[PATCH v4 3/3] btrfs: assert that relocation is protected with sb_start_write()

[Naohiro Aota <naohiro.aota@wdc.com>]

Relocation of a data block group creates ordered extents. They can cause a
hang when a process is trying to thaw the filesystem.

We should have called sb_start_write(), so the filesystem is not being
frozen. Add an ASSERT to check it is protected.

Signed-off-by: Naohiro Aota <naohiro.aota@wdc.com>
---
 fs/btrfs/relocation.c | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/fs/btrfs/relocation.c b/fs/btrfs/relocation.c
index fdc2c4b411f0..5e52cd8d5f23 100644
--- a/fs/btrfs/relocation.c
+++ b/fs/btrfs/relocation.c
@@ -3977,6 +3977,16 @@ int btrfs_relocate_block_group(struct btrfs_fs_info *fs_info, u64 group_start)
 	if (!bg)
 		return -ENOENT;
 
+	/*
+	 * Relocation of a data block group creates ordered extents.
+	 * Without sb_start_write(), we can freeze the FS while unfinished
+	 * ordered extents are left. Such ordered extents can cause a
+	 * deadlock e.g, when syncfs() is trying to finish them because
+	 * they never finish as the FS is already frozen.
+	 */
+	if (bg->flags & BTRFS_BLOCK_GROUP_DATA)
+		ASSERT(sb_write_started(fs_info->sb));
+
 	if (btrfs_pinned_by_swapfile(fs_info, bg)) {
 		btrfs_put_block_group(bg);
 		return -ETXTBSY;
-- 
2.35.1