[PATCH net] net: phy: Fix deadlocking in phy_error() invocation

[Serge Semin <fancer.lancer@gmail.com>]

Since commit 91a7cda1f4b8 ("net: phy: Fix race condition on link status
change") all the phy_error() method invocations have been causing the
nested-mutex-lock deadlock because it's normally done in the PHY-driver
threaded IRQ handlers which since that change have been called with the
phydev->lock mutex held. Here is the calls thread:

IRQ: phy_interrupt()
     +-> mutex_lock(&phydev->lock); <--------------------+
         drv->handle_interrupt()                         | Deadlock due
         +-> ERROR: phy_error()                          + to the nested
                    +-> phy_process_error()              | mutex lock
                        +-> mutex_lock(&phydev->lock); <-+
                            phydev->state = PHY_ERROR;
                            mutex_unlock(&phydev->lock);
         mutex_unlock(&phydev->lock);

The problem can be easily reproduced just by calling phy_error() from any
PHY-device threaded interrupt handler. Fix it by dropping the phydev->lock
mutex lock from the phy_process_error() method and printing a nasty error
message to the system log if the mutex isn't held in the caller execution
context.

Note for the fix to work correctly in the PHY-subsystem itself the
phydev->lock mutex locking must be added to the phy_error_precise()
function.

Link: https://lore.kernel.org/netdev/20230816180944.19262-1-fancer.lancer@gmail.com
Fixes: 91a7cda1f4b8 ("net: phy: Fix race condition on link status change")
Suggested-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Serge Semin <fancer.lancer@gmail.com>
---
 drivers/net/phy/phy.c | 11 +++++++----
 1 file changed, 7 insertions(+), 4 deletions(-)

diff --git a/drivers/net/phy/phy.c b/drivers/net/phy/phy.c
index bdf00b2b2c1d..a9ecfdd19624 100644
--- a/drivers/net/phy/phy.c
+++ b/drivers/net/phy/phy.c
@@ -1184,9 +1184,11 @@ void phy_stop_machine(struct phy_device *phydev)
 
 static void phy_process_error(struct phy_device *phydev)
 {
-	mutex_lock(&phydev->lock);
+	/* phydev->lock must be held for the state change to be safe */
+	if (!mutex_is_locked(&phydev->lock))
+		phydev_err(phydev, "PHY-device data unsafe context\n");
+
 	phydev->state = PHY_ERROR;
-	mutex_unlock(&phydev->lock);
 
 	phy_trigger_machine(phydev);
 }
@@ -1195,7 +1197,9 @@ static void phy_error_precise(struct phy_device *phydev,
 			      const void *func, int err)
 {
 	WARN(1, "%pS: returned: %d\n", func, err);
+	mutex_lock(&phydev->lock);
 	phy_process_error(phydev);
+	mutex_unlock(&phydev->lock);
 }
 
 /**
@@ -1204,8 +1208,7 @@ static void phy_error_precise(struct phy_device *phydev,
  *
  * Moves the PHY to the ERROR state in response to a read
  * or write error, and tells the controller the link is down.
- * Must not be called from interrupt context, or while the
- * phydev->lock is held.
+ * Must be called with phydev->lock held.
  */
 void phy_error(struct phy_device *phydev)
 {
-- 
2.41.0