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lukaszraczylo
b26afa2a5d
Replaces snapshot-of-tx_tail comparison in the watchdog with a tx_stall_tail_moved flag set/cleared under tx_ptr_lock. Avoids the index-aliasing false-positive Phil Elwell flagged on the raspberrypi/linux PR review. Anchored against the rpi-6.18.y vendor fork so it applies cleanly on top of patches 0001..0003. See talos-kernel-patches/0004-*.patch for the mainline-anchored variant intended for netdev v2.
116 lines
5.0 KiB
Diff
116 lines
5.0 KiB
Diff
From 0000000000000000000000000000000000000000 Mon Sep 17 00:00:00 2001
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From: Lukasz Raczylo <lukasz@raczylo.com>
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Date: Sat, 2 May 2026 14:42:29 +0100
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Subject: [PATCH] net: macb: track TX-tail movement via boolean to avoid index
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aliasing
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Address review feedback from pelwell on raspberrypi/linux#7340.
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Per-queue ring indices are bounded and reused, so the previous
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snapshot-of-tx_tail comparison in macb_tx_stall_watchdog() can
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false-positive when the index lands on the same value between
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two ticks under sustained load.
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Replace the snapshot with a tx_stall_tail_moved boolean that
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macb_tx_complete() sets under tx_ptr_lock whenever tx_tail
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advances and the watchdog clears under the same lock at each
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tick. No atomic is required (single producer / single consumer
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already serialised by tx_ptr_lock).
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Initialise the flag to true in macb_open() so the first tick
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after open is a grace period rather than a synthetic stall.
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Anchored against the rpi-6.18.y vendor fork (which carries the
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extra `bool tx_pending` field on struct macb_queue) — the
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mainline-anchored variant lives at
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home-cluster/talos-kernel-patches/0004-*.patch.
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Link: https://github.com/raspberrypi/linux/pull/7340#pullrequestreview-4198291045
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Signed-off-by: Lukasz Raczylo <lukasz@raczylo.com>
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---
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drivers/net/ethernet/cadence/macb.h | 10 ++++++++--
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drivers/net/ethernet/cadence/macb_main.c | 23 +++++++++++++++--------
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2 files changed, 23 insertions(+), 10 deletions(-)
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diff --git a/drivers/net/ethernet/cadence/macb.h b/drivers/net/ethernet/cadence/macb.h
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index 827075e4b..bdbafe097 100644
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--- a/drivers/net/ethernet/cadence/macb.h
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+++ b/drivers/net/ethernet/cadence/macb.h
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@@ -1295,9 +1295,15 @@ struct macb_queue {
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bool txubr_pending;
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bool tx_pending;
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- /* TX stall watchdog -- see macb_tx_stall_watchdog() in macb_main.c */
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+ /* TX stall watchdog -- see macb_tx_stall_watchdog() in macb_main.c.
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+ * tx_stall_tail_moved is set by macb_tx_complete() under tx_ptr_lock
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+ * whenever tx_tail advances, and cleared by the watchdog tick on the
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+ * same lock. A bool avoids the index-aliasing false-positive that a
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+ * snapshot-of-tx_tail comparison would have when the ring index space
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+ * happens to wrap to the same value between two ticks.
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+ */
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struct delayed_work tx_stall_watchdog_work;
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- unsigned int tx_stall_last_tail;
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+ bool tx_stall_tail_moved;
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struct napi_struct napi_tx;
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diff --git a/drivers/net/ethernet/cadence/macb_main.c b/drivers/net/ethernet/cadence/macb_main.c
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index 77fbcbab8..423aac02a 100644
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--- a/drivers/net/ethernet/cadence/macb_main.c
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+++ b/drivers/net/ethernet/cadence/macb_main.c
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@@ -1505,6 +1505,8 @@ static int macb_tx_complete(struct macb_queue *queue, int budget)
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packets, bytes);
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queue->tx_tail = tail;
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+ if (packets)
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+ queue->tx_stall_tail_moved = true;
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if (__netif_subqueue_stopped(bp->dev, queue_index) &&
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CIRC_CNT(queue->tx_head, queue->tx_tail,
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bp->tx_ring_size) <= MACB_TX_WAKEUP_THRESH(bp))
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@@ -2036,9 +2038,16 @@ static int macb_tx_poll(struct napi_struct *napi, int budget)
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* recovery chain (tx_pending -> txubr_pending -> macb_tx_restart())
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* that fires on TCOMP; if TCOMP itself is lost the TX ring stalls
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* silently until something else kicks TSTART. This watchdog runs
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- * once per second per queue, snapshots tx_tail, and calls
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- * macb_tx_restart() if the ring is non-empty and tx_tail has not
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- * advanced since the previous tick.
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+ * once per second per queue and calls macb_tx_restart() if the ring
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+ * is non-empty and tx_tail has not advanced since the previous tick.
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+ *
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+ * Movement is tracked via the tx_stall_tail_moved boolean rather
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+ * than by snapshotting tx_tail. Per-queue ring indices are bounded
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+ * (and reused), so a snapshot comparison can false-positive when the
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+ * index happens to land on the same value between two ticks under
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+ * sustained load. The boolean is set by macb_tx_complete() whenever
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+ * tx_tail advances and cleared by this watchdog after each tick;
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+ * both writes are under tx_ptr_lock, so no atomic is required.
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*
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* macb_tx_restart() already checks the hardware's TBQP against the
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* driver's head index before re-asserting TSTART, so on a healthy
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@@ -2061,11 +2070,9 @@ static void macb_tx_stall_watchdog(struct work_struct *work)
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spin_lock_irqsave(&queue->tx_ptr_lock, flags);
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cur_tail = queue->tx_tail;
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cur_head = queue->tx_head;
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- if (cur_head != cur_tail &&
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- cur_tail == queue->tx_stall_last_tail)
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+ if (cur_head != cur_tail && !queue->tx_stall_tail_moved)
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stalled = true;
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- else
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- queue->tx_stall_last_tail = cur_tail;
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+ queue->tx_stall_tail_moved = false;
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spin_unlock_irqrestore(&queue->tx_ptr_lock, flags);
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if (stalled) {
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@@ -3346,7 +3353,7 @@ static int macb_open(struct net_device *dev)
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for (q = 0, queue = bp->queues; q < bp->num_queues; ++q, ++queue) {
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napi_enable(&queue->napi_rx);
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napi_enable(&queue->napi_tx);
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- queue->tx_stall_last_tail = queue->tx_tail;
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+ queue->tx_stall_tail_moved = true;
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schedule_delayed_work(&queue->tx_stall_watchdog_work,
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msecs_to_jiffies(MACB_TX_STALL_INTERVAL_MS));
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}
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--
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2.54.0
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