usbnet: smsc95xx: Fix deadlock on runtime resume

Commit 05b35e7e ("smsc95xx: add phylib support") amended
smsc95xx_resume() to call phy_init_hw().  That function waits for the
device to runtime resume even though it is placed in the runtime resume
path, causing a deadlock.

The problem is that phy_init_hw() calls down to smsc95xx_mdiobus_read(),
which never uses the _nopm variant of usbnet_read_cmd().

Commit b4df480f ("usbnet: smsc95xx: add reset_resume function with
reset operation") causes a similar deadlock on resume if the device was
already runtime suspended when entering system sleep:

That's because the commit introduced smsc95xx_reset_resume(), which
calls down to smsc95xx_reset(), which neglects to use _nopm accessors.

Fix by auto-detecting whether a device access is performed by the
suspend/resume task_struct and use the _nopm variant if so.  This works
because the PM core guarantees that suspend/resume callbacks are run in
task context.

Stacktrace for posterity:

  INFO: task kworker/2:1:49 blocked for more than 122 seconds.
  Workqueue: usb_hub_wq hub_event
  schedule
  rpm_resume
  __pm_runtime_resume
  usb_autopm_get_interface
  usbnet_read_cmd
  __smsc95xx_read_reg
  __smsc95xx_phy_wait_not_busy
  __smsc95xx_mdio_read
  smsc95xx_mdiobus_read
  __mdiobus_read
  mdiobus_read
  smsc_phy_reset
  phy_init_hw
  smsc95xx_resume
  usb_resume_interface
  usb_resume_both
  usb_runtime_resume
  __rpm_callback
  rpm_callback
  rpm_resume
  __pm_runtime_resume
  usb_autoresume_device
  hub_event
  process_one_work

Fixes: b4df480f ("usbnet: smsc95xx: add reset_resume function with reset operation")
Signed-off-by: Lukas Wunner <lukas@wunner.de>
Cc: stable@vger.kernel.org # v3.16+
Cc: Andre Edich <andre.edich@microchip.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/usb/smsc95xx.c

@@ -71,6 +71,7 @@ struct smsc95xx_priv {
 	struct fwnode_handle *irqfwnode;
 	struct mii_bus *mdiobus;
 	struct phy_device *phydev;
+	struct task_struct *pm_task;
 };
 
 static bool turbo_mode = true;
@@ -80,13 +81,14 @@ MODULE_PARM_DESC(turbo_mode, "Enable multiple frames per Rx transaction");
 static int __must_check __smsc95xx_read_reg(struct usbnet *dev, u32 index,
 					    u32 *data, int in_pm)
 {
+	struct smsc95xx_priv *pdata = dev->driver_priv;
 	u32 buf;
 	int ret;
 	int (*fn)(struct usbnet *, u8, u8, u16, u16, void *, u16);
 
 	BUG_ON(!dev);
 
-	if (!in_pm)
+	if (current != pdata->pm_task)
 		fn = usbnet_read_cmd;
 	else
 		fn = usbnet_read_cmd_nopm;
@@ -110,13 +112,14 @@ static int __must_check __smsc95xx_read_reg(struct usbnet *dev, u32 index,
 static int __must_check __smsc95xx_write_reg(struct usbnet *dev, u32 index,
 					     u32 data, int in_pm)
 {
+	struct smsc95xx_priv *pdata = dev->driver_priv;
 	u32 buf;
 	int ret;
 	int (*fn)(struct usbnet *, u8, u8, u16, u16, const void *, u16);
 
 	BUG_ON(!dev);
 
-	if (!in_pm)
+	if (current != pdata->pm_task)
 		fn = usbnet_write_cmd;
 	else
 		fn = usbnet_write_cmd_nopm;
@@ -1490,9 +1493,12 @@ static int smsc95xx_suspend(struct usb_interface *intf, pm_message_t message)
 	u32 val, link_up;
 	int ret;
 
+	pdata->pm_task = current;
+
 	ret = usbnet_suspend(intf, message);
 	if (ret < 0) {
 		netdev_warn(dev->net, "usbnet_suspend error\n");
+		pdata->pm_task = NULL;
 		return ret;
 	}
 
@@ -1732,6 +1738,7 @@ static int smsc95xx_suspend(struct usb_interface *intf, pm_message_t message)
 	if (ret && PMSG_IS_AUTO(message))
 		usbnet_resume(intf);
 
+	pdata->pm_task = NULL;
 	return ret;
 }
 
@@ -1752,29 +1759,31 @@ static int smsc95xx_resume(struct usb_interface *intf)
 	/* do this first to ensure it's cleared even in error case */
 	pdata->suspend_flags = 0;
 
+	pdata->pm_task = current;
+
 	if (suspend_flags & SUSPEND_ALLMODES) {
 		/* clear wake-up sources */
 		ret = smsc95xx_read_reg_nopm(dev, WUCSR, &val);
 		if (ret < 0)
-			return ret;
+			goto done;
 
 		val &= ~(WUCSR_WAKE_EN_ | WUCSR_MPEN_);
 
 		ret = smsc95xx_write_reg_nopm(dev, WUCSR, val);
 		if (ret < 0)
-			return ret;
+			goto done;
 
 		/* clear wake-up status */
 		ret = smsc95xx_read_reg_nopm(dev, PM_CTRL, &val);
 		if (ret < 0)
-			return ret;
+			goto done;
 
 		val &= ~PM_CTL_WOL_EN_;
 		val |= PM_CTL_WUPS_;
 
 		ret = smsc95xx_write_reg_nopm(dev, PM_CTRL, val);
 		if (ret < 0)
-			return ret;
+			goto done;
 	}
 
 	phy_init_hw(pdata->phydev);
@@ -1783,15 +1792,20 @@ static int smsc95xx_resume(struct usb_interface *intf)
 	if (ret < 0)
 		netdev_warn(dev->net, "usbnet_resume error\n");
 
+done:
+	pdata->pm_task = NULL;
 	return ret;
 }
 
 static int smsc95xx_reset_resume(struct usb_interface *intf)
 {
 	struct usbnet *dev = usb_get_intfdata(intf);
+	struct smsc95xx_priv *pdata = dev->driver_priv;
 	int ret;
 
+	pdata->pm_task = current;
 	ret = smsc95xx_reset(dev);
+	pdata->pm_task = NULL;
 	if (ret < 0)
 		return ret;