Newlib/Send string over UART with multiple threads cuts string of thread with lower priority

[biboc]

Description

If two threads with different priorities try to write a string over UART using Ethos, the string of thread with lower priority will be cut.

[EDIT]
As I show below with uart-thread-mutex_pb_simple, the problem does not come from Ethos.
If you enable mutex in main.c, output is good. If you use only mutex in stdio_uart.c, string are cut.
Between these two calls, it is newlib? Am I right?

Is newlib and printf thread safe?

Steps to reproduce the issue

Checkout my branch: https://github.com/biboc/RIOT/tree/uart_mutex_thread_pb
And run example uart-thread-mutex_pb_BR on samr21-xpro (make flash BOARD=samr21-xpro)
OR
Run example uart-thread-mutex_pb_simple on samr21-xpro (make flash BOARD=samr21-xpro)

My problem first appeared on STM32F4 as a Border Router, I didn't try the example above on this board but it should behave the same.

Expected results

The string of thread with lower priority should be fully printed before other thread can print string

Actual results

Output from example uart-thread-mutex_pb_BR :

2018-11-28 11:58:50.71 ~~33     `@0909]!UHCP@~~}!main(): This is RIOT!
(Version: 2018.10-RC1-330-gd8cfe-uart_mutex_thread_pb)
2018-11-28 11:58:50.71 ~~}!RIOT border router example application
2018-11-28 11:58:50.72 ~~}!THREAD 2-|+2-|+2-|+2-|+2-|+2-|+2-|+2-|+2-|+2-|+2-|+2-|+2-|+2-|+2-|+2-|+2-|+2-|+2-|+2-|+2-|+2-|+2-|+2-|+2-|+2-|+2-|+2-|+2-|+2-|+
2018-11-28 11:58:50.74 ~~}!THREAD 2-|+2-|+2-|+2-|+THREAD
1_!*11_!*11_!*11_!*11_!*11_!*11_!*11_!*11_!*11_!*11_!*11_!*11_!*11_!*11_!*11_!*11_!*11_!*11_!*11_!*11_!*11_!*11_!*11_!*1
2018-11-28 11:58:50.75
~~}!2-|+2-|+2-|+2-|+2-|+2-|+2-|+2-|+2-|+2-|+2-|+2-|+2-|+2-|+2-|+2-|+2-|+2-|+2-|+2-|+2-|+2-|+2-|+2-|+2-|+2-|+
2018-11-28 11:58:50.76 ~~}!THREAD
2-|+2-|+2-|+2-|+2-|+2-|+2-|+2-|+2-|+2-|+2-|+2-|+2-|+2-|+2-|+2-|+2-|+2-|+2-|+2-|+2-|+2-|+2-|+2-|+2-|+2-|+2-|+2-|+2-|+2-|+

Output from example uart-thread-mutex_pb_simple :

main(): This is RIOT! (Version: 2018.10-RC1-330-gd8cfe)
RIOT border router example application
THREAD 2-|+2-|+2-|+2-|+2-|+2-|+2-|+2-|+2-|+2-|+2-|+2-|+2-|+2-|+2-|+2-|+2-|+2-|+2-|+2-|+2-|+2-|+2-|+2-|+2-|+2-|+2-|+2-|+2-|+2-|+
THREAD 2-|+2-|+2-|+2-11_!*11_!*11_!*11_!*11_!*11_!*11_!*11_!*11_!*11_!*11_!*11_!*11_!*11_!*11_!*11_!*11_!*11_!*11_!*11_!*11_!*1
THREAD 1_!*11_!*11_!*11_!*11_!*11_!*11_!*11_!*11_!*11_!*11_!*11_!*11_!*11_!*11_!*11_!*11_!*11_!*11_!*11_!*11_!*11_!*11_!*11_!*1

Versions

Ubuntu 16.04 and RIOT master (28/11/2018)

[kaspar030]

Newlib is not thread-safe. Could you try merging #8619?

[biboc]

#8619 Will try to merge

Do you have a list of functions that are not thread safe?
Are va_start, va_end and vsnprintf thread safe?

[biboc]

@kaspar030, could I replace printf newlib function by another which is thread safe? Is is possible with newlib?

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. If you want me to ignore this issue, please mark it with the "State: don't stale" label. Thank you for your contributions.