[PATCH net-next v3] net: phy: Fix suspicious rcu_dereference usage

The phy_detach function can be called with or without the rtnl lock held.
When the rtnl lock is not held, using rtnl_dereference() triggers a
warning due to the lack of lock context.

Add an rcu_read_lock() to ensure the lock is acquired and to maintain
synchronization.

The path reported to not having RTNL lock acquired is the suspend path of
the ravb MAC driver. Without this fix we got this warning:

[   39.032969] =============================
[   39.032983] WARNING: suspicious RCU usage
[   39.033019] -----------------------------
[   39.033033] drivers/net/phy/phy_device.c:2004 suspicious
rcu_dereference_protected() usage!
...
[   39.033597] stack backtrace:
[   39.033613] CPU: 0 UID: 0 PID: 174 Comm: python3 Not tainted
6.13.0-rc7-next-20250116-arm64-renesas-00002-g35245dfdc62c #7
[   39.033623] Hardware name: Renesas SMARC EVK version 2 based on
r9a08g045s33 (DT)
[   39.033628] Call trace:
[   39.033633]  show_stack+0x14/0x1c (C)
[   39.033652]  dump_stack_lvl+0xb4/0xc4
[   39.033664]  dump_stack+0x14/0x1c
[   39.033671]  lockdep_rcu_suspicious+0x16c/0x22c
[   39.033682]  phy_detach+0x160/0x190
[   39.033694]  phy_disconnect+0x40/0x54
[   39.033703]  ravb_close+0x6c/0x1cc
[   39.033714]  ravb_suspend+0x48/0x120
[   39.033721]  dpm_run_callback+0x4c/0x14c
[   39.033731]  device_suspend+0x11c/0x4dc
[   39.033740]  dpm_suspend+0xdc/0x214
[   39.033748]  dpm_suspend_start+0x48/0x60
[   39.033758]  suspend_devices_and_enter+0x124/0x574
[   39.033769]  pm_suspend+0x1ac/0x274
[   39.033778]  state_store+0x88/0x124
[   39.033788]  kobj_attr_store+0x14/0x24
[   39.033798]  sysfs_kf_write+0x48/0x6c
[   39.033808]  kernfs_fop_write_iter+0x118/0x1a8
[   39.033817]  vfs_write+0x27c/0x378
[   39.033825]  ksys_write+0x64/0xf4
[   39.033833]  __arm64_sys_write+0x18/0x20
[   39.033841]  invoke_syscall+0x44/0x104
[   39.033852]  el0_svc_common.constprop.0+0xb4/0xd4
[   39.033862]  do_el0_svc+0x18/0x20
[   39.033870]  el0_svc+0x3c/0xf0
[   39.033880]  el0t_64_sync_handler+0xc0/0xc4
[   39.033888]  el0t_64_sync+0x154/0x158
[   39.041274] ravb 11c30000.ethernet eth0: Link is Down

Tested-by: Claudiu Beznea <claudiu.beznea.uj@bp.renesas.com>
Reported-by: Claudiu Beznea <claudiu.beznea.uj@bp.renesas.com>
Closes: https://lore.kernel.org/netdev/4c6419d8-c06b-495c-b987-d66c2e1ff848@tuxon.dev/ (local)
Fixes: 35f7cad1743e ("net: Add the possibility to support a selected hwtstamp in netdevice")
Signed-off-by: Kory Maincent <kory.maincent@bootlin.com>
---

Changes in v3:
- Update the commit message with the stack trace.

Changes in v2:
- Add a missing ;
---
 drivers/net/phy/phy_device.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/drivers/net/phy/phy_device.c b/drivers/net/phy/phy_device.c
index 5b34d39d1d52..3eeee7cba923 100644
--- a/drivers/net/phy/phy_device.c
+++ b/drivers/net/phy/phy_device.c

@@ -2001,12 +2001,14 @@ void phy_detach(struct phy_device *phydev)
 	if (dev) {
 		struct hwtstamp_provider *hwprov;
 
-		hwprov = rtnl_dereference(dev->hwprov);
+		rcu_read_lock();
+		hwprov = rcu_dereference(dev->hwprov);
 		/* Disable timestamp if it is the one selected */
 		if (hwprov && hwprov->phydev == phydev) {
 			rcu_assign_pointer(dev->hwprov, NULL);
 			kfree_rcu(hwprov, rcu_head);
 		}
+		rcu_read_unlock();
 
 		phydev->attached_dev->phydev = NULL;
 		phydev->attached_dev = NULL;

-- 
2.34.1