Re: [PATCH 2/3] xfs: fix an AGI lock acquisition ordering problem in xrep_dinode_findmode

[Christoph Hellwig <hch@infradead.org>]

On Tue, Apr 02, 2024 at 10:18:31PM -0700, Darrick J. Wong wrote:
> From: Darrick J. Wong <djwong@kernel.org>
> 
> While reviewing the next patch which fixes an ABBA deadlock between the
> AGI and a directory ILOCK, someone asked a question about why we're
> holding the AGI in the first place.  The reason for that is to quiesce
> the inode structures for that AG while we do a repair.
> 
> I then realized that the xrep_dinode_findmode invokes xchk_iscan_iter,
> which walks the inobts (and hence the AGIs) to find all the inodes.
> This itself is also an ABBA vector, since the damaged inode could be in
> AG 5, which we hold while we scan AG 0 for directories.  5 -> 0 is not
> allowed.
> 
> To address this, modify the iscan to allow trylock of the AGI buffer
> using the flags argument to xfs_ialloc_read_agi that the previous patch
> added.

Well, I guess we need this as a quick fix, but any scheme based on
trylock and return is just fundamentally broken.

Same for the next one.