btrfs: Remove btrfs_inode::delayed_iput_count

delayed_iput_count wa supposed to be used to implement, well, delayed
iput. The idea is that we keep accumulating the number of iputs we do
until eventually the inode is deleted. Turns out we never really
switched the delayed_iput_count from 0 to 1, hence all conditional
code relying on the value of that member being different than 0 was
never executed. This, as it turns out, didn't cause any problem due
to the simple fact that the generic inode's i_count member was always
used to count the number of iputs. So let's just remove the unused
member and all unused code. This patch essentially provides no
functional changes. While at it, also add proper documentation for
btrfs_add_delayed_iput

Signed-off-by: Nikolay Borisov <nborisov@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
[ reformat comment ]
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/btrfs_inode.h

@@ -195,7 +195,6 @@ struct btrfs_inode {
 
 	/* Hook into fs_info->delayed_iputs */
 	struct list_head delayed_iput;
-	long delayed_iput_count;
 
 	/*
 	 * To avoid races between lockless (i_mutex not held) direct IO writes

fs/btrfs/inode.c

@@ -3242,6 +3242,16 @@ static int btrfs_readpage_end_io_hook(struct btrfs_io_bio *io_bio,
 				      start, (size_t)(end - start + 1));
 }
 
+/*
+ * btrfs_add_delayed_iput - perform a delayed iput on @inode
+ *
+ * @inode: The inode we want to perform iput on
+ *
+ * This function uses the generic vfs_inode::i_count to track whether we should
+ * just decrement it (in case it's > 1) or if this is the last iput then link
+ * the inode to the delayed iput machinery. Delayed iputs are processed at
+ * transaction commit time/superblock commit/cleaner kthread.
+ */
 void btrfs_add_delayed_iput(struct inode *inode)
 {
 	struct btrfs_fs_info *fs_info = btrfs_sb(inode->i_sb);
@@ -3251,12 +3261,8 @@ void btrfs_add_delayed_iput(struct inode *inode)
 		return;
 
 	spin_lock(&fs_info->delayed_iput_lock);
-	if (binode->delayed_iput_count == 0) {
-		ASSERT(list_empty(&binode->delayed_iput));
-		list_add_tail(&binode->delayed_iput, &fs_info->delayed_iputs);
-	} else {
-		binode->delayed_iput_count++;
-	}
+	ASSERT(list_empty(&binode->delayed_iput));
+	list_add_tail(&binode->delayed_iput, &fs_info->delayed_iputs);
 	spin_unlock(&fs_info->delayed_iput_lock);
 }
 
@@ -3269,13 +3275,7 @@ void btrfs_run_delayed_iputs(struct btrfs_fs_info *fs_info)
 
 		inode = list_first_entry(&fs_info->delayed_iputs,
 				struct btrfs_inode, delayed_iput);
-		if (inode->delayed_iput_count) {
-			inode->delayed_iput_count--;
-			list_move_tail(&inode->delayed_iput,
-					&fs_info->delayed_iputs);
-		} else {
-			list_del_init(&inode->delayed_iput);
-		}
+		list_del_init(&inode->delayed_iput);
 		spin_unlock(&fs_info->delayed_iput_lock);
 		iput(&inode->vfs_inode);
 		spin_lock(&fs_info->delayed_iput_lock);
@@ -9333,7 +9333,6 @@ struct inode *btrfs_alloc_inode(struct super_block *sb)
 	ei->dir_index = 0;
 	ei->last_unlink_trans = 0;
 	ei->last_log_commit = 0;
-	ei->delayed_iput_count = 0;
 
 	spin_lock_init(&ei->lock);
 	ei->outstanding_extents = 0;