Previously, this was subject to races incrementing/decrementing
the counter variable pendsv_lock.
Technically, all that's needed here would be to make pendsv_lock an atomic
counter.
This implementation fulfils a stronger guarantee: it also provides mutual
exclusion for the core which calls pendsv_suspend(). This is because the
current use of pendsv_suspend/resume in MicroPython is to ensure exclusive
access to softtimer data structures, and this does require mutual
exclusion.
The conceptually cleaner implementation would split the mutual exclusion
part out into a softtimer-specific spinlock, but this increases the
complexity and doesn't seem like it makes for a better implementation in
the long run.
This work was funded through GitHub Sponsors.
Signed-off-by: Angus Gratton <angus@redyak.com.au>
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