On Sat, Dec 30, 2017 at 06:00:57PM -0500, Theodore Ts'o wrote: > On Sat, Dec 30, 2017 at 05:40:28PM -0500, Theodore Ts'o wrote: > > On Sat, Dec 30, 2017 at 12:44:17PM -0800, Matthew Wilcox wrote: > > > > > > I'm not sure I agree with this part. What if we add a new TCP lock class > > > for connections which are used for filesystems/network block devices/...? > > > Yes, it'll be up to each user to set the lockdep classification correctly, > > > but that's a relatively small number of places to add annotations, > > > and I don't see why it wouldn't work. > > > > I was exagerrating a bit for effect, I admit. (but only a bit). I feel like there's been rather too much of that recently. Can we stick to facts as far as possible, please? > > It can probably be for all TCP connections that are used by kernel > > code (as opposed to userspace-only TCP connections). But it would > > probably have to be each and every device-mapper instance, each and > > every block device, each and every mounted file system, each and every > > bdi object, etc. > > Clarification: all TCP connections that are used by kernel code would > need to be in their own separate lock class. All TCP connections used > only by userspace could be in their own shared lock class. You can't > use a one lock class for all kernel-used TCP connections, because of > the Network Block Device mounted on a local file system which is then > exported via NFS and squirted out yet another TCP connection problem. So the false positive you're concerned about is write-comes-in-over-NFS (with socket lock held), NFS sends a write request to local filesystem, local filesystem sends write to block device, block device sends a packet to a socket which takes that socket lock. I don't think we need to be as drastic as giving each socket its own lock class to solve this. All NFS sockets can be in lock class A; all NBD sockets can be in lock class B; all user sockets can be in lock class C; etc. > Also, what to do with TCP connections which are created in userspace > (with some authentication exchanges happening in userspace), and then > passed into kernel space for use in kernel space, is an interesting > question. Yes! I'd love to have a lockdep expert weigh in here. I believe it's legitimate to change a lock's class after it's been used, essentially destroying it and reinitialising it. If not, it should be because it's a reasonable design for an object to need different lock classes for different phases of its existance. > So "all you have to do is classify the locks 'properly'" is much like > the apocrophal, "all you have to do is bell the cat"[1]. Or like the > saying, "colonizing the stars is *easy*; all you have to do is figure > out faster than light travel." This is only computer programming, not rocket surgery :-)
