Fixes a regression introduced in 3af006efb3
where WFE never blocked in `mp_wfe_or_timeout()` function and would
busy-wait instead. This increases power consumption measurably.
Root cause is that `mp_wfe_or_timeout()` calls soft timer functions that
(after the regression) call `recursive_mutex_enter()` and
`recursive_mutex_exit()`. The exit calls
`lock_internal_spin_unlock_with_notify()` and the default pico-sdk
implementation of this macro issues a SEV which negates the WFE that
follows it, meaning the CPU never suspends.
See https://forums.raspberrypi.com/viewtopic.php?p=2233908 for more
details.
The fix in this comment adds a custom "nowait" variant mutex that doesn't
do WFE/SEV, and uses this one for PendSV. This will use more power when
there's contention for the PendSV mutex as the other core will spin, but
this shouldn't happen very often.
This work was funded through GitHub Sponsors.
Signed-off-by: Angus Gratton <angus@redyak.com.au>
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>
