mp_thread_begin_atomic_section() is expected to be recursive (i.e. for
nested machine.disable_irq() calls, or if Python code calls disable_irq()
and then the Python runtime calls mp_handle_pending() which also enters an
atomic section to check the scheduler state).
On rp2 when not using core1 the atomic sections are recursive.
However when core1 was active (i.e. _thread) then there was a bug that
caused the core to live-lock if an atomic section recursed.
Adds a test case specifically for mutual exclusion and recursive atomic
sections when using two threads. Without this fix the test immediately
hangs on rp2.
This work was funded through GitHub Sponsors.
Signed-off-by: Angus Gratton <angus@redyak.com.au>
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