These did not cooperate with the hash-locked scheme of the other
atomic operations, with the effect that, for instance, a typical
naive spin lock based on atomic_*,
volatile unsigned locked = 0;
lock()
{
while (atomic_swap_uint(&locked, 1))
continue;
membar_acquire();
}
unlock()
{
membar_release();
atomic_store_relaxed(&locked, 0);
}
would fail to achieve mutual exclusion.
For this case, we need to use atomic_swap_* (or, for 8- or 16-bit
objects, atomic_cas_32 loops, since there is no atomic_swap_8 or
atomic_swap_16).
The new machine/types.h macro __HAVE_HASHLOCKED_ATOMICS says whether
these contortions are necessary.
Note that this _requires_ the use of atomic_store_*(p, v), not
regular stores *p = v, to work with the r/m/w atomic operations.
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