Re: About the try to remove cross-release feature entirely by Ingo

[Byungchul Park]

On 1/1/2018 7:18 PM, Matthew Wilcox wrote:
> 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.

I also think it should be done ultimately. And I think it's very much
hard since it requires to change the dependency graph of lockdep but
anyway possible. It's up to lockdep maintainer's will though..

--
Thanks,
Byungchul