net: fix backlog_unlock_irq_restore() vs CONFIG_PREEMPT_RT

CONFIG_PREEMPT_RT is special, make this clear in backlog_lock_irq_save()
and backlog_unlock_irq_restore().

The issue shows up with CONFIG_DEBUG_IRQFLAGS=y

raw_local_irq_restore() called with IRQs enabled
 WARNING: kernel/locking/irqflag-debug.c:10 at warn_bogus_irq_restore+0xc/0x20 kernel/locking/irqflag-debug.c:10, CPU#1: aoe_tx0/1321
Modules linked in:
CPU: 1 UID: 0 PID: 1321 Comm: aoe_tx0 Not tainted syzkaller #0 PREEMPT_{RT,(full)}
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/24/2026
 RIP: 0010:warn_bogus_irq_restore+0xc/0x20 kernel/locking/irqflag-debug.c:10
Call Trace:
 <TASK>
  backlog_unlock_irq_restore net/core/dev.c:253 [inline]
  enqueue_to_backlog+0x525/0xcf0 net/core/dev.c:5347
  netif_rx_internal+0x120/0x550 net/core/dev.c:5659
  __netif_rx+0xa9/0x110 net/core/dev.c:5679
  loopback_xmit+0x43a/0x660 drivers/net/loopback.c:90
  __netdev_start_xmit include/linux/netdevice.h:5275 [inline]
  netdev_start_xmit include/linux/netdevice.h:5284 [inline]
  xmit_one net/core/dev.c:3864 [inline]
  dev_hard_start_xmit+0x2df/0x830 net/core/dev.c:3880
  __dev_queue_xmit+0x16f4/0x3990 net/core/dev.c:4829
  dev_queue_xmit include/linux/netdevice.h:3384 [inline]

Fixes: 27a01c1 ("net: fully inline backlog_unlock_irq_restore()")
Signed-off-by: Eric Dumazet <[email protected]>
Link: https://patch.msgid.link/[email protected]
Signed-off-by: Paolo Abeni <[email protected]>

Author: edumazet

net/core/dev.c

@@ -231,10 +231,13 @@ static bool use_backlog_threads(void)
 static inline void backlog_lock_irq_save(struct softnet_data *sd,
 					 unsigned long *flags)
 {
-	if (IS_ENABLED(CONFIG_RPS) || use_backlog_threads())
+	if (IS_ENABLED(CONFIG_PREEMPT_RT)) {
 		spin_lock_irqsave(&sd->input_pkt_queue.lock, *flags);
-	else
+	} else {
 		local_irq_save(*flags);
+		if (IS_ENABLED(CONFIG_RPS) || use_backlog_threads())
+			spin_lock(&sd->input_pkt_queue.lock);
+	}
 }
 
 static inline void backlog_lock_irq_disable(struct softnet_data *sd)
@@ -248,9 +251,13 @@ static inline void backlog_lock_irq_disable(struct softnet_data *sd)
 static inline void backlog_unlock_irq_restore(struct softnet_data *sd,
 					      unsigned long flags)
 {
-	if (IS_ENABLED(CONFIG_RPS) || use_backlog_threads())
-		spin_unlock(&sd->input_pkt_queue.lock);
-	local_irq_restore(flags);
+	if (IS_ENABLED(CONFIG_PREEMPT_RT)) {
+		spin_unlock_irqrestore(&sd->input_pkt_queue.lock, flags);
+	} else {
+		if (IS_ENABLED(CONFIG_RPS) || use_backlog_threads())
+			spin_unlock(&sd->input_pkt_queue.lock);
+		local_irq_restore(flags);
+	}
 }
 
 static inline void backlog_unlock_irq_enable(struct softnet_data *sd)