btrfs: assert that relocation is protected with sb_start_write()

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.

Reviewed-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: Naohiro Aota <naohiro.aota@wdc.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

diff --git a/fs/btrfs/relocation.c b/fs/btrfs/relocation.c
index b1c36fc72ffa7b48b48bfea02c439564df847b9b..79af494a316abc39134e293ef336196dc2a4f1ab 100644 (file)
--- a/fs/btrfs/relocation.c
+++ b/fs/btrfs/relocation.c
@@ -3978,6 +3978,17 @@ 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 filesystem while unfinished
+        * ordered extents are left. Such ordered extents can cause a deadlock
+        * e.g. when syncfs() is waiting for their completion but they can't
+        * finish because they block when joining a transaction, due to the
+        * fact that the freeze locks are being held in write mode.
+        */
+       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;