[Ocfs2-devel] [PATCH] ocfs2: fix missing reset j_num_trans for sync

[Heming Zhao]

fstest generic case 266 272 281 trigger hanging issue when umount.

I use 266 to describe the root cause.

```
 49 _dmerror_unmount
 50 _dmerror_mount
 51
 52 echo "Compare files"
 53 md5sum $testdir/file1 | _filter_scratch
 54 md5sum $testdir/file2 | _filter_scratch
 55
 56 echo "CoW and unmount"
 57 sync
 58 _dmerror_load_error_table
 59 urk=$($XFS_IO_PROG -f -c "pwrite -S 0x63 -b $bufsize 0 $filesize" \
 60     -c "fdatasync" $testdir/file2 2>&1)
 61 echo $urk >> $seqres.full
 62 echo "$urk" | grep -q "error" || _fail "pwrite did not fail"
 63
 64 echo "Clean up the mess"
 65 _dmerror_unmount
```

After line 49 50 umount & mount ocfs2 dev, this case run md5sum to
verify target file. line 57 run 'sync' before line 58 changes the dm
target from dm-linear to dm-error. this case is hanging at line 65.

In 266, md5sum calls jbd2 trans pair: ocfs2_[start|commit]_trans to
do journal job. but there is only ->j_num_trans+1 in ocfs2_start_trans,
the ocfs2_commit_trans doesn't do reduction operation.

call flow:
```
[md5sum]
 vfs_read
  ocfs2_file_read_iter
   ocfs2_inode_lock_atime
    ocfs2_update_inode_atime
     + ocfs2_start_trans //atomic_inc j_num_trans
     + ...
     + ocfs2_commit_trans//no modify j_num_trans

sync //no modify j_num_trans

_dmerror_load_error_table //all write will return error after this line

_dmerror_unmount //found j_num_trans is not zero, run commit thread
               //but the underlying dev is dm-error, journaling IO
               //failed all the time and keep going to retry.
```

*** How to fix ***

kick commit thread in sync path, which can reset j_num_trans to 0.

Signed-off-by: Heming Zhao <heming.zhao at suse.com>
---
 fs/ocfs2/super.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/fs/ocfs2/super.c b/fs/ocfs2/super.c
index 0b0e6a132101..9929320bd967 100644
--- a/fs/ocfs2/super.c
+++ b/fs/ocfs2/super.c
@@ -408,6 +408,9 @@ static int ocfs2_sync_fs(struct super_block *sb, int wait)
 
 	if (jbd2_journal_start_commit(osb->journal->j_journal,
 				      &target)) {
+		/* kick commit thread to reset journal->j_num_trans */
+		wake_up(&osb->checkpoint_event);
+
 		if (wait)
 			jbd2_log_wait_commit(osb->journal->j_journal,
 					     target);
-- 
2.39.2