Re: [PATCH 0/7] Eliminate delegation self-conflicts

Jeff Layton
2019-02-09T12:44:03Z

On Fri, 2019-02-08 at 15:10 -0500, J. Bruce Fields wrote:
> From: "J. Bruce Fields" <bfields@redhat.com>
>
> These patches allow NFSv4 clients holding delegations to keep them when
> the operation that would break a delegation comes from the same client.
>
> To do that, we somehow need to pass the identity of the
> delegation-breaker down through the VFS.
>
> This series uses the tgid, a solution suggested by Trond. To do that we
> need nfsd tasks to share the same tgid. I do that by extending the
> kthread code slightly to allow knfsd to run the kthreadd main loop in a
> task of its own, and spawn its server threads off of that task.
>
> Part of Trond's thinking was that this would work for userspace too.
> Delegations are currently only available to knfsd, but Ganesha and Samba
> may eventually be interested in a userspace interface (probably a minor
> variation on the fcntl F_{GET,SET}LEASE interface). A threaded
> userspace server would first resolve conflicts between its own clients,
> and then call into the kernel to break any leases acquired by other
> processes. That may require some careful locking of the server's own
> data structures, but it should work.
>
> Previously I considered instead adding a new field somewhere in the
> struct task. That might require a new system call to expose to user
> space. Or we might be able to put this in a keyring, if David Howells
> thought that would work.
>
> Before that I tried passing the identity of the breaker explicitly, but
> that looks like it would require passing the new argument around to huge
> swaths of the VFS.
>
> I'm testing this with some a locally modified pynfs; I'll fix that up
> and push it out at some point, but pynfs has a number of bugs in this
> area.
>
> I wasn't sure who to ask about the kthread.c changes, so I'm cc'ing a
> random assortment of developers in recent changelogs, hope that's OK.
>
> --b.
>
> J. Bruce Fields (7):
> kthreads: minor kthreadd refactoring
> kthreads: Simplify tsk_fork_get_node
> kthreads: allow multiple kthreadd's
> kthreads: allow cloning threads with different flags
> rpc: separate out body of svc_start_kthreads
> rpc: move rpc server threads into their own thread group
> nfsd: ignore delegation self-conflicts
>
> fs/locks.c | 39 +++++++++++
> fs/nfsd/nfs4state.c | 61 ++++++++++++++++
> fs/nfsd/state.h | 2 +
> fs/nfsd/vfs.c | 32 +++++++--
> include/linux/fs.h | 2 +
> include/linux/kthread.h | 21 +++++-
> include/linux/sunrpc/svc.h | 1 +
> init/init_task.c | 3 +
> init/main.c | 4 +-
> kernel/fork.c | 4 ++
> kernel/kthread.c | 140 +++++++++++++++++++++++++++----------
> net/sunrpc/svc.c | 83 ++++++++++++++--------
> 12 files changed, 317 insertions(+), 75 deletions(-)
>

Nice work! I like the basic idea, the changes seem to be well-organized,
and the tgid semantics are clear and make sense.

Would this preclude us from moving to a workqueue-based model for knfsd
later? It's likely to still be worth it, but it'd be good to understand
the potential drawbacks.

Thanks,
--
Jeff Layton <jlayton@redhat.com>