tests: some tests don't work with `newlib` lock functions.

[gschorcht]

Description

The following test applications call puts inside an ISR which can lead to problems when newlib's locking functions are implemented by the platform:

tests/isr_yield_higher
tests/periph_rtc

Background

newlib's reentrant versions of system calls use lock functions for different lock objects. For example, puts locks the stdout file using _lock_acquire_recursive. While the lock functions in single-thread environments are only dummies, these functions would have to be implemented by the operating system in multi-thread environments.

The only reasonable way in RIOT to realize these lock functions

_lock_acquire
_lock_acquire_reusrive
_lock_try_acquire
_lock_try_acquire_reusrive

seems to be RIOT's built-in mechanism mutex and rmutex. Since mutex and rmutex don't work in interrupt context, system calls must not be used in ISRs.

The problem was found out in PR #11108.

Steps to reproduce

Enable locking functions in cpu/esp8266/syscalls.c in PR #11108 and flash tests/isr_yield_higher

make BOARD=esp8266-esp-12x -C tests/isr_yield_higher

[gschorcht]

The question is, how we should deal with it. It is more a general problem.

[gschorcht]

@kaspar030 For the moment, I could find a hack which solves the problem with the puts call in the ISR of tests/isr_yield_higher

RIOT/cpu/esp_common/syscalls.c

Lines 374 to 379 in d71b98c

	/*
	* disable the locking for stdout/stderr to avoid rmutex based locking
	* when puts/printf are called from an ISR
	*/
	__fsetlocking(_GLOBAL_REENT->_stdout, FSETLOCKING_BYCALLER);
	__fsetlocking(_GLOBAL_REENT->_stderr, FSETLOCKING_BYCALLER);

It also solves the problem that assert(!irq_is_in()) in locking functions doesn't work.

RIOT/cpu/esp_common/syscalls.c

Line 182 in d71b98c

assert(!irq_is_in());

I'm not really happy with this hack since it breaks the thread safety for file I/O operations on stdout/stderr. But, at least it's no worse than on other platforms that use newlib_nano, which isn't thread safe at all.

Do you think, this hack is an acceptable way or do we need a more general solution?