siz
 	u64 start, end, i_size;
 	int ret;
 
+	spin_lock(&inode->lock);
 	i_size = new_i_size ?: i_size_read(&inode->vfs_inode);
 	if (btrfs_fs_incompat(fs_info, NO_HOLES)) {
 		inode->disk_i_size = i_size;
-		return;
+		goto out_unlock;
 	}
 
-	spin_lock(&inode->lock);
 	ret = find_contiguous_extent_bit(&inode->file_extent_tree, 0, &start,
 					 &end, EXTENT_DIRTY);
 	if (!ret && start == 0)
@@ -66,6 +66,7 @@ void btrfs_inode_safe_disk_i_size_write(struct btrfs_inode *inode, u64 new_i_siz
 	else
 		i_size = 0;
 	inode->disk_i_size = i_size;
+out_unlock:
 	spin_unlock(&inode->lock);
 }