xfs: fix stale inode flush avoidance

When reclaiming stale inodes, we need to guarantee that inodes are
unpinned before returning with a "clean" status. If we don't we can
reclaim inodes that are pinned, leading to use after free in the
transaction subsystem as transactions complete.
Signed-off-by: Dave Chinner <david@fromorbit.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Alex Elder <aelder@sgi.com>

fs/xfs/xfs_inode.c

@@ -2842,13 +2842,9 @@ xfs_iflush(
 
 	/*
 	 * If the inode isn't dirty, then just release the inode flush lock and
-	 * do nothing. Treat stale inodes the same; we cannot rely on the
-	 * backing buffer remaining stale in cache for the remaining life of
-	 * the stale inode and so xfs_itobp() below may give us a buffer that
-	 * no longer contains inodes below. Doing this stale check here also
-	 * avoids forcing the log on pinned, stale inodes.
+	 * do nothing.
 	 */
-	if (xfs_inode_clean(ip) || xfs_iflags_test(ip, XFS_ISTALE)) {
+	if (xfs_inode_clean(ip)) {
 		xfs_ifunlock(ip);
 		return 0;
 	}
@@ -2871,6 +2867,19 @@ xfs_iflush(
 	}
 	xfs_iunpin_wait(ip);
 
+	/*
+	 * For stale inodes we cannot rely on the backing buffer remaining
+	 * stale in cache for the remaining life of the stale inode and so
+	 * xfs_itobp() below may give us a buffer that no longer contains
+	 * inodes below. We have to check this after ensuring the inode is
+	 * unpinned so that it is safe to reclaim the stale inode after the
+	 * flush call.
+	 */
+	if (xfs_iflags_test(ip, XFS_ISTALE)) {
+		xfs_ifunlock(ip);
+		return 0;
+	}
+
 	/*
 	 * This may have been unpinned because the filesystem is shutting
 	 * down forcibly. If that's the case we must not write this inode