[XFS] Use a cursor for AIL traversal.

To replace the current generation number ensuring sanity of the AIL
traversal, replace it with an external cursor that is linked to the AIL.

Basically, we store the next item in the cursor whenever we want to drop
the AIL lock to do something to the current item. When we regain the lock.
the current item may already be free, so we can't reference it, but the
next item in the traversal is already held in the cursor.

When we move or delete an object, we search all the active cursors and if
there is an item match we clear the cursor(s) that point to the object.
This forces the traversal to restart transparently.

We don't invalidate the cursor on insert because the cursor still points
to a valid item. If the intem is inserted between the current item and the
cursor it does not matter; the traversal is considered to be past the
insertion point so it will be picked up in the next traversal.

Hence traversal restarts pretty much disappear altogether with this method
of traversal, which should substantially reduce the overhead of pushing on
a busy AIL.

Version 2 o add restart logic o comment cursor interface o minor cleanups

SGI-PV: 988143

SGI-Modid: xfs-linux-melb:xfs-kern:32347a

Signed-off-by: David Chinner <david@fromorbit.com>
Signed-off-by: Lachlan McIlroy <lachlan@sgi.com>
Signed-off-by: Christoph Hellwig <hch@infradead.org>

1 files changed, 44 insertions, 11 deletions

diff --git a/fs/xfs/xfs_trans_priv.h b/fs/xfs/xfs_trans_priv.h
index 98317fdc33b..f114d388570 100644
--- a/fs/xfs/xfs_trans_priv.h
+++ b/fs/xfs/xfs_trans_priv.h
@@ -44,20 +44,33 @@ xfs_log_busy_slot_t		*xfs_trans_add_busy(xfs_trans_t *tp,
 						    xfs_extlen_t idx);
 
 /*
- * From xfs_trans_ail.c
+ * AIL traversal cursor.
+ *
+ * Rather than using a generation number for detecting changes in the ail, use
+ * a cursor that is protected by the ail lock. The aild cursor exists in the
+ * struct xfs_ail, but other traversals can declare it on the stack and link it
+ * to the ail list.
+ *
+ * When an object is deleted from or moved int the AIL, the cursor list is
+ * searched to see if the object is a designated cursor item. If it is, it is
+ * deleted from the cursor so that the next time the cursor is used traversal
+ * will return to the start.
+ *
+ * This means a traversal colliding with a removal will cause a restart of the
+ * list scan, rather than any insertion or deletion anywhere in the list. The
+ * low bit of the item pointer is set if the cursor has been invalidated so
+ * that we can tell the difference between invalidation and reaching the end
+ * of the list to trigger traversal restarts.
  */
-void			xfs_trans_update_ail(struct xfs_mount *mp,
-				     struct xfs_log_item *lip, xfs_lsn_t lsn)
-				     __releases(mp->m_ail_lock);
-void			xfs_trans_delete_ail(struct xfs_mount *mp,
-				     struct xfs_log_item *lip)
-				     __releases(mp->m_ail_lock);
-struct xfs_log_item	*xfs_trans_first_ail(struct xfs_mount *, int *);
-struct xfs_log_item	*xfs_trans_next_ail(struct xfs_mount *,
-				     struct xfs_log_item *, int *, int *);
+struct xfs_ail_cursor {
+	struct xfs_ail_cursor	*next;
+	struct xfs_log_item	*item;
+};
 
 /*
- * AIL push thread support
+ * Private AIL structures.
+ *
+ * Eventually we need to drive the locking in here as well.
  */
 struct xfs_ail {
 	struct xfs_mount	*xa_mount;
@@ -65,8 +78,28 @@ struct xfs_ail {
 	uint			xa_gen;
 	struct task_struct	*xa_task;
 	xfs_lsn_t		xa_target;
+	struct xfs_ail_cursor	xa_cursors;
 };
 
+/*
+ * From xfs_trans_ail.c
+ */
+void			xfs_trans_update_ail(struct xfs_mount *mp,
+				     struct xfs_log_item *lip, xfs_lsn_t lsn)
+				     __releases(mp->m_ail_lock);
+void			xfs_trans_delete_ail(struct xfs_mount *mp,
+				     struct xfs_log_item *lip)
+				     __releases(mp->m_ail_lock);
+struct xfs_log_item	*xfs_trans_first_ail(struct xfs_mount *mp,
+					struct xfs_ail_cursor *cur);
+struct xfs_log_item	*xfs_trans_next_ail(struct xfs_mount *mp,
+					struct xfs_ail_cursor *cur);
+
+void xfs_trans_ail_cursor_init(struct xfs_ail *ailp,
+					struct xfs_ail_cursor *cur);
+void xfs_trans_ail_cursor_done(struct xfs_ail *ailp,
+					struct xfs_ail_cursor *cur);
+
 long	xfsaild_push(struct xfs_ail *, xfs_lsn_t *);
 void	xfsaild_wakeup(struct xfs_ail *, xfs_lsn_t);
 int	xfsaild_start(struct xfs_ail *);