Re: [PATCH] xfs: recheck appropriateness of map_shared lock

[Christoph Hellwig <hch@infradead.org>]

On Wed, Jan 18, 2023 at 05:24:58PM -0800, Darrick J. Wong wrote:
>  	xfs_ilock(ip, lock_mode);
> +
> +	/*
> +	 * It's possible that the unlocked access of the data fork to determine
> +	 * the lock mode could have raced with another thread that was failing
> +	 * to load the bmbt but hadn't yet torn down the iext tree.  Recheck
> +	 * the lock mode and upgrade to an exclusive lock if we need to.
> +	 */
> +	if (lock_mode == XFS_ILOCK_SHARED &&
> +	    xfs_need_iread_extents(&ip->i_df)) {

Eww.  I think the proper fix here is to make sure
xfs_need_iread_extents does not return false until we're actually
read the extents.  So I think we'll need a new inode flag
XFS_INEED_READ - gets set when reading inode in btree format,
and gets cleared at the very end of xfs_iread_extents once we know
the read succeeded.