On Wed, Oct 30, 2013 at 05:39:04PM -0500, Ben Myers wrote: > 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 Yeah, that's not the obvious or common path, but it has the same cause of allocation - it's a bmbt block that gets allocated. i.e. removing a block from the middle of a contiguous extent can result in the extent tree growing, and hence needing allocation of block for the new entry. This is the path I was hitting: .... xfs_dir2_shrink_inode xfs_bunmapi xfs_bmap_del_extent case 0: /* delete middle of extent */ xfs_btree_update xfs_btree_increment xfs_btree_insert xfs_btree_insrec xfs_btree_make_block_unfull xfs_btree_split .alloc_block xfs_bmbt_alloc_block xfs_alloc_vextent * takes agf > I was thinking I'd find something in .free_block, but I didn't. Right, data extents are added to the free list that is later walked and freed via xfs_bmap_finish() after it adds an EFI to match the free list to the current transaction the free list belongs to and commits it. > 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... Yup, but with the above call chain, any random block removal can cause a bmbt allocation to occur, so we don't really need to look any further. Indeed, you should just assume that any call to xfs_bunmapi() to free an extent will require block allocation.... Cheers, Dave. -- Dave Chinner david@xxxxxxxxxxxxx _______________________________________________ xfs mailing list xfs@xxxxxxxxxxx http://oss.sgi.com/mailman/listinfo/xfs