Deadlock in USB_DEVICE_Detach()

[js-nano]

There's a deadlock when calling USB_DEVICE_Detach()

Cause of the deadlock

If USB_DEVICE_Detach() is called from a non-interrupt context, a mutex is locked:

usb/driver/usbdp/src/dynamic/drv_usbdp_device.c

Line 842 in 204d5f2

if(OSAL_MUTEX_Lock((OSAL_MUTEX_HANDLE_TYPE *)&hDriver->mutexID, OSAL_WAIT_FOREVER) == OSAL_RESULT_TRUE)

USB_DEVICE_Detach() then calls DRV_USBDP_EndpointDisable()...

usb/driver/usbdp/src/dynamic/drv_usbdp_device.c

Line 859 in 204d5f2

DRV_USBDP_EndpointDisable((DRV_HANDLE)hDriver, DRV_USB_DEVICE_ENDPOINT_ALL);

... which then proceeds to try to lock the same mutex, with a timeout of OSAL_WAIT_FOREVER:

usb/driver/usbdp/src/dynamic/drv_usbdp_device.c

Line 1232 in 204d5f2

if(OSAL_MUTEX_Lock((OSAL_MUTEX_HANDLE_TYPE *)&hDriver->mutexID, OSAL_WAIT_FOREVER) == OSAL_RESULT_TRUE)

This sequence is guaranteed to cause a deadlock (assuming USB_DEVICE_Detach() is not called from an interrupt context).

Steps leading to the deadlock

I encountered this problem when trying to use both FreeRTOS and the default USB stack on a SAM G55 XPlained Pro board.

The docs say to call USB_DEVICE_Detach() in response to the USB_DEVICE_EVENT_POWER_REMOVED (1).

But the VBUS monitoring occurrs in DRV_USBDP_Tasks(), which is called in a task context (not an interrupt context) when using FreeRTOS. This means that when the USB_DEVICE_EVENT_POWER_REMOVED event is passed into the task event handler, the task is not running in an interrupt context

Why does the demo project work?

Calling USB_DEVICE_Detach() in the G55 cdc_com_port_single demo project works, because the OSAL implementation used is osal_implementation_basic.h. In this implementation, the mutex locking doesn't take the timeout into account, so the deadlock doesn't occur:
https://github.com/Microchip-MPLAB-Harmony/usb_apps_device/blob/e8089912b07a7c4eaec5d5ee81c6929badfc9a9f/apps/cdc_com_port_single/firmware/src/config/sam_g55_xpro/osal/osal_impl_basic.h#L274-L282

I've not got the hardware to test one of the FreeRTOS demo projects, so I can't say if they work

Thanks,
-Jon

[js-nano]

Modifying osal_freertos.h so that all the OSAL_Mutex functions use a recursive mutex solves the problem, indicating that the problem is indeed caused by the mutex locking.

[otonoton]

Are you using polled usb mode instead of the default interrupt mode? I believe that is not recommended, but I agree this shouldn't happen either way. FWIW my event handler by default runs in an interrupt context and Detach() works fine.