Re: [PATCH] xfs: fix incorrect i_nlink caused by inode racing

["Darrick J. Wong" <djwong@kernel.org>]

On Mon, Nov 07, 2022 at 10:36:48PM +0800, Long Li wrote:
> The following error occurred during the fsstress test:

> XFS: Assertion failed: VFS_I(ip)->i_nlink >= 2, file: fs/xfs/xfs_inode.c, line: 2925

What kernel is this?  xfs_inode.c line 2925 is in the middle of
xfs_rename and doesn't have any assertions on nlink.

The only assertion on nlink in the entire xfs codebase is in xfs_remove,
and that's not what's going on here.

<confused>

--D

> 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
> 
> So skip inodes that being flushed and markded as XFS_IRECLAIMABLE, prevent
> concurrent read and write to inodes.
> 
> Signed-off-by: Long Li <leo.lilong@huawei.com>
> ---
>  fs/xfs/xfs_icache.c | 5 +++++
>  1 file changed, 5 insertions(+)
> 
> diff --git a/fs/xfs/xfs_icache.c b/fs/xfs/xfs_icache.c
> index eae7427062cf..cc68b0ff50ce 100644
> --- a/fs/xfs/xfs_icache.c
> +++ b/fs/xfs/xfs_icache.c
> @@ -514,6 +514,11 @@ xfs_iget_cache_hit(
>  	    (ip->i_flags & XFS_IRECLAIMABLE))
>  		goto out_skip;
>  
> +	/* Skip inodes that being flushed */
> +	if ((ip->i_flags & XFS_IFLUSHING) &&
> +	    (ip->i_flags & XFS_IRECLAIMABLE))
> +		goto out_skip;
> +
>  	/* The inode fits the selection criteria; process it. */
>  	if (ip->i_flags & XFS_IRECLAIMABLE) {
>  		/* Drops i_flags_lock and RCU read lock. */
> -- 
> 2.31.1
>