On Thu, Jul 11, 2013 at 10:04:06AM -0400, Jeff Layton wrote: > On Wed, 10 Jul 2013 17:26:21 -0400 > "J. Bruce Fields" <bfields@xxxxxxxxxx> wrote: > > > On Wed, Jul 10, 2013 at 01:38:53PM +1000, Dave Chinner wrote: > > > On Tue, Jul 09, 2013 at 10:40:59PM -0400, J. Bruce Fields wrote: > > > > On Wed, Jul 10, 2013 at 12:09:21PM +1000, Dave Chinner wrote: > > > > > Sure. I'd prefer ordering by inode number, because then ordering is > > > > > deterministic rather than being dependent on memory allocation > > > > > results. It makes forensic analysis of deadlocks and corruptions > > > > > easier because you can look at on-disk structures and accurately > > > > > predict locking behaviour and therefore determine the order of > > > > > operations that should occur. With lock ordering determined by > > > > > memory addresses, you can't easily predict the lock ordering two > > > > > particular inodes might take from one operation to another. > > > > > > > > Hm, OK, not having done this I don't have a good feeling for how > > > > important that is, but I can take your word for it. > > > > > > > > But the ext4 code actually originally used i_ino order and was changed > > > > by 03bd8b9b896c8e "ext4: move_extent code cleanup", possibly on Linus's > > > > suggestion?: > > > > > > > > http://mid.gmane.org/<CA+55aFwdh_QWG-R2FQ71kDXiNYZ04qPANBsY_PssVUwEBH4uSw@xxxxxxxxxxxxxx> > > > > > > > > "And the only sane order is comparing inode pointers, not inode > > > > numbers like ext4 apparently does." > > > > > > Interesting. What has worked for the last 20 years must be wrong if > > > Linus says so ;) > > > > > > > > > > > (Uh, I thought I also remembered some rationale but can't dig up the > > > > email now.) > > > > > > Probably duplicate inode numbers on inodes in different filesystems. > > > But rename doesn't allow that, and I don't we ever want to allow > > > arbitrary nested inode locking across superblocks. Hence I can't > > > think of a reason why it's a problem... > > > > I have some vague memory the argument was rather that inode numbers > > could fail to be unique within a fs due to bugs, but I may be making > > that up. I've got no strong opinion here. > > > > There are also legitimate cases where inode numbers can collide, > particularly on network filesystems. That's one of the main reasons we > have iget5_locked(). > > One possibility might be to order by i_ino first, and then fall back to > using the inode pointer value if they are equal. As long as no one ever modifies i_ino. Which I'd think would be a shooting offense. But it sure looks like fuse allows this--see fuse_do_getattr->fuse_change_attributes->fuse_change_attributes_common. Maybe I'm misunderstanding.... As long as there's a chance filesystems (even if only due to bugs) could mess with this sort of guarantee I'm really inclined to stick with the obviously-well-defined pointer ordering even if it means giving up the determinism Dave wants. Argh. > > > FWIW - gfs2 does multiple glock locking similar to XFS inode locking > > > - it sorts the locks in lock number order and the locks them all one > > > at a time... Taking a look--I don't think I'm going to begin to understand how that's used in any reasonable amount of time. Cc'ing Steve in case he can. > > > A quick grep shows lock_2_inodes() in fs/ubifs/dir.c. I don't see > > > any other obvious ones. Which isn't bothering with consistent lock ordering because (says a comment) its only called after taking the vfs locks. Which looks correct--the only callers are in link, unlink, and rmdir methods. And a similar lock_3_inodes is called from the rename method. --b. -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
