mptcp: add accept_subflow re-check

The re-check of pm->accept_subflow with pm->lock held was missing, this
patch fixed it.

Suggested-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Reviewed-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: Geliang Tang <geliangtang@gmail.com>

net/mptcp/pm.c

@@ -48,7 +48,7 @@ void mptcp_pm_new_connection(struct mptcp_sock *msk, int server_side)
 bool mptcp_pm_allow_new_subflow(struct mptcp_sock *msk)
 {
  struct mptcp_pm_data *pm = &msk->pm;
- int ret;
+ int ret = 0;
 
  pr_debug("msk=%p subflows=%d max=%d allow=%d", msk, pm->subflows,
  pm->subflows_max, READ_ONCE(pm->accept_subflow));
@@ -58,9 +58,11 @@ bool mptcp_pm_allow_new_subflow(struct mptcp_sock *msk)
  return false;
 
  spin_lock_bh(&pm->lock);
- ret = pm->subflows < pm->subflows_max;
- if (ret && ++pm->subflows == pm->subflows_max)
- WRITE_ONCE(pm->accept_subflow, false);
+ if (READ_ONCE(pm->accept_subflow)) {
+ ret = pm->subflows < pm->subflows_max;
+ if (ret && ++pm->subflows == pm->subflows_max)
+ WRITE_ONCE(pm->accept_subflow, false);
+ }
  spin_unlock_bh(&pm->lock);
 
  return ret;