net: phy: add locking to phy_read_mmd_indirect()/phy_write_mmd_indirect()

The phy layer is missing locking for the above two functions - it
has been observed that two threads (userspace and the phy worker
thread) can race, entering the bus ->write or ->read functions
simultaneously.

This causes the FEC driver to initialise a completion while another
thread is waiting on it or while the interrupt is calling complete()
on it, which causes spinlock unlock-without-lock, spinlock lockups,
and completion timeouts.

Fixes: a59a4d19 ("phy: add the EEE support and the way to access to the MMD registers.")
Fixes: 0c1d77df ("net: libphy: Add phy specific function to access mmd phy registers")
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
Acked-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/phy/phy.c

@@ -1038,10 +1038,14 @@ int phy_read_mmd_indirect(struct phy_device *phydev, int prtad,
 	int value = -1;
 
 	if (phydrv->read_mmd_indirect == NULL) {
-		mmd_phy_indirect(phydev->bus, prtad, devad, addr);
+		struct mii_bus *bus = phydev->bus;
+
+		mutex_lock(&bus->mdio_lock);
+		mmd_phy_indirect(bus, prtad, devad, addr);
 
 		/* Read the content of the MMD's selected register */
-		value = phydev->bus->read(phydev->bus, addr, MII_MMD_DATA);
+		value = bus->read(bus, addr, MII_MMD_DATA);
+		mutex_unlock(&bus->mdio_lock);
 	} else {
 		value = phydrv->read_mmd_indirect(phydev, prtad, devad, addr);
 	}
@@ -1071,10 +1075,14 @@ void phy_write_mmd_indirect(struct phy_device *phydev, int prtad,
 	struct phy_driver *phydrv = phydev->drv;
 
 	if (phydrv->write_mmd_indirect == NULL) {
-		mmd_phy_indirect(phydev->bus, prtad, devad, addr);
+		struct mii_bus *bus = phydev->bus;
+
+		mutex_lock(&bus->mdio_lock);
+		mmd_phy_indirect(bus, prtad, devad, addr);
 
 		/* Write the data into MMD's selected register */
-		phydev->bus->write(phydev->bus, addr, MII_MMD_DATA, data);
+		bus->write(bus, addr, MII_MMD_DATA, data);
+		mutex_unlock(&bus->mdio_lock);
 	} else {
 		phydrv->write_mmd_indirect(phydev, prtad, devad, addr, data);
 	}