xfs: fix incorrect i_nlink caused by inode racing

The following error occurred during the fsstress test:

XFS: Assertion failed: VFS_I(ip)->i_nlink >= 2, file: fs/xfs/xfs_inode.c, line: 2452

The problem was that inode race condition causes incorrect i_nlink to be
written to disk, and then it is read into memory. Consider the following
call graph, inodes that are marked as both XFS_IFLUSHING and
XFS_IRECLAIMABLE, i_nlink will be reset to 1 and then restored to original
value in xfs_reinit_inode(). Therefore, the i_nlink of directory on disk
may be set to 1.

  xfsaild
      xfs_inode_item_push
          xfs_iflush_cluster
              xfs_iflush
                  xfs_inode_to_disk

  xfs_iget
      xfs_iget_cache_hit
          xfs_iget_recycle
              xfs_reinit_inode
                  inode_init_always

xfs_reinit_inode() needs to hold the ILOCK_EXCL as it is changing internal
inode state and can race with other RCU protected inode lookups. On the
read side, xfs_iflush_cluster() grabs the ILOCK_SHARED while under rcu +
ip->i_flags_lock, and so xfs_iflush/xfs_inode_to_disk() are protected from
racing inode updates (during transactions) by that lock.

Fixes: ff7bebeb ("xfs: refactor the inode recycling code") # goes further back than this
Signed-off-by: Long Li <leo.lilong@huawei.com>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Darrick J. Wong <djwong@kernel.org>

fs/xfs/xfs_icache.c

@@ -342,6 +342,9 @@ xfs_iget_recycle(
 
 	trace_xfs_iget_recycle(ip);
 
+	if (!xfs_ilock_nowait(ip, XFS_ILOCK_EXCL))
+		return -EAGAIN;
+
 	/*
 	 * We need to make it look like the inode is being reclaimed to prevent
 	 * the actual reclaim workers from stomping over us while we recycle
@@ -355,6 +358,7 @@ xfs_iget_recycle(
 
 	ASSERT(!rwsem_is_locked(&inode->i_rwsem));
 	error = xfs_reinit_inode(mp, inode);
+	xfs_iunlock(ip, XFS_ILOCK_EXCL);
 	if (error) {
 		/*
 		 * Re-initializing the inode failed, and we are in deep
@@ -518,6 +522,8 @@ xfs_iget_cache_hit(
 	if (ip->i_flags & XFS_IRECLAIMABLE) {
 		/* Drops i_flags_lock and RCU read lock. */
 		error = xfs_iget_recycle(pag, ip);
+		if (error == -EAGAIN)
+			goto out_skip;
 		if (error)
 			return error;
 	} else {