net: fix possible race in skb_attempt_defer_free()

A cpu can observe sd->defer_count reaching 128,
and call smp_call_function_single_async()

Problem is that the remote CPU can clear sd->defer_count
before the IPI is run/acknowledged.

Other cpus can queue more packets and also decide
to call smp_call_function_single_async() while the pending
IPI was not yet delivered.

This is a common issue with smp_call_function_single_async().
Callers must ensure correct synchronization and serialization.

I triggered this issue while experimenting smaller threshold.
Performing the call to smp_call_function_single_async()
under sd->defer_lock protection did not solve the problem.

Commit 5a18ceca ("smp: Allow smp_call_function_single_async()
to insert locked csd") replaced an informative WARN_ON_ONCE()
with a return of -EBUSY, which is often ignored.
Test of CSD_FLAG_LOCK presence is racy anyway.

Fixes: 68822bdf ("net: generalize skb freeing deferral to per-cpu lists")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/netdevice.h

@@ -3136,6 +3136,7 @@ struct softnet_data {
 	/* Another possibly contended cache line */
 	spinlock_t		defer_lock ____cacheline_aligned_in_smp;
 	int			defer_count;
+	int			defer_ipi_scheduled;
 	struct sk_buff		*defer_list;
 	call_single_data_t	defer_csd;
 };

net/core/dev.c

@@ -4582,9 +4582,12 @@ static void rps_trigger_softirq(void *data)
 #endif /* CONFIG_RPS */
 
 /* Called from hardirq (IPI) context */
-static void trigger_rx_softirq(void *data __always_unused)
+static void trigger_rx_softirq(void *data)
 {
+	struct softnet_data *sd = data;
+
 	__raise_softirq_irqoff(NET_RX_SOFTIRQ);
+	smp_store_release(&sd->defer_ipi_scheduled, 0);
 }
 
 /*
@@ -11382,7 +11385,7 @@ static int __init net_dev_init(void)
 		INIT_CSD(&sd->csd, rps_trigger_softirq, sd);
 		sd->cpu = i;
 #endif
-		INIT_CSD(&sd->defer_csd, trigger_rx_softirq, NULL);
+		INIT_CSD(&sd->defer_csd, trigger_rx_softirq, sd);
 		spin_lock_init(&sd->defer_lock);
 
 		init_gro_hash(&sd->backlog);

net/core/skbuff.c

@@ -6516,8 +6516,7 @@ void skb_attempt_defer_free(struct sk_buff *skb)
 	sd->defer_count++;
 
 	/* kick every time queue length reaches 128.
-	 * This should avoid blocking in smp_call_function_single_async().
-	 * This condition should hardly be bit under normal conditions,
+	 * This condition should hardly be hit under normal conditions,
 	 * unless cpu suddenly stopped to receive NIC interrupts.
 	 */
 	kick = sd->defer_count == 128;
@@ -6527,6 +6526,6 @@ void skb_attempt_defer_free(struct sk_buff *skb)
 	/* Make sure to trigger NET_RX_SOFTIRQ on the remote CPU
 	 * if we are unlucky enough (this seems very unlikely).
 	 */
-	if (unlikely(kick))
+	if (unlikely(kick) && !cmpxchg(&sd->defer_ipi_scheduled, 0, 1))
 		smp_call_function_single_async(cpu, &sd->defer_csd);
 }