From: Naohiro Aota <naohiro.aota@wdc.com>
Date: Tue, 29 Mar 2022 06:56:00 +0000 (+0900)
Subject: btrfs: assert that relocation is protected with sb_start_write()
X-Git-Url: http://git.maquefel.me/?a=commitdiff_plain;h=0320b3538b2b819c24daa699a26aa184f3d3cc6c;p=linux.git

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 b1c36fc72ffa7..79af494a316ab 100644
--- 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;