Currently phylink_resolve() protects itself against concurrent
phylink_bringup_phy() or phylink_disconnect_phy() calls which modify
pl->phydev by relying on pl->state_mutex.

The problem is that in phylink_resolve(), pl->state_mutex is in a lock
inversion state with pl->phydev->lock. So pl->phydev->lock needs to be
acquired prior to pl->state_mutex. But that requires dereferencing
pl->phydev in the first place, and without pl->state_mutex, that is
racy.

Hence the reason for the extra lock. Currently it is redundant, but it
will serve a functional purpose once mutex_lock(&phy->lock) will be
moved outside of the mutex_lock(&pl->state_mutex) section.

Another alternative considered would have been to let phylink_resolve()
acquire the rtnl_mutex, which is also held when phylink_bringup_phy()
and phylink_disconnect_phy() are called. But since phylink_disconnect_phy()
runs under rtnl_lock(), it would deadlock with phylink_resolve() when
calling flush_work(&pl->resolve). Additionally, it would have been
undesirable because it would have unnecessarily blocked many other call
paths as well in the entire kernel, so the smaller-scoped lock was
preferred.

Link: https://lore.kernel.org/netdev/aLb6puGVzR29GpPx@shell.armlinux.org.uk/
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
---
v2->v3:
- clarify in the commit message that rtnl_lock() in phylink_resolve()
  wouldn't have worked
- s/phy_lock/phydev_mutex/
- expand and refactor critical section in phylink_disconnect_phy()
- make use of the local "phy" variable in phylink_resolve()
v2 at:
https://lore.kernel.org/netdev/20250903152348.2998651-1-vladimir.oltean@nxp.com/
v1->v2: patch is new

 drivers/net/phy/phylink.c | 19 ++++++++++++++++---
 1 file changed, 16 insertions(+), 3 deletions(-)

diff --git a/drivers/net/phy/phylink.c b/drivers/net/phy/phylink.c
index c7f867b361dd..386d37f6bad4 100644
--- a/drivers/net/phy/phylink.c
+++ b/drivers/net/phy/phylink.c
@@ -67,6 +67,8 @@ struct phylink {
 	struct timer_list link_poll;
 
 	struct mutex state_mutex;
+	/* Serialize updates to pl->phydev with phylink_resolve() */
+	struct mutex phydev_mutex;
 	struct phylink_link_state phy_state;
 	unsigned int phy_ib_mode;
 	struct work_struct resolve;
@@ -1582,8 +1584,11 @@ static void phylink_resolve(struct work_struct *w)
 	struct phylink_link_state link_state;
 	bool mac_config = false;
 	bool retrigger = false;
+	struct phy_device *phy;
 	bool cur_link_state;
 
+	mutex_lock(&pl->phydev_mutex);
+	phy = pl->phydev;
 	mutex_lock(&pl->state_mutex);
 	cur_link_state = phylink_link_is_up(pl);
 
@@ -1617,11 +1622,11 @@ static void phylink_resolve(struct work_struct *w)
 		/* If we have a phy, the "up" state is the union of both the
 		 * PHY and the MAC
 		 */
-		if (pl->phydev)
+		if (phy)
 			link_state.link &= pl->phy_state.link;
 
 		/* Only update if the PHY link is up */
-		if (pl->phydev && pl->phy_state.link) {
+		if (phy && pl->phy_state.link) {
 			/* If the interface has changed, force a link down
 			 * event if the link isn't already down, and re-resolve.
 			 */
@@ -1685,6 +1690,7 @@ static void phylink_resolve(struct work_struct *w)
 		queue_work(system_power_efficient_wq, &pl->resolve);
 	}
 	mutex_unlock(&pl->state_mutex);
+	mutex_unlock(&pl->phydev_mutex);
 }
 
 static void phylink_run_resolve(struct phylink *pl)
@@ -1820,6 +1826,7 @@ struct phylink *phylink_create(struct phylink_config *config,
 	if (!pl)
 		return ERR_PTR(-ENOMEM);
 
+	mutex_init(&pl->phydev_mutex);
 	mutex_init(&pl->state_mutex);
 	INIT_WORK(&pl->resolve, phylink_resolve);
 
@@ -2080,6 +2087,7 @@ static int phylink_bringup_phy(struct phylink *pl, struct phy_device *phy,
 		     dev_name(&phy->mdio.dev), phy->drv->name, irq_str);
 	kfree(irq_str);
 
+	mutex_lock(&pl->phydev_mutex);
 	mutex_lock(&phy->lock);
 	mutex_lock(&pl->state_mutex);
 	pl->phydev = phy;
@@ -2125,6 +2133,7 @@ static int phylink_bringup_phy(struct phylink *pl, struct phy_device *phy,
 
 	mutex_unlock(&pl->state_mutex);
 	mutex_unlock(&phy->lock);
+	mutex_unlock(&pl->phydev_mutex);
 
 	phylink_dbg(pl,
 		    "phy: %s setting supported %*pb advertising %*pb\n",
@@ -2303,6 +2312,7 @@ void phylink_disconnect_phy(struct phylink *pl)
 
 	ASSERT_RTNL();
 
+	mutex_lock(&pl->phydev_mutex);
 	phy = pl->phydev;
 	if (phy) {
 		mutex_lock(&phy->lock);
@@ -2312,8 +2322,11 @@ void phylink_disconnect_phy(struct phylink *pl)
 		pl->mac_tx_clk_stop = false;
 		mutex_unlock(&pl->state_mutex);
 		mutex_unlock(&phy->lock);
-		flush_work(&pl->resolve);
+	}
+	mutex_unlock(&pl->phydev_mutex);
 
+	if (phy) {
+		flush_work(&pl->resolve);
 		phy_disconnect(phy);
 	}
 }
-- 
2.34.1