improvement request:
as already stated by M. Patel and P. Stoffregen in #5914 there is an urgent need of refactoring the current Arduino libs and cores (especially for ARM, ARMD, STM32) to make them multithreading-safe.

Having MT-safe cores and libs is indispensible especially for more powerful MCUs , i.e. ARM, ARMD, STM32-cores like e.g., the Zero, M0, Due, Teensy, and the new STAR OTTO.

They all have lot of memory and cpu-power but that can be utilized reasonably only by powerful multilthreading libraries (e.g. std::thread (C++11/14) and even possibly POSIX pthread (ANSI-C) ) which require compellingly reliable thread-safe core and lib fundaments.