quota: simplify drop_dquot_ref()

As Honza said, remove_inode_dquot_ref() currently does not release the
last dquot reference but instead adds the dquot to tofree_head list. This
is because dqput() can sleep while dropping of the last dquot reference
(writing back the dquot and calling ->release_dquot()) and that must not
happen under dq_list_lock. Now that dqput() queues the final dquot cleanup
into a workqueue, remove_inode_dquot_ref() can call dqput() unconditionally
and we can significantly simplify it.

Here we open code the simplified code of remove_inode_dquot_ref() into
remove_dquot_ref() and remove the function put_dquot_list() which is no
longer used.

Signed-off-by: Baokun Li <libaokun1@huawei.com>
Signed-off-by: Jan Kara <jack@suse.cz>
Message-Id: <20230630110822.3881712-6-libaokun1@huawei.com>

Author: LiBaokun96

fs/quota/dquot.c

@@ -1072,59 +1072,7 @@ static int add_dquot_ref(struct super_block *sb, int type)
 	return err;
 }
 
-/*
- * Remove references to dquots from inode and add dquot to list for freeing
- * if we have the last reference to dquot
- */
-static void remove_inode_dquot_ref(struct inode *inode, int type,
-				   struct list_head *tofree_head)
-{
-	struct dquot **dquots = i_dquot(inode);
-	struct dquot *dquot = dquots[type];
-
-	if (!dquot)
-		return;
-
-	dquots[type] = NULL;
-	if (list_empty(&dquot->dq_free)) {
-		/*
-		 * The inode still has reference to dquot so it can't be in the
-		 * free list
-		 */
-		spin_lock(&dq_list_lock);
-		list_add(&dquot->dq_free, tofree_head);
-		spin_unlock(&dq_list_lock);
-	} else {
-		/*
-		 * Dquot is already in a list to put so we won't drop the last
-		 * reference here.
-		 */
-		dqput(dquot);
-	}
-}
-
-/*
- * Free list of dquots
- * Dquots are removed from inodes and no new references can be got so we are
- * the only ones holding reference
- */
-static void put_dquot_list(struct list_head *tofree_head)
-{
-	struct list_head *act_head;
-	struct dquot *dquot;
-
-	act_head = tofree_head->next;
-	while (act_head != tofree_head) {
-		dquot = list_entry(act_head, struct dquot, dq_free);
-		act_head = act_head->next;
-		/* Remove dquot from the list so we won't have problems... */
-		list_del_init(&dquot->dq_free);
-		dqput(dquot);
-	}
-}
-
-static void remove_dquot_ref(struct super_block *sb, int type,
-		struct list_head *tofree_head)
+static void remove_dquot_ref(struct super_block *sb, int type)
 {
 	struct inode *inode;
 #ifdef CONFIG_QUOTA_DEBUG
@@ -1141,11 +1089,16 @@ static void remove_dquot_ref(struct super_block *sb, int type,
 		 */
 		spin_lock(&dq_data_lock);
 		if (!IS_NOQUOTA(inode)) {
+			struct dquot **dquots = i_dquot(inode);
+			struct dquot *dquot = dquots[type];
+
 #ifdef CONFIG_QUOTA_DEBUG
 			if (unlikely(inode_get_rsv_space(inode) > 0))
 				reserved = 1;
 #endif
-			remove_inode_dquot_ref(inode, type, tofree_head);
+			dquots[type] = NULL;
+			if (dquot)
+				dqput(dquot);
 		}
 		spin_unlock(&dq_data_lock);
 	}
@@ -1162,13 +1115,8 @@ static void remove_dquot_ref(struct super_block *sb, int type,
 /* Gather all references from inodes and drop them */
 static void drop_dquot_ref(struct super_block *sb, int type)
 {
-	LIST_HEAD(tofree_head);
-
-	if (sb->dq_op) {
-		remove_dquot_ref(sb, type, &tofree_head);
-		synchronize_srcu(&dquot_srcu);
-		put_dquot_list(&tofree_head);
-	}
+	if (sb->dq_op)
+		remove_dquot_ref(sb, type);
 }
 
 static inline