On Tue, Oct 29, 2013 at 10:11:44PM +1100, Dave Chinner wrote:
> From: Dave Chinner <dchinner@xxxxxxxxxx>
>
> Removing an inode from the namespace involves removing the directory
> entry and dropping the link count on the inode. Removing the
> directory entry can result in locking an AGF (directory blocks were
> freed) and removing a link count can result in placing the inode on
> an unlinked list which results in locking an AGI.
>
> The big problem here is that we have an ordering constraint on AGF
> and AGI locking - inode allocation locks the AGI, then can allocate
> a new extent for new inodes, locking the AGF after the AGI.
> Similarly, freeing the inode removes the inode from the unlinked
> list, requiring that we lock the AGI first, and then freeing the
> inode can result in an inode chunk being freed and hence freeing
> disk space requiring that we lock an AGF.
>
> Hence the ordering that is imposed by other parts of the code is AGI
> before AGF. This means we cannot remove the directory entry before
> we drop the inode reference count and put it on the unlinked list as
> this results in a lock order of AGF then AGI, and this can deadlock
> against inode allocation and freeing. Therefore we must drop the
> link counts before we remove the directory entry.
>
> This is still safe from a transactional point of view - it is not
> until we get to xfs_bmap_finish() that we have the possibility of
> multiple transactions in this operation. Hence as long as we remove
> the directory entry and drop the link count in the first transaction
> of the remove operation, there are no transactional constraints on
> the ordering here.
>
> Change the ordering of the operations in the xfs_remove() function
> to align the ordering of AGI and AGF locking to match that of the
> rest of the code.
>
> Signed-off-by: Dave Chinner <dchinner@xxxxxxxxxx>
These two codepaths look plausible for the deadlock you described:
inode allocation locking:
xfs_create
xfs_dir_ialloc
xfs_ialloc
xfs_dialloc
xfs_ialloc_read_agi * takes agi
xfs_ialloc_ag_alloc
xfs_alloc_vextent
xfs_alloc_fix_freelist
xfs_alloc_read_agf * takes agf
vs
xfs_remove
xfs_dir_removename
xfs_dir2_node_removename
xfs_dir2_leafn_remove
xfs_dir2_shrink_inode
xfs_bunmapi
. xfs_bmap_del_extent
. xfs_btree_delete
. xfs_btree_delrec
. .free_block
. xfs_bmbt_free_block
. xfs_bmap_add_free * adds to free list, doesn't take agf
xfs_bmap_extents_to_btree
xfs_alloc_vextent * takes agf
xfs_droplink
xfs_iunlink
xfs_read_agi * takes agi
I was thinking I'd find something in .free_block, but I didn't. But it does
look like we'll take the agf if we have to convert between directory formats in
xfs_dir2_leafn_remove, and it looks like there are a few more opportunities to
take the agf in xfs_bunmapi...
Looks good.
Reviewed-by: Ben Myers <bpm@xxxxxxx>
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