There is a hung_task issue with running generic/068 on an SMR device. The hang occurs while a process is trying to thaw the filesystem. The process is trying to take sb->s_umount to thaw the FS. The lock is held by fsstress, which calls btrfs_sync_fs() and is waiting for an ordered extent to finish. However, as the FS is frozen, the ordered extent never finish.

Having an ordered extent while the FS is frozen is the root cause of the hang. The ordered extent is initiated from btrfs_relocate_chunk() which is called from btrfs_reclaim_bgs_work().

This commit add sb_*_write() around btrfs_relocate_chunk() call site. For the usual "btrfs balance" command, we already call it with mnt_want_file() in btrfs_ioctl_balance().

Additionally, add an ASSERT in btrfs_relocate_chunk() to check it is properly called.

Fixes: 18bb8bbf13c1 ("btrfs: zoned: automatically reclaim zones") Cc: stable@vger.kernel.org # 5.13+ Link: https://github.com/naota/linux/issues/56 Signed-off-by: Naohiro Aota naohiro.aota@wdc.com --- fs/btrfs/block-group.c | 8 +++++++- fs/btrfs/volumes.c | 6 ++++++ 2 files changed, 13 insertions(+), 1 deletion(-)

diff --git a/fs/btrfs/block-group.c b/fs/btrfs/block-group.c index 68feabc83a27..f9ca58a90630 100644 --- a/fs/btrfs/block-group.c +++ b/fs/btrfs/block-group.c @@ -1513,8 +1513,12 @@ void btrfs_reclaim_bgs_work(struct work_struct *work) if (!test_bit(BTRFS_FS_OPEN, &fs_info->flags)) return;

- if (!btrfs_exclop_start(fs_info, BTRFS_EXCLOP_BALANCE)) + sb_start_write(fs_info->sb); + + if (!btrfs_exclop_start(fs_info, BTRFS_EXCLOP_BALANCE)) { + sb_end_write(fs_info->sb); return; + }

/* * Long running balances can keep us blocked here for eternity, so @@ -1522,6 +1526,7 @@ void btrfs_reclaim_bgs_work(struct work_struct *work) */ if (!mutex_trylock(&fs_info->reclaim_bgs_lock)) { btrfs_exclop_finish(fs_info); + sb_end_write(fs_info->sb); return; }

@@ -1596,6 +1601,7 @@ void btrfs_reclaim_bgs_work(struct work_struct *work) spin_unlock(&fs_info->unused_bgs_lock); mutex_unlock(&fs_info->reclaim_bgs_lock); btrfs_exclop_finish(fs_info); + sb_end_write(fs_info->sb); }

void btrfs_reclaim_bgs(struct btrfs_fs_info *fs_info) diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c index b07d382d53a8..e415fb4a698e 100644 --- a/fs/btrfs/volumes.c +++ b/fs/btrfs/volumes.c @@ -3251,6 +3251,9 @@ int btrfs_relocate_chunk(struct btrfs_fs_info *fs_info, u64 chunk_offset) u64 length; int ret;

+ /* Assert we called sb_start_write(), not to race with FS freezing */ + sb_assert_write_started(fs_info->sb); + /* * Prevent races with automatic removal of unused block groups. * After we relocate and before we remove the chunk with offset @@ -8299,10 +8302,12 @@ static int relocating_repair_kthread(void *data) target = cache->start; btrfs_put_block_group(cache);

+ sb_start_write(fs_info->sb); if (!btrfs_exclop_start(fs_info, BTRFS_EXCLOP_BALANCE)) { btrfs_info(fs_info, "zoned: skip relocating block group %llu to repair: EBUSY", target); + sb_end_write(fs_info->sb); return -EBUSY; }

@@ -8330,6 +8335,7 @@ static int relocating_repair_kthread(void *data) btrfs_put_block_group(cache); mutex_unlock(&fs_info->reclaim_bgs_lock); btrfs_exclop_finish(fs_info); + sb_end_write(fs_info->sb);

return ret; }