xfs: aborted buf items can be in the AIL.

Saw this on generic/270 after a DQALLOC transaction overrun
shutdown:

XFS: Assertion failed: !(bip->bli_item.li_flags & XFS_LI_IN_AIL), file: fs/xfs/xfs_buf_item.c, line: 952
.....
 xfs_buf_item_relse+0x4f/0xd0
 xfs_buf_item_unlock+0x1b4/0x1e0
 xfs_trans_free_items+0x7d/0xb0
 xfs_trans_cancel+0x13c/0x1b0
 xfs_symlink+0x37e/0xa60
....

When a transaction abort occured.

If we are aborting a transaction and trigger this code path, then
the item may be dirty. If the item is dirty, then it may be in the
AIL. Hence if we are aborting, we need to check if the item is in
the AIL and remove it before freeing it.
Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Mark Tinguely <tinguely@sgi.com>
Signed-off-by: Ben Myers <bpm@sgi.com>

fs/xfs/xfs_buf_item.c

@@ -613,13 +613,27 @@ xfs_buf_item_unlock(
 			}
 		}
 	}
-	if (clean || aborted) {
+
+	/*
+	 * Clean buffers, by definition, cannot be in the AIL. However, aborted
+	 * buffers may be dirty and hence in the AIL. Therefore if we are
+	 * aborting a buffer and we've just taken the last refernce away, we
+	 * have to check if it is in the AIL before freeing it. We need to free
+	 * it in this case, because an aborted transaction has already shut the
+	 * filesystem down and this is the last chance we will have to do so.
+	 */
 	if (atomic_dec_and_test(&bip->bli_refcount)) {
-			ASSERT(!aborted || XFS_FORCED_SHUTDOWN(lip->li_mountp));
+		if (clean)
 			xfs_buf_item_relse(bp);
+		else if (aborted) {
+			ASSERT(XFS_FORCED_SHUTDOWN(lip->li_mountp));
+			if (lip->li_flags & XFS_LI_IN_AIL) {
+				xfs_trans_ail_delete(lip->li_ailp, lip,
+						     SHUTDOWN_LOG_IO_ERROR);
+			}
+			xfs_buf_item_relse(bp);
+		}
 	}
-	} else
-		atomic_dec(&bip->bli_refcount);
 
 	if (!(flags & XFS_BLI_HOLD))
 		xfs_buf_relse(bp);