Zephyr uarte library missing UART_RX_BUF_RELEASED events

[Kyrre Aalerud]

I run newest nRF53 chip as of writing with 1.6.0 nRF Connect SDK from nRF Connect Toolchain manager 3.6.1 using SES 5.50c.

I use a array of 5 rx-buffers (Increased from 3 to figure out what was going on) and handle UART_RX_BUF_REQUEST and UART_RX_BUF_RELEASED events in the registered callback.
Most of the time it runs fine but during a continuous test I notice that at random intervalls it may skip a UART_RX_BUF_RELEASED event which leads my application to detect a request of using a buffer that is not yet freed. If these buffers had been allocated dynamically this would have caused a memory leak. Instead I catch it and log it while actually allowing the reassigning of already assigned buffer.

Since I use the buffers round-robin and process data (transfer to other threads) during any UART_RX_RDY events I do not miss data but this behavior is worrying.
Is there a race-condition in the library?

[Susheel Nuguru]

Hi Kyrre,

Sounds very much like a race condition. I need to understand more about the chain of events that happened before digging into the code. Could you please help me understand with these questions.

1. Your application seems to be tracking which buffer is free and not instead of relying solely on these events. Is it possible that the application tracking logic might have a race condition? Could you rule that out?
2. Could you provide me the chain of UART events that are received by your application (a log file with a way to see EVENTS/buffer mapping would help).
3. What other application peripheral interrupts are enabled and being used when your application detected that a request arrived for a buffer that is not yet free?

[Kyrre Aalerud]

The logs are very long before things happen so I had to use pastebin. Bottom of the log is the relevant part but all is included.

Log of REQ and FREE

The app keeps track of this array separately but it prints each event as they occur in the log.
It is clear that a free event does not happen. It does ask for two buffers in quick succession without freeing buffer 3 though, but frees 3 right after.