[MPTCP] [RFC PATCH 0/6] dd locking to PM APis, implement PM netlink

[Paolo Abeni <pabeni at redhat.com>]

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Sending as an early RFC to gather feedback, as this is proving to be more
intrusive that what I initially hoped for.

This implement the locking required by the PM to allow concurent subflow
accessing the msk PM data in a safe way. Such data is protected via a msk-level
spinlock. It relies on the Double checked locking to avoid contention in the
most common scenarion (no or very little PM "actions").

Overall the above adds quite a bit of code churn, is that a too premature
optimization? On the flip side, the whole PM processing should be quite
light on the CPU in most cases.

Additionally changes more than a bit the PM hooks/APIs, to be hopefully simpler
and avoiding duplicate code (no v4/v6 variant, work queue management done by the
core PM infra).

It also introduce the PM netlink - should replace the current basic ones.
Completely untested but please have a look at the specified netlink uAPI.

TODO:
 - implement U/S helper
 - self-tests
 - do we need NL dump operation to inspect the PM netlink status?

 include/uapi/linux/mptcp.h |   42 +++
 net/mptcp/Makefile         |    3 
 net/mptcp/options.c        |   59 ++---
 net/mptcp/pm.c             |  340 +++++++++++++----------------
 net/mptcp/pm_netlink.c     |  524 +++++++++++++++++++++++++++++++++++++++++++++
 net/mptcp/protocol.c       |    2 
 net/mptcp/protocol.h       |  110 +++++----
 net/mptcp/subflow.c        |   63 +++--
 8 files changed, 872 insertions(+), 271 deletions(-)