From: Christian Taedcke macb_interrupt() defers TX completion handling to NAPI, but when it schedules the poll it only masks TCOMP, even though TXUBR is enabled alongside it (both are part of MACB_TX_INT_FLAGS). macb_tx_poll() is asymmetric in the same way and only re-enables TCOMP. TXUBR is thus left unmasked while responsibility for handling it has been deferred to NAPI. Unlike an edge event, TXUBR is a persistent condition: the controller keeps it asserted for as long as the transmitter reads a buffer descriptor whose used bit is set. Leaving a level-triggered source enabled while NAPI owns its processing means the interrupt refires immediately after the handler returns, before the poll has had a chance to clear the underlying condition. This turns into a hard interrupt storm that pegs a CPU in the (threaded) MAC IRQ handler and, on PREEMPT_RT, triggers RT throttling ("sched: RT throttling activated"), taking the network interface down. Several situations can keep the used-bit read asserted across a poll - for example unreaped completed descriptors still sitting at tx_tail, or a transmit restart racing with macb_start_xmit(). The specific trigger does not matter: as long as the source stays unmasked, any persistent assertion is enough to storm, so the interrupt handling itself must be made self-limiting. Mask TXUBR together with TCOMP in the IDR write when scheduling the TX NAPI, and re-enable both from the napi_complete path in macb_tx_poll(), making the TX interrupt mask/unmask symmetric and consistent with how the driver already treats every other NAPI-serviced source. The pending TXUBR is still recorded in queue->txubr_pending before masking and acted on by macb_tx_restart(), so no event is lost. A persistent TXUBR now degrades to NAPI-paced polling instead of a CPU-pegging hard interrupt storm. Fixes: 138badbc21a0 ("net: macb: use NAPI for TX completion path") Cc: stable@vger.kernel.org Assisted-by: Claude:claude-opus-4-8 Signed-off-by: Christian Taedcke --- drivers/net/ethernet/cadence/macb_main.c | 5 +++-- 1 file changed, 3 insertions(+), 2 deletions(-) diff --git a/drivers/net/ethernet/cadence/macb_main.c b/drivers/net/ethernet/cadence/macb_main.c index b11cb8f068b7..f75cf2ffdf6f 100644 --- a/drivers/net/ethernet/cadence/macb_main.c +++ b/drivers/net/ethernet/cadence/macb_main.c @@ -1971,7 +1971,7 @@ static int macb_tx_poll(struct napi_struct *napi, int budget) (unsigned int)(queue - bp->queues), work_done, budget); if (work_done < budget && napi_complete_done(napi, work_done)) { - queue_writel(queue, IER, MACB_BIT(TCOMP)); + queue_writel(queue, IER, MACB_BIT(TCOMP) | MACB_BIT(TXUBR)); /* Packet completions only seem to propagate to raise * interrupts when interrupts are enabled at the time, so if @@ -2161,7 +2161,8 @@ static irqreturn_t macb_interrupt(int irq, void *dev_id) if (status & (MACB_BIT(TCOMP) | MACB_BIT(TXUBR))) { - queue_writel(queue, IDR, MACB_BIT(TCOMP)); + queue_writel(queue, IDR, MACB_BIT(TCOMP) | + MACB_BIT(TXUBR)); macb_queue_isr_clear(bp, queue, MACB_BIT(TCOMP) | MACB_BIT(TXUBR)); if (status & MACB_BIT(TXUBR)) { -- 2.54.0