net: napi: Avoid gro timer misfiring at end of busypoll

When in irq deferral mode (defer-hard-irqs > 0), a short enough
gro-flush timeout can trigger before NAPI_STATE_SCHED is cleared if the
last poll in busy_poll_stop() takes too long. This can have the effect
of leaving the queue stuck with interrupts disabled and no timer armed
which results in a tx timeout if there is no subsequent busypoll cycle.

To prevent this, defer the gro-flush timer arm after the last poll.

Fixes: 7fd3253a7de6 ("net: Introduce preferred busy-polling")
Co-developed-by: Martin Karsten <mkarsten@uwaterloo.ca>
Signed-off-by: Martin Karsten <mkarsten@uwaterloo.ca>
Signed-off-by: Dragos Tatulea <dtatulea@nvidia.com>
Reviewed-by: Tariq Toukan <tariqt@nvidia.com>
Reviewed-by: Cosmin Ratiu <cratiu@nvidia.com>
Reviewed-by: Joe Damato <joe@dama.to>
Link: https://patch.msgid.link/20260506090808.820559-2-dtatulea@nvidia.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

diff --git a/net/core/dev.c b/net/core/dev.c
index 8bfa8313ef62eda9fe6aa037c6a5408df54e91cf..0c6c270d9f7d115feb824f4ebe6be122c40d745f 100644 (file)
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -6862,9 +6862,9 @@ static void skb_defer_free_flush(void)
 
 #if defined(CONFIG_NET_RX_BUSY_POLL)
 
-static void __busy_poll_stop(struct napi_struct *napi, bool skip_schedule)
+static void __busy_poll_stop(struct napi_struct *napi, unsigned long timeout)
 {
-       if (!skip_schedule) {
+       if (!timeout) {
                gro_normal_list(&napi->gro);
                __napi_schedule(napi);
                return;
@@ -6874,6 +6874,8 @@ static void __busy_poll_stop(struct napi_struct *napi, bool skip_schedule)
        gro_flush_normal(&napi->gro, HZ >= 1000);
 
        clear_bit(NAPI_STATE_SCHED, &napi->state);
+       hrtimer_start(&napi->timer, ns_to_ktime(timeout),
+                     HRTIMER_MODE_REL_PINNED);
 }
 
 enum {
@@ -6885,8 +6887,7 @@ static void busy_poll_stop(struct napi_struct *napi, void *have_poll_lock,
                           unsigned flags, u16 budget)
 {
        struct bpf_net_context __bpf_net_ctx, *bpf_net_ctx;
-       bool skip_schedule = false;
-       unsigned long timeout;
+       unsigned long timeout = 0;
        int rc;
 
        /* Busy polling means there is a high chance device driver hard irq
@@ -6906,10 +6907,12 @@ static void busy_poll_stop(struct napi_struct *napi, void *have_poll_lock,
 
        if (flags & NAPI_F_PREFER_BUSY_POLL) {
                napi->defer_hard_irqs_count = napi_get_defer_hard_irqs(napi);
-               timeout = napi_get_gro_flush_timeout(napi);
-               if (napi->defer_hard_irqs_count && timeout) {
-                       hrtimer_start(&napi->timer, ns_to_ktime(timeout), HRTIMER_MODE_REL_PINNED);
-                       skip_schedule = true;
+               if (napi->defer_hard_irqs_count) {
+                       /* A short enough gro flush timeout and long enough
+                        * poll can result in timer firing too early.
+                        * Timer will be armed later if necessary.
+                        */
+                       timeout = napi_get_gro_flush_timeout(napi);
                }
        }
 
@@ -6924,7 +6927,7 @@ static void busy_poll_stop(struct napi_struct *napi, void *have_poll_lock,
        trace_napi_poll(napi, rc, budget);
        netpoll_poll_unlock(have_poll_lock);
        if (rc == budget)
-               __busy_poll_stop(napi, skip_schedule);
+               __busy_poll_stop(napi, timeout);
        bpf_net_ctx_clear(bpf_net_ctx);
        local_bh_enable();
 }