Umm, what non-atomic state could the bit be in? Half-set, half-cleared? Or do you think that if some other bit in proximity is changed, the other bit would temporarily flip 0->1->0, or something like that? I don't think that should happen.
Since the lock is not held, the bit can be flipped while we are reading, isn't it ? IOW, the test is not reliable is what I fear.
