xfs: consume iodone callback items on buffers as they are processed

To allow buffer iodone callbacks to consume multiple items off the
callback list, first we need to convert the xfs_buf_do_callbacks()
to consume items and always pull the next item from the head of the
list.

The means the item list walk is never dependent on knowing the
next item on the list and hence allows callbacks to remove items
from the list as well. This allows callbacks to do bulk operations
by scanning the list for identical callbacks, consuming them all
and then processing them in bulk, negating the need for multiple
callbacks of that type.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>

fs/xfs/xfs_buf_item.c

@@ -142,7 +142,7 @@ xfs_buf_item_log_check(
 #endif
 
 STATIC void	xfs_buf_error_relse(xfs_buf_t *bp);
-STATIC void	xfs_buf_do_callbacks(xfs_buf_t *bp, xfs_log_item_t *lip);
+STATIC void	xfs_buf_do_callbacks(struct xfs_buf *bp);
 
 /*
  * This returns the number of log iovecs needed to log the
@@ -450,7 +450,7 @@ xfs_buf_item_unpin(
 		 * xfs_trans_ail_delete() drops the AIL lock.
 		 */
 		if (bip->bli_flags & XFS_BLI_STALE_INODE) {
-			xfs_buf_do_callbacks(bp, (xfs_log_item_t *)bip);
+			xfs_buf_do_callbacks(bp);
 			XFS_BUF_SET_FSPRIVATE(bp, NULL);
 			XFS_BUF_CLR_IODONE_FUNC(bp);
 		} else {
@@ -918,15 +918,26 @@ xfs_buf_attach_iodone(
 	XFS_BUF_SET_IODONE_FUNC(bp, xfs_buf_iodone_callbacks);
 }
 
+/*
+ * We can have many callbacks on a buffer. Running the callbacks individually
+ * can cause a lot of contention on the AIL lock, so we allow for a single
+ * callback to be able to scan the remaining lip->li_bio_list for other items
+ * of the same type and callback to be processed in the first call.
+ *
+ * As a result, the loop walking the callback list below will also modify the
+ * list. it removes the first item from the list and then runs the callback.
+ * The loop then restarts from the new head of the list. This allows the
+ * callback to scan and modify the list attached to the buffer and we don't
+ * have to care about maintaining a next item pointer.
+ */
 STATIC void
 xfs_buf_do_callbacks(
-	xfs_buf_t	*bp,
-	xfs_log_item_t	*lip)
+	struct xfs_buf		*bp)
 {
-	xfs_log_item_t	*nlip;
+	struct xfs_log_item	*lip;
 
-	while (lip != NULL) {
-		nlip = lip->li_bio_list;
+	while ((lip = XFS_BUF_FSPRIVATE(bp, xfs_log_item_t *)) != NULL) {
+		XFS_BUF_SET_FSPRIVATE(bp, lip->li_bio_list);
 		ASSERT(lip->li_cb != NULL);
 		/*
 		 * Clear the next pointer so we don't have any
@@ -936,7 +947,6 @@ xfs_buf_do_callbacks(
 		 */
 		lip->li_bio_list = NULL;
 		lip->li_cb(bp, lip);
-		lip = nlip;
 	}
 }
 
@@ -970,7 +980,7 @@ xfs_buf_iodone_callbacks(
 			ASSERT(XFS_BUF_TARGET(bp) == mp->m_ddev_targp);
 			XFS_BUF_SUPER_STALE(bp);
 			trace_xfs_buf_item_iodone(bp, _RET_IP_);
-			xfs_buf_do_callbacks(bp, lip);
+			xfs_buf_do_callbacks(bp);
 			XFS_BUF_SET_FSPRIVATE(bp, NULL);
 			XFS_BUF_CLR_IODONE_FUNC(bp);
 			xfs_buf_ioend(bp, 0);
@@ -1029,7 +1039,7 @@ xfs_buf_iodone_callbacks(
 		return;
 	}
 
-	xfs_buf_do_callbacks(bp, lip);
+	xfs_buf_do_callbacks(bp);
 	XFS_BUF_SET_FSPRIVATE(bp, NULL);
 	XFS_BUF_CLR_IODONE_FUNC(bp);
 	xfs_buf_ioend(bp, 0);
@@ -1063,7 +1073,7 @@ xfs_buf_error_relse(
 	 * We have to unpin the pinned buffers so do the
 	 * callbacks.
 	 */
-	xfs_buf_do_callbacks(bp, lip);
+	xfs_buf_do_callbacks(bp);
 	XFS_BUF_SET_FSPRIVATE(bp, NULL);
 	XFS_BUF_CLR_IODONE_FUNC(bp);
 	XFS_BUF_SET_BRELSE_FUNC(bp,NULL);