[PATCH 2/2] usb: misc: onboard_hub: Move 'attach' work to the driver

[Matthias Kaehlcke]

Currently each onboard_hub platform device owns an 'attach' work,
which is scheduled when the device probes. With this deadlocks
have been reported on a Raspberry Pi 3 B+ [1], which has nested
onboard hubs.

The flow of the deadlock is something like this (with the onboard_hub
driver built as a module) [2]:

- USB root hub is instantiated
- core hub driver calls onboard_hub_create_pdevs(), which creates the
  'raw' platform device for the 1st level hub
- 1st level hub is probed by the core hub driver
- core hub driver calls onboard_hub_create_pdevs(), which creates
  the 'raw' platform device for the 2nd level hub

- onboard_hub platform driver is registered
- platform device for 1st level hub is probed
  - schedules 'attach' work
- platform device for 2nd level hub is probed
  - schedules 'attach' work

- onboard_hub USB driver is registered
- device (and parent) lock of hub is held while the device is
  re-probed with the onboard_hub driver

- 'attach' work (running in another thread) calls driver_attach(), which
   blocks on one of the hub device locks

- onboard_hub_destroy_pdevs() is called by the core hub driver when one
  of the hubs is detached
- destroying the pdevs invokes onboard_hub_remove(), which waits for the
  'attach' work to complete
  - waits forever, since the 'attach' work can't acquire the device lock

Use a single work struct for the driver instead of having a work struct
per onboard hub platform driver instance. With that it isn't necessary
to cancel the work in onboard_hub_remove(), which fixes the deadlock.
The work is only cancelled when the driver is unloaded.

[1] https://lore.kernel.org/r/d04bcc45-3471-4417-b30b-5cf9880d785d@xxxxxxxx/
[2] https://lore.kernel.org/all/Y6OrGbqaMy2iVDWB@xxxxxxxxxx/

Cc: stable@xxxxxxxxxxxxxxx
Fixes: 8bc063641ceb ("usb: misc: Add onboard_usb_hub driver")
Link: https://lore.kernel.org/r/d04bcc45-3471-4417-b30b-5cf9880d785d@xxxxxxxx/
Link: https://lore.kernel.org/all/Y6OrGbqaMy2iVDWB@xxxxxxxxxx/
Reported-by: Stefan Wahren <stefan.wahren@xxxxxxxx>
Signed-off-by: Matthias Kaehlcke <mka@xxxxxxxxxxxx>
---

 drivers/usb/misc/onboard_usb_hub.c | 19 +++++++++++++------
 1 file changed, 13 insertions(+), 6 deletions(-)

diff --git a/drivers/usb/misc/onboard_usb_hub.c b/drivers/usb/misc/onboard_usb_hub.c
index db0844b30bbd..8bc4deb465f0 100644
--- a/drivers/usb/misc/onboard_usb_hub.c
+++ b/drivers/usb/misc/onboard_usb_hub.c
@@ -27,7 +27,10 @@
 
 #include "onboard_usb_hub.h"
 
+static void onboard_hub_attach_usb_driver(struct work_struct *work);
+
 static struct usb_device_driver onboard_hub_usbdev_driver;
+static DECLARE_WORK(attach_usb_driver_work, onboard_hub_attach_usb_driver);
 
 /************************** Platform driver **************************/
 
@@ -45,7 +48,6 @@ struct onboard_hub {
 	bool is_powered_on;
 	bool going_away;
 	struct list_head udev_list;
-	struct work_struct attach_usb_driver_work;
 	struct mutex lock;
 };
 
@@ -270,9 +272,15 @@ static int onboard_hub_probe(struct platform_device *pdev)
 	 *
 	 * This needs to be done deferred to avoid self-deadlocks on systems
 	 * with nested onboard hubs.
+	 *
+	 * If the work is already running wait for it to complete, then
+	 * schedule it again to ensure that the USB devices of this onboard
+	 * hub instance are bound to the USB driver.
 	 */
-	INIT_WORK(&hub->attach_usb_driver_work, onboard_hub_attach_usb_driver);
-	schedule_work(&hub->attach_usb_driver_work);
+	while (work_busy(&attach_usb_driver_work) & WORK_BUSY_RUNNING)
+		msleep(10);
+
+	schedule_work(&attach_usb_driver_work);
 
 	return 0;
 }
@@ -285,9 +293,6 @@ static int onboard_hub_remove(struct platform_device *pdev)
 
 	hub->going_away = true;
 
-	if (&hub->attach_usb_driver_work != current_work())
-		cancel_work_sync(&hub->attach_usb_driver_work);
-
 	mutex_lock(&hub->lock);
 
 	/* unbind the USB devices to avoid dangling references to this device */
@@ -449,6 +454,8 @@ static void __exit onboard_hub_exit(void)
 {
 	usb_deregister_device_driver(&onboard_hub_usbdev_driver);
 	platform_driver_unregister(&onboard_hub_driver);
+
+	cancel_work_sync(&attach_usb_driver_work);
 }
 module_exit(onboard_hub_exit);
 
-- 
2.39.0.314.g84b9a713c41-goog