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

[Dave Chinner <david@fromorbit.com>]

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
> 
> 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.

urk.

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. Have a look at what xfs_iflush_cluster() does - it
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.

Hence it looks to me that I_FLUSHING isn't the problem here - it's
that we have a transient modified inode state in xfs_reinit_inode()
that is externally visisble...

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com