xfs: convert AIL cursors to use struct list_head

The list of active AIL cursors uses a roll-your-own linked list with
special casing for the AIL push cursor. Simplify this code by
replacing the list with standard struct list_head lists, and use a
separate list_head to track the active cursors. This allows us to
treat the AIL push cursor as a generic cursor rather than as a
special case, further simplifying the code.

Further, fix the duplicate push cursor initialisation that the
special case handling was hiding, and clean up all the comments
around the active cursor list handling.
Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Alex Elder <aelder@sgi.com>

fs/xfs/xfs_trans_ail.c

@@ -163,17 +163,11 @@ xfs_ail_max_lsn(
 }
 
 /*
- * AIL traversal cursor initialisation.
- *
- * The cursor keeps track of where our current traversal is up
- * to by tracking the next ƣtem in the list for us. However, for
- * this to be safe, removing an object from the AIL needs to invalidate
- * any cursor that points to it. hence the traversal cursor needs to
- * be linked to the struct xfs_ail so that deletion can search all the
- * active cursors for invalidation.
- *
- * We don't link the push cursor because it is embedded in the struct
- * xfs_ail and hence easily findable.
+ * The cursor keeps track of where our current traversal is up to by tracking
+ * the next item in the list for us. However, for this to be safe, removing an
+ * object from the AIL needs to invalidate any cursor that points to it. hence
+ * the traversal cursor needs to be linked to the struct xfs_ail so that
+ * deletion can search all the active cursors for invalidation.
  */
 STATIC void
 xfs_trans_ail_cursor_init(
@@ -181,17 +175,12 @@ xfs_trans_ail_cursor_init(
 	struct xfs_ail_cursor	*cur)
 {
 	cur->item = NULL;
-	if (cur == &ailp->xa_cursors)
-		return;
-
-	cur->next = ailp->xa_cursors.next;
-	ailp->xa_cursors.next = cur;
+	list_add_tail(&cur->list, &ailp->xa_cursors);
 }
 
 /*
- * Get the next item in the traversal and advance the cursor.
- * If the cursor was invalidated (inidicated by a lip of 1),
- * restart the traversal.
+ * Get the next item in the traversal and advance the cursor.  If the cursor
+ * was invalidated (indicated by a lip of 1), restart the traversal.
  */
 struct xfs_log_item *
 xfs_trans_ail_cursor_next(
@@ -208,40 +197,25 @@ xfs_trans_ail_cursor_next(
 }
 
 /*
- * Now that the traversal is complete, we need to remove the cursor
- * from the list of traversing cursors. Avoid removing the embedded
- * push cursor, but use the fact it is always present to make the
- * list deletion simple.
+ * When the traversal is complete, we need to remove the cursor from the list
+ * of traversing cursors.
  */
 void
 xfs_trans_ail_cursor_done(
 	struct xfs_ail		*ailp,
-	struct xfs_ail_cursor	*done)
+	struct xfs_ail_cursor	*cur)
 {
-	struct xfs_ail_cursor	*prev = NULL;
-	struct xfs_ail_cursor	*cur;
-
-	done->item = NULL;
-	if (done == &ailp->xa_cursors)
-		return;
-	prev = &ailp->xa_cursors;
-	for (cur = prev->next; cur; prev = cur, cur = prev->next) {
-		if (cur == done) {
-			prev->next = cur->next;
-			break;
-		}
-	}
-	ASSERT(cur);
+	cur->item = NULL;
+	list_del_init(&cur->list);
 }
 
 /*
- * Invalidate any cursor that is pointing to this item. This is
- * called when an item is removed from the AIL. Any cursor pointing
- * to this object is now invalid and the traversal needs to be
- * terminated so it doesn't reference a freed object. We set the
- * cursor item to a value of 1 so we can distinguish between an
- * invalidation and the end of the list when getting the next item
- * from the cursor.
+ * Invalidate any cursor that is pointing to this item. This is called when an
+ * item is removed from the AIL. Any cursor pointing to this object is now
+ * invalid and the traversal needs to be terminated so it doesn't reference a
+ * freed object. We set the low bit of the cursor item pointer so we can
+ * distinguish between an invalidation and the end of the list when getting the
+ * next item from the cursor.
  */
 STATIC void
 xfs_trans_ail_cursor_clear(
@@ -250,8 +224,7 @@ xfs_trans_ail_cursor_clear(
 {
 	struct xfs_ail_cursor	*cur;
 
-	/* need to search all cursors */
-	for (cur = &ailp->xa_cursors; cur; cur = cur->next) {
+	list_for_each_entry(cur, &ailp->xa_cursors, list) {
 		if (cur->item == lip)
 			cur->item = (struct xfs_log_item *)
 					((__psint_t)cur->item | 1);
@@ -405,7 +378,7 @@ xfs_ail_worker(
 	struct xfs_ail		*ailp = container_of(to_delayed_work(work),
 					struct xfs_ail, xa_work);
 	xfs_mount_t		*mp = ailp->xa_mount;
-	struct xfs_ail_cursor	*cur = &ailp->xa_cursors;
+	struct xfs_ail_cursor	cur;
 	xfs_log_item_t		*lip;
 	xfs_lsn_t		lsn;
 	xfs_lsn_t		target;
@@ -417,13 +390,12 @@ xfs_ail_worker(
 
 	spin_lock(&ailp->xa_lock);
 	target = ailp->xa_target;
-	xfs_trans_ail_cursor_init(ailp, cur);
-	lip = xfs_trans_ail_cursor_first(ailp, cur, ailp->xa_last_pushed_lsn);
+	lip = xfs_trans_ail_cursor_first(ailp, &cur, ailp->xa_last_pushed_lsn);
 	if (!lip || XFS_FORCED_SHUTDOWN(mp)) {
 		/*
 		 * AIL is empty or our push has reached the end.
 		 */
-		xfs_trans_ail_cursor_done(ailp, cur);
+		xfs_trans_ail_cursor_done(ailp, &cur);
 		spin_unlock(&ailp->xa_lock);
 		goto out_done;
 	}
@@ -511,12 +483,12 @@ xfs_ail_worker(
 		if (stuck > 100)
 			break;
 
-		lip = xfs_trans_ail_cursor_next(ailp, cur);
+		lip = xfs_trans_ail_cursor_next(ailp, &cur);
 		if (lip == NULL)
 			break;
 		lsn = lip->li_lsn;
 	}
-	xfs_trans_ail_cursor_done(ailp, cur);
+	xfs_trans_ail_cursor_done(ailp, &cur);
 	spin_unlock(&ailp->xa_lock);
 
 	if (flush_log) {
@@ -848,6 +820,7 @@ xfs_trans_ail_init(
 
 	ailp->xa_mount = mp;
 	INIT_LIST_HEAD(&ailp->xa_ail);
+	INIT_LIST_HEAD(&ailp->xa_cursors);
 	spin_lock_init(&ailp->xa_lock);
 	INIT_DELAYED_WORK(&ailp->xa_work, xfs_ail_worker);
 	mp->m_ail = ailp;

fs/xfs/xfs_trans_priv.h

@@ -53,7 +53,7 @@ void	xfs_trans_committed_bulk(struct xfs_ail *ailp, struct xfs_log_vec *lv,
  * of the list to trigger traversal restarts.
  */
 struct xfs_ail_cursor {
-	struct xfs_ail_cursor	*next;
+	struct list_head	list;
 	struct xfs_log_item	*item;
 };
 
@@ -66,7 +66,7 @@ struct xfs_ail {
 	struct xfs_mount	*xa_mount;
 	struct list_head	xa_ail;
 	xfs_lsn_t		xa_target;
-	struct xfs_ail_cursor	xa_cursors;
+	struct list_head	xa_cursors;
 	spinlock_t		xa_lock;
 	struct delayed_work	xa_work;
 	xfs_lsn_t		xa_last_pushed_lsn;