[PATCH 0/1] wintun driver get stalled for few sec until another packet is being sent -- approach 2

[odedkatz <katz.oded@gmail.com>]

## Problem
High-speed TCP uploads stall for 4-5 seconds when the sender and receiver are close (~1ms RTT). The root cause is a missed-wakeup race in the Receive ring's Alertable/Tail signaling protocol between the userspace API (WintunSendPacket) and the kernel0 consumer thread (TunProcessReceiveData).

On x86-64, WriteRelease/ReadAcquire provide acquire-release semantics but do not emit MFENCE — the only instruction that prevents store-load reordering (the one reordering x86 permits). Both cores can simultaneously observe stale values of the other's store, causing the userspace to skip SetEvent while the driver enters KeWaitForMultipleObjects indefinitely.

The driver sleeps until a TCP retransmission (RTO ~1s + exponential backoff) coincidentally wins the race, producing the observed 4-5 second stalls.

## Fix
Insert MemoryBarrier() between each store-load pair on both sides. This guarantees that at least one side always sees the other's store, eliminating the missed wakeup. The Alertable optimization (avoiding SetEvent syscalls when the driver is actively spinning) is fully preserved.
Alexey Lapuka (1):
  Fix missed-wakeup race in ring buffer Alertable signaling

 api/session.c   | 1 +
 driver/wintun.c | 1 +
 2 files changed, 2 insertions(+)

-- 
2.43.0