How does a ros node handle multiple subscribers?

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Does the node deal with these subscribers (or publishers) sequentially (i.e., first comes, first serves), or does it parallelize them? This question just popped out from my head while I was dealing with callbacks of multiple subscribers which try to access and modify same variable simultaneously and I'm not sure about this even after reading http://wiki.ros.org/Nodes. I appreciate if someone can explain to me.