Re: [PATCH net v2 2/2] net: macb: Protect access to net_device::ip_ptr with RCU lock

Access to net_device::ip_ptr and its associated members must be
protected by an RCU lock. Since we are modifying this piece of code,
let's also move it to execute only when WAKE_ARP is enabled.

To minimize the duration of the RCU lock, a local variable is used to
temporarily store the IP address. This change resolves the following
RCU check warning:
  WARNING: suspicious RCU usage
  7.0.0-rc3-next-20260310-yocto-standard+ #122 Not tainted
  -----------------------------
  drivers/net/ethernet/cadence/macb_main.c:5944 suspicious rcu_dereference_check() usage!

  other info that might help us debug this:

  rcu_scheduler_active = 2, debug_locks = 1
  5 locks held by rtcwake/518:
   #0: ffff000803ab1408 (sb_writers#5){.+.+}-{0:0}, at: vfs_write+0xf8/0x368
   #1: ffff0008090bf088 (&of->mutex#2){+.+.}-{4:4}, at: kernfs_fop_write_iter+0xbc/0x1c8
   #2: ffff00080098d588 (kn->active#70){.+.+}-{0:0}, at: kernfs_fop_write_iter+0xcc/0x1c8
   #3: ffff800081c84888 (system_transition_mutex){+.+.}-{4:4}, at: pm_suspend+0x1ec/0x290
   #4: ffff0008009ba0f8 (&dev->mutex){....}-{4:4}, at: device_suspend+0x118/0x4f0

  stack backtrace:
  CPU: 3 UID: 0 PID: 518 Comm: rtcwake Not tainted 7.0.0-rc3-next-20260310-yocto-standard+ #122 PREEMPT
  Hardware name: ZynqMP ZCU102 Rev1.1 (DT)
  Call trace:
   show_stack+0x24/0x38 (C)
   __dump_stack+0x28/0x38
   dump_stack_lvl+0x64/0x88
   dump_stack+0x18/0x24
   lockdep_rcu_suspicious+0x134/0x1d8
   macb_suspend+0xd8/0x4c0
   device_suspend+0x218/0x4f0
   dpm_suspend+0x244/0x3a0
   dpm_suspend_start+0x50/0x78
   suspend_devices_and_enter+0xec/0x560
   pm_suspend+0x194/0x290
   state_store+0x110/0x158
   kobj_attr_store+0x1c/0x30
   sysfs_kf_write+0xa8/0xd0
   kernfs_fop_write_iter+0x11c/0x1c8
   vfs_write+0x248/0x368
   ksys_write+0x7c/0xf8
   __arm64_sys_write+0x28/0x40
   invoke_syscall+0x4c/0xe8
   el0_svc_common+0x98/0xf0
   do_el0_svc+0x28/0x40
   el0_svc+0x54/0x1e0
   el0t_64_sync_handler+0x84/0x130
   el0t_64_sync+0x198/0x1a0

Fixes: 0cb8de39a776 ("net: macb: Add ARP support to WOL")
Signed-off-by: Kevin Hao <redacted>
Cc: stable@vger.kernel.org
---
 drivers/net/ethernet/cadence/macb_main.c | 25 ++++++++++++++++---------
 1 file changed, 16 insertions(+), 9 deletions(-)

diff --git a/drivers/net/ethernet/cadence/macb_main.c b/drivers/net/ethernet/cadence/macb_main.c
index 2d4304331e297accd91ab48813a9bd4722ce72dc..9856764402b17397928d0a61da61865a3b10484f 100644
--- a/drivers/net/ethernet/cadence/macb_main.c
+++ b/drivers/net/ethernet/cadence/macb_main.c

@@ -5909,9 +5909,9 @@ static int __maybe_unused macb_suspend(struct device *dev)
 	struct macb_queue *queue;
 	struct in_device *idev;
 	unsigned long flags;
+	u32 tmp, ifa_local;
 	unsigned int q;
 	int err;
-	u32 tmp;
 
 	if (!device_may_wakeup(&bp->dev->dev))
 		phy_exit(bp->phy);

@@ -5920,14 +5920,21 @@ static int __maybe_unused macb_suspend(struct device *dev)
 		return 0;
 
 	if (bp->wol & MACB_WOL_ENABLED) {
-		/* Check for IP address in WOL ARP mode */
-		idev = __in_dev_get_rcu(bp->dev);
-		if (idev)
-			ifa = rcu_dereference(idev->ifa_list);
-		if ((bp->wolopts & WAKE_ARP) && !ifa) {
-			netdev_err(netdev, "IP address not assigned as required by WoL walk ARP\n");
-			return -EOPNOTSUPP;
+		if (bp->wolopts & WAKE_ARP) {
+			/* Check for IP address in WOL ARP mode */
+			rcu_read_lock();
+			idev = __in_dev_get_rcu(bp->dev);
+			if (idev)
+				ifa = rcu_dereference(idev->ifa_list);
+			if (!ifa) {
+				rcu_read_unlock();
+				netdev_err(netdev, "IP address not assigned as required by WoL walk ARP\n");
+				return -EOPNOTSUPP;
+			}
+			ifa_local = be32_to_cpu(ifa->ifa_local);
+			rcu_read_unlock();
 		}
+
 		spin_lock_irqsave(&bp->lock, flags);
 
 		/* Disable Tx and Rx engines before  disabling the queues,

@@ -5966,7 +5973,7 @@ static int __maybe_unused macb_suspend(struct device *dev)
 		if (bp->wolopts & WAKE_ARP) {
 			tmp |= MACB_BIT(ARP);
 			/* write IP address into register */
-			tmp |= MACB_BFEXT(IP, be32_to_cpu(ifa->ifa_local));
+			tmp |= MACB_BFEXT(IP, ifa_local);
 		}
 		spin_unlock_irqrestore(&bp->lock, flags);