The memory barrier is not needed here. Declaring poll_msg as volatile
should help with preveting some unwanted optimizations. Most likely
even that is unnecessary.
This came up when testing maxadmin "show servers", which is
surprisingly slow compared to the other commands. This change does
not help because the slowness is caused by the polling loop sleeping.
In a more busy environment the command would probably complete faster.
This should be looked at later.
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