Prior to this commit there is a potential deadlock in
mp_thread_begin_atomic_section(), when obtaining the atomic_mutex, in the
following situation:
- main thread calls mp_thread_begin_atomic_section() (for whatever reason,
doesn't matter)
- the second core is running so the main thread grabs the mutex via the
call mp_thread_mutex_lock(&atomic_mutex, 1), and this succeeds
- before the main thread has a chance to run save_and_disable_interrupts()
a USB IRQ comes in and the main thread jumps off to process this IRQ
- that USB processing triggers a call to the dcd_event_handler() wrapper
from commit bcbdee2357
- that then calls mp_sched_schedule_node()
- that then attempts to obtain the atomic section, calling
mp_thread_begin_atomic_section()
- that call then blocks trying to obtain atomic_mutex
- core0 is now deadlocked on itself, because the main thread has the mutex
but the IRQ handler (which preempted the main thread) is blocked waiting
for the mutex, which will never be free
The solution in this commit is to use mutex enter/exit functions that also
atomically disable/restore interrupts.
Fixes issues #12980 and #13288.
Signed-off-by: Damien George <damien@micropython.org>
dc2a4e3cbd