fence_wait returned with error -512?

[George Mitchell]

Any idea what this means?

Jun  9 19:57:43 haymarket kernel: fence_wait returned with error -512
Jun  9 19:59:52 haymarket kernel: fence_wait returned with error -512
Jun  9 20:00:40 haymarket kernel: fence_wait returned with error -512
Jun  9 20:11:30 haymarket last message repeated 19 times
Jun  9 20:21:46 haymarket last message repeated 14 times
Jun  9 20:29:30 haymarket last message repeated 13 times
Jun  9 20:55:28 haymarket kernel: fence_wait returned with error -512
Jun  9 21:19:44 haymarket kernel: fence_wait returned with error -512
Jun  9 21:23:20 haymarket last message repeated 2 times
Jun  9 21:39:23 haymarket last message repeated 3 times
Jun  9 21:44:28 haymarket last message repeated 5 times
Jun  9 21:59:42 haymarket last message repeated 5 times
Jun  9 22:09:16 haymarket last message repeated 4 times
Jun  9 22:18:57 haymarket last message repeated 8 times
Jun  9 23:47:45 haymarket kernel: fence_wait returned with error -512
Jun 10 05:50:07 haymarket kernel: fence_wait returned with error -512
Jun 10 05:54:42 haymarket kernel: fence_wait returned with error -512
Jun 10 07:10:39 haymarket kernel: fence_wait returned with error -512

FreeBSD 11.2-RC1 #0 r334470 on an amd64; machine was doing an overnight
build of a bunch of ports.  No obvious sign of distress.    -- George

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