On Tue, Mar 29, 2022 at 03:56:00PM +0900, Naohiro Aota wrote:
> 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@xxxxxxx>
> ---
> 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
syncfs() is not trying to finish them, it's waiting for them to complete.
> + * they never finish as the FS is already frozen.
More specifically 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.
Anyway, I won't make you send yet another version. Perhaps this is something
David can fixup when he picks the patch.
Reviewed-by: Filipe Manana <fdmanana@xxxxxxxx>
Thanks.
> + */
> + 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
>
