[PATCH 5.15 035/148] btrfs: fix hang during unmount when stopping block group reclaim worker

Greg Kroah-Hartman
2022-09-26T11:29:31Z

From: Filipe Manana <fdmanana@suse.com>

commit 8a1f1e3d1eecf9d2359a2709e276743a67e145db upstream.

During early unmount, at close_ctree(), we try to stop the block group
reclaim task with cancel_work_sync(), but that may hang if the block group
reclaim task is currently at btrfs_relocate_block_group() waiting for the
flag BTRFS_FS_UNFINISHED_DROPS to be cleared from fs_info->flags. During
unmount we only clear that flag later, after trying to stop the block
group reclaim task.

Fix that by clearing BTRFS_FS_UNFINISHED_DROPS before trying to stop the
block group reclaim task and after setting BTRFS_FS_CLOSING_START, so that
if the reclaim task is waiting on that bit, it will stop immediately after
being woken, because it sees the filesystem is closing (with a call to
btrfs_fs_closing()), and then returns immediately with -EINTR.

Fixes: 31e70e527806c5 ("btrfs: fix hang during unmount when block group reclaim task is running")
CC: stable@vger.kernel.org # 5.15+
Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
fs/btrfs/disk-io.c | 17 +++++++++++------
1 file changed, 11 insertions(+), 6 deletions(-)

--- a/fs/btrfs/disk-io.c
+++ b/fs/btrfs/disk-io.c
@@ -4298,6 +4298,17 @@ void __cold close_ctree(struct btrfs_fs_
set_bit(BTRFS_FS_CLOSING_START, &fs_info->flags);

/*
+ * If we had UNFINISHED_DROPS we could still be processing them, so
+ * clear that bit and wake up relocation so it can stop.
+ * We must do this before stopping the block group reclaim task, because
+ * at btrfs_relocate_block_group() we wait for this bit, and after the
+ * wait we stop with -EINTR if btrfs_fs_closing() returns non-zero - we
+ * have just set BTRFS_FS_CLOSING_START, so btrfs_fs_closing() will
+ * return 1.
+ */
+ btrfs_wake_unfinished_drop(fs_info);
+
+ /*
* We may have the reclaim task running and relocating a data block group,
* in which case it may create delayed iputs. So stop it before we park
* the cleaner kthread otherwise we can get new delayed iputs after
@@ -4315,12 +4326,6 @@ void __cold close_ctree(struct btrfs_fs_
*/
kthread_park(fs_info->cleaner_kthread);

- /*
- * If we had UNFINISHED_DROPS we could still be processing them, so
- * clear that bit and wake up relocation so it can stop.
- */
- btrfs_wake_unfinished_drop(fs_info);
-
/* wait for the qgroup rescan worker to stop */
btrfs_qgroup_wait_for_completion(fs_info, false);