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_InterlockedExchange_rel() is required for correctness on ARM because the _ReadWriteBarrier() macro is only a compiler memory barrier, not a hardware memory barrier. Due to ARM's relaxed memory model, this means the '*lock = 0' write may be observed before the operations inside the lock, causing possible corruption of data protected by the lock. _InterlockedExchange_acq() is more efficient on ARM because it avoids an expensive full memory barrier that _InterlockedExchange() does.