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The network thread waited on the timer and nothing else, so a frame that landed just after a poll sat in the ring for up to 10 ms -- the largest fixed latency in the system. That was deliberate when the stack was new (a receive path woken by an interrupt is much harder to debug than one that is asked) and there is now a working stack to compare against, which is exactly when it becomes safe to do. The frame is still *pulled* by the thread through .receive. It has to be: a call into lwIP can reach the driver, the driver can sleep on the buffer cache, and an interrupt handler may do neither. netdev_rx_signal() delivers nothing and reads no ring -- it only ends the wait early. The wait stays on the timer tick rather than moving to a channel of its own, and that is the design. lwIP's timers must still run when the link is silent, so the clock has to remain a floor under the wait, and sleep() cannot wait on two things at once. Joining the tick's existing wakeup costs nothing and leaves one wait with one predicate. rx_signal lives under tickslock for the same reason it gets no lock of its own: sleep() releases tickslock atomically, so a driver setting the flag and waking under that lock cannot lose the wakeup by arriving between the predicate test and the sleep. A private lock would not be held across the sleep, and that window would be back. Calling it from interrupt context is safe on the same terms the timer handler already relies on -- acquire() disables interrupts, so a CPU holding tickslock cannot be interrupted into it. The e1000 signals on RXT0 (a frame or burst has landed) and RXDMT0 (the ring has fallen below its threshold, which is when waiting for the clock hurts most). Measured rather than asserted: /proc/netif reports wake_rx and wake_tick, and across three pings rx_frames and wake_rx both went 2 -> 6 -- every received frame woke the thread through the interrupt. wake_tick keeps advancing as the idle floor, which is what lwIP's timers need. socktest still passes and ping is still 0% loss. Co-Authored-By: Claude Opus 5 <noreply@anthropic.com> Claude-Session: https://claude.ai/code/session_01VQ7Qx8E1q1M1bbCq8QLHS2 |
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