- Use isync/lwsync unconditionally for acquire/release. Use of isync
guarantees a complete memory barrier, which is important for serialization
of bus space accesses with mutexes on multi-processor systems.
- Go back to using sync as the I/O memory barrier, which solves the same
problem as above with respect to mutex release using lwsync, while not
penalizing non-I/O operations like a return to sync on the atomic release
operations would.
- Place an acquisition barrier around thread lock acquisition in
cpu_switchin().
sync performs a strict superset of the functions of eieio, so using both
is redundant. While here, expand bus barriers to all bus_space operations,
since many drivers do not correctly use bus_space_barrier().
In principle, we can also replace sync just with eieio, for a significant
performance increase, but it remains to be seen whether any poorly-written
drivers currently depend on the side effects of sync to properly function.
MFC after: 1 week
