MSC + CDC and stdout overflow/stall?

[MattMills]

Hello,

I'm trying to use an MSC for writing flash as well as several simultaneous CDC devices, including one for STDOUT.

I seem to have run into an issue during an MSC read where tud_cdc_n_write_available for the CDC STDOUT interface seems to run out of space and eventually deadlock, because every time tud_task() to get the CDC interface to output data and make space, an mscd_xfer_cb is received with the incoming data, which gets processed, printing more info to stdout.

I suppose to simplify my question, if I am writing all data received by mscd_xfer_cb plus some additional data to tud_cdc_n_write(), and the write to CDC is blocking, is it possible to do that without deadlocking? Having dug through the code a bit it seems like each time tud_task() is called it fires all the drivers and callbacks, so it seems like this use case would deadlock, because we need to output cdc data faster than msc data, or delay/repeat the incoming msc callback after a cdc write has completed to flush the buffer, unless there is something I'm missing.

To be clear, this isn't what I'm doing, I'm just outputting a lot of debug data into stdout, which is going into a CDC interface, during a file write to an MSC interface, which is then being written to flash on an RP2040 or ESP32, depending on file format.

Or should tud_task() not be called to try to process the cdc_write buffer?