From: Chenguang Zhao <zhaochenguang@kylinos.cn>

When a valid MP_FAIL is received and infinite fallback is still allowed
(single contiguous subflow), RFC8684 section 3.7 requires leaving MPTCP
mode. Today the stack only clears allow_subflows and defers the real
fallback to the later infinite-map transmit path. If no data is sent in
between, the peer can still complete the 4th ACK as MPTCP and keep using
MPTCP options.

This series falls back immediately after the MP_FAIL response, rejects
later joins via mptcp_is_fully_established(), annotates allow_subflows
with READ/WRITE_ONCE, and adds an MPFailFallback MIB while switching to
mptcp_try_fallback().

Patch 1 fixes the fallback behaviour.
Patch 2 annotates allow_subflows accesses.
Patch 3 adds the dedicated MIB and uses the common fallback helper.

v1 -> v2:
- Split the previous single patch as suggested by Gang Yan
- Keep the MIB as a separate patch so the functional fix can be reviewed
  on its own

Chenguang Zhao (3):
  mptcp: fallback to TCP on MP_FAIL with a single subflow
  mptcp: use READ/WRITE_ONCE for allow_subflows
  mptcp: add MPFailFallback MIB for MP_FAIL TCP fallback

 net/mptcp/mib.c      |  1 +
 net/mptcp/mib.h      |  1 +
 net/mptcp/pm.c       | 33 +++++++++++++++++++++++++++++++--
 net/mptcp/protocol.c | 19 ++++++++++++-------
 net/mptcp/protocol.h | 18 +++++++++++-------
 net/mptcp/subflow.c  |  2 +-
 6 files changed, 57 insertions(+), 17 deletions(-)

---
v1:
 https://lore.kernel.org/all/20260713064134.914507-1-chenguang.zhao@linux.dev/
-- 
2.25.1