[PATCH] writeback livelock fix

To avoid various livelocks, the writeback code parks all the dirty inodes onto
sb->s_io and then works through that list until it is empty.  This assumes
that each inode will be moved to some other list as it is processed.

But there's a loophole: if the ->writepages() implementation does nothing at
all, the inode is not redirtied (which would move it to s_dirty).  This causes
s_io to not empty and pdflush goes nuts.

So when this happens, move the inode onto s_dirty within
__sync_single_inode().  Use list_move_tail() to attempt to preserve the
time-ordering of the s_dirty list.

fs/fs-writeback.c

@@ -170,17 +170,19 @@ __sync_single_inode(struct inode *inode, struct writeback_control *wbc)
 		if (!(inode->i_state & I_DIRTY) &&
 		    mapping_tagged(mapping, PAGECACHE_TAG_DIRTY)) {
 			/*
-			 * We didn't write back all the pages.  Redirty the
-			 * inode.  It is still on sb->s_dirty.
+			 * We didn't write back all the pages.  nfs_writepages()
+			 * sometimes bales out without doing anything. Redirty
+			 * the inode.  It is still on sb->s_io.
 			 */
 			if (wbc->for_kupdate) {
 				/*
 				 * For the kupdate function we leave the inode
-				 * where it is on sb_dirty so it will get more
+				 * at the head of sb_dirty so it will get more
 				 * writeout as soon as the queue becomes
 				 * uncongested.
 				 */
 				inode->i_state |= I_DIRTY_PAGES;
+				list_move_tail(&inode->i_list, &sb->s_dirty);
 			} else {
 				/*
 				 * Otherwise fully redirty the inode so that