Fast commit can hold s_fc_lock while writing journal blocks. Mapping the
journal inode can take its i_data_sem. Normal inode update paths can take a
data inode i_data_sem and then s_fc_lock, which makes lockdep report a
circular dependency.

lockdep treats all i_data_sem instances as one lock class and cannot
distinguish the journal inode i_data_sem from a regular inode i_data_sem.
The journal inode is not tracked by fast commit and no FC waiters ever
depend on it, so this is not a real ABBA deadlock. Assign the journal inode
a dedicated i_data_sem lockdep subclass to avoid the false positive.

Inode cache objects can be recycled, so also reset i_data_sem to
I_DATA_SEM_NORMAL when allocating an ext4 inode. Otherwise a new inode may
inherit an old subclass (journal/quota/ea) and trigger lockdep warnings.

Signed-off-by: Li Chen <me@linux.beauty>
---
 fs/ext4/ext4.h  | 4 +++-
 fs/ext4/super.c | 8 ++++++++
 2 files changed, 11 insertions(+), 1 deletion(-)

diff --git a/fs/ext4/ext4.h b/fs/ext4/ext4.h
index bd30c24d4f94..2e1681057196 100644
--- a/fs/ext4/ext4.h
+++ b/fs/ext4/ext4.h
@@ -1025,12 +1025,14 @@ do {										\
  *			  than the first
  *  I_DATA_SEM_QUOTA  - Used for quota inodes only
  *  I_DATA_SEM_EA     - Used for ea_inodes only
+ *  I_DATA_SEM_JOURNAL - Used for journal inode only
  */
 enum {
 	I_DATA_SEM_NORMAL = 0,
 	I_DATA_SEM_OTHER,
 	I_DATA_SEM_QUOTA,
-	I_DATA_SEM_EA
+	I_DATA_SEM_EA,
+	I_DATA_SEM_JOURNAL
 };
 
 struct ext4_fc_inode_snap;
diff --git a/fs/ext4/super.c b/fs/ext4/super.c
index 79762c3e0dff..4f5f0c21d436 100644
--- a/fs/ext4/super.c
+++ b/fs/ext4/super.c
@@ -1423,6 +1423,9 @@ static struct inode *ext4_alloc_inode(struct super_block *sb)
 	INIT_WORK(&ei->i_rsv_conversion_work, ext4_end_io_rsv_work);
 	ext4_fc_init_inode(&ei->vfs_inode);
 	spin_lock_init(&ei->i_fc_lock);
+#ifdef CONFIG_LOCKDEP
+	lockdep_set_subclass(&ei->i_data_sem, I_DATA_SEM_NORMAL);
+#endif
 	return &ei->vfs_inode;
 }
 
@@ -5863,6 +5866,11 @@ static struct inode *ext4_get_journal_inode(struct super_block *sb,
 		return ERR_PTR(-EFSCORRUPTED);
 	}
 
+#ifdef CONFIG_LOCKDEP
+	lockdep_set_subclass(&EXT4_I(journal_inode)->i_data_sem,
+			     I_DATA_SEM_JOURNAL);
+#endif
+
 	ext4_debug("Journal inode found at %p: %lld bytes\n",
 		  journal_inode, journal_inode->i_size);
 	return journal_inode;
-- 
2.52.0