From 06e3f54e8b22040ada01a28343badf1990b4dd7d Mon Sep 17 00:00:00 2001 From: Eric Dumazet Date: Wed, 23 Sep 2026 13:03:18 +0000 Subject: [PATCH] net: flush skb_defer_nodes in dev_cpu_dead() When a CPU goes offline, dev_cpu_dead() drains its softnet queues (completion_queue, output_queue, poll_list, process_queue, and input_pkt_queue), but leaves net_hotdata.skb_defer_nodes untouched. If oldcpu goes offline while holding pending skbs in its skb_defer_nodes lists (e.g. below the sysctl_skb_defer_max >> 1 IPI threshold, or if the IPI races with CPU teardown), those skbs remain stranded until oldcpu is brought back online. If any of these skbs hold page_pool fragments, page_pool_destroy() will stall indefinitely waiting for inflight pages to be returned when a netdev or driver is torn down while oldcpu is offline. Additionally, if smp_call_function_single_async() fails in kick_defer_list_purge() because the target CPU went offline, reset defer_ipi_scheduled to 0 so future IPI kicks are not blocked when the CPU comes back online. Also, if oldcpu was the last online CPU on its NUMA node, drain that node's slot across all CPUs so no skbs deferred from that node remain stranded on idle remote CPUs (or if the node itself is subsequently offlined). Finally, in skb_attempt_defer_free(), re-check cpu_online(cpu) and whether the caller migrated CPUs after llist_add(), flushing the node list if so, to close the preemption TOCTOU race against CPU/node teardown. Fixes: 68822bdf76f1 ("net: generalize skb freeing deferral to per-cpu lists") Fixes: 5628f3fe3b16 ("net: add NUMA awareness to skb_attempt_defer_free()") Closes: https://lore.kernel.org/netdev/20260916003430.3612956-1-kris.pan@intel.com/ Signed-off-by: Eric Dumazet Cc: Kris Pan Link: https://patch.msgid.link/20260923130318.607255-1-edumazet@google.com Signed-off-by: Jakub Kicinski --- net/core/dev.c | 47 +++++++++++++++++++++++++++++++++++------------ net/core/dev.h | 2 ++ net/core/skbuff.c | 12 +++++++++--- 3 files changed, 46 insertions(+), 15 deletions(-) diff --git a/net/core/dev.c b/net/core/dev.c index 0292a16e16c2..f660fccfc0db 100644 --- a/net/core/dev.c +++ b/net/core/dev.c @@ -5376,7 +5376,8 @@ void kick_defer_list_purge(unsigned int cpu) backlog_unlock_irq_restore(sd, flags); } else if (!cmpxchg(&sd->defer_ipi_scheduled, 0, 1)) { - smp_call_function_single_async(cpu, &sd->defer_csd); + if (smp_call_function_single_async(cpu, &sd->defer_csd)) + WRITE_ONCE(sd->defer_ipi_scheduled, 0); } } @@ -6900,25 +6901,35 @@ bool napi_complete_done(struct napi_struct *n, int work_done) } EXPORT_SYMBOL(napi_complete_done); -static void skb_defer_free_flush(void) +static void __skb_defer_free_flush(struct skb_defer_node *sdn, int budget) { struct llist_node *free_list; struct sk_buff *skb, *next; + + if (llist_empty(&sdn->defer_list)) + return; + atomic_long_set(&sdn->defer_count, 0); + free_list = llist_del_all(&sdn->defer_list); + + llist_for_each_entry_safe(skb, next, free_list, ll_node) { + prefetch(next); + napi_consume_skb(skb, budget); + } +} + +void skb_defer_node_flush(struct skb_defer_node *sdn) +{ + __skb_defer_free_flush(sdn, 0); +} + +static void skb_defer_free_flush(void) +{ struct skb_defer_node *sdn; int node; for_each_node(node) { sdn = this_cpu_ptr(net_hotdata.skb_defer_nodes) + node; - - if (llist_empty(&sdn->defer_list)) - continue; - atomic_long_set(&sdn->defer_count, 0); - free_list = llist_del_all(&sdn->defer_list); - - llist_for_each_entry_safe(skb, next, free_list, ll_node) { - prefetch(next); - napi_consume_skb(skb, 1); - } + __skb_defer_free_flush(sdn, 1); } } @@ -12897,6 +12908,7 @@ static int dev_cpu_dead(unsigned int oldcpu) struct sk_buff **list_skb; struct sk_buff *skb; unsigned int cpu; + int node; struct softnet_data *sd, *oldsd, *remsd = NULL; local_irq_disable(); @@ -12957,6 +12969,17 @@ static int dev_cpu_dead(unsigned int oldcpu) rps_input_queue_head_incr(oldsd); } + for_each_node(node) + skb_defer_node_flush(per_cpu_ptr(net_hotdata.skb_defer_nodes, + oldcpu) + node); + node = cpu_to_node(oldcpu); + if (node_possible(node) && + !cpumask_intersects(cpumask_of_node(node), cpu_online_mask)) { + for_each_possible_cpu(cpu) + skb_defer_node_flush(per_cpu_ptr(net_hotdata.skb_defer_nodes, + cpu) + node); + } + return 0; } diff --git a/net/core/dev.h b/net/core/dev.h index b757faead4d1..04fb0e9a571e 100644 --- a/net/core/dev.h +++ b/net/core/dev.h @@ -399,6 +399,8 @@ static inline void napi_assert_will_not_race(const struct napi_struct *napi) WARN_ON(READ_ONCE(napi->list_owner) != -1); } +struct skb_defer_node; +void skb_defer_node_flush(struct skb_defer_node *sdn); void kick_defer_list_purge(unsigned int cpu); int dev_set_hwtstamp_phylib(struct net_device *dev, diff --git a/net/core/skbuff.c b/net/core/skbuff.c index b4edbd06655e..c3042d822afa 100644 --- a/net/core/skbuff.c +++ b/net/core/skbuff.c @@ -7360,8 +7360,8 @@ void skb_attempt_defer_free(struct sk_buff *skb) struct skb_defer_node *sdn; unsigned long defer_count; unsigned int defer_max; + int cpu, my_cpu; bool kick; - int cpu; if (static_branch_unlikely(&skb_defer_disable_key)) goto nodefer; @@ -7371,7 +7371,8 @@ void skb_attempt_defer_free(struct sk_buff *skb) goto nodefer; cpu = skb->alloc_cpu; - if (cpu == raw_smp_processor_id() || + my_cpu = raw_smp_processor_id(); + if (cpu == my_cpu || WARN_ON_ONCE(cpu >= nr_cpu_ids) || !cpu_online(cpu)) { nodefer: kfree_skb_napi_cache(skb); @@ -7382,7 +7383,7 @@ nodefer: kfree_skb_napi_cache(skb); DEBUG_NET_WARN_ON_ONCE(skb->destructor); DEBUG_NET_WARN_ON_ONCE(skb_nfct(skb)); - sdn = per_cpu_ptr(net_hotdata.skb_defer_nodes, cpu) + numa_node_id(); + sdn = per_cpu_ptr(net_hotdata.skb_defer_nodes, cpu) + cpu_to_node(my_cpu); defer_max = READ_ONCE(net_hotdata.sysctl_skb_defer_max); defer_count = atomic_long_inc_return(&sdn->defer_count); @@ -7392,6 +7393,11 @@ nodefer: kfree_skb_napi_cache(skb); llist_add(&skb->ll_node, &sdn->defer_list); + if (unlikely(!cpu_online(cpu) || my_cpu != raw_smp_processor_id())) { + skb_defer_node_flush(sdn); + return; + } + /* Send an IPI every time queue reaches half capacity. */ kick = (defer_count - 1) == (defer_max >> 1);