[PATCH] wifi: brcmfmac: fix lost 802.1x TX completion wakeup

[Karl Mehltretter]

brcmf_txfinalize() decrements pend_8021x_cnt before a lockless
waitqueue_active() check. atomic_dec() does not order the decrement
against the check.

The waiter can therefore observe a nonzero count while the waker observes
an empty queue, losing the final wakeup and delaying key installation
until the 950 ms timeout.

Add smp_mb__after_atomic() to order the decrement before the queue
check. wait_event_timeout() provides the matching barrier. LKMM confirms
that this forbids the lost-wakeup outcome.

Fixes: 21fff75d2fb6 ("brcmfmac: use wait_event_timeout for 8021x pending count")
Assisted-by: Claude:claude-fable-5
Signed-off-by: Karl Mehltretter <kmehltretter@xxxxxxxxx>
---
LKMM results with herdtools7 7.58:

  C brcmf-wakeup-buggy
  { atomic_t cnt = ATOMIC_INIT(1); }
  P0(atomic_t *cnt, int *wq)
  {
  	int r0;
  	atomic_dec(cnt);
  	r0 = READ_ONCE(*wq);
  }
  P1(atomic_t *cnt, int *wq)
  {
  	int r1;
  	WRITE_ONCE(*wq, 1);
  	smp_mb();
  	r1 = atomic_read(cnt);
  }
  exists (0:r0=0 /\ 1:r1=1)

The fixed variant adds smp_mb__after_atomic() immediately after
atomic_dec().

  Observation brcmf-wakeup-buggy Sometimes 1 3
  Observation brcmf-wakeup-fixed Never 0 3

 drivers/net/wireless/broadcom/brcm80211/brcmfmac/core.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/drivers/net/wireless/broadcom/brcm80211/brcmfmac/core.c b/drivers/net/wireless/broadcom/brcm80211/brcmfmac/core.c
index dad6f4563d146..b192947ab4735 100644
--- a/drivers/net/wireless/broadcom/brcm80211/brcmfmac/core.c
+++ b/drivers/net/wireless/broadcom/brcm80211/brcmfmac/core.c
@@ -555,6 +555,8 @@ void brcmf_txfinalize(struct brcmf_if *ifp, struct sk_buff *txp, bool success)

 	if (type == ETH_P_PAE) {
 		atomic_dec(&ifp->pend_8021x_cnt);
+		/* Order the decrement before waitqueue_active() */
+		smp_mb__after_atomic();
 		if (waitqueue_active(&ifp->pend_8021x_wait))
 			wake_up(&ifp->pend_8021x_wait);
 	}
--
2.53.0