[Bug 254333] sysctl net.inet.tcp.hostcache.list hangs

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=254333

            Bug ID: 254333
           Summary: sysctl net.inet.tcp.hostcache.list hangs
           Product: Base System
           Version: 11.4-STABLE
          Hardware: amd64
                OS: Any
            Status: New
          Severity: Affects Only Me
          Priority: ---
         Component: kern
          Assignee: bugs at FreeBSD.org
          Reporter: maxim.shalomikhin at kaspersky.com

I'm running 'sysctl net.inet.tcp.hostcache.list' from operator's crontab every
5 minutes for gathering tcp statistics. 

After 3 or 4 months of work sysctl hangs. Kill (and kill -9) doesn't help.
Running one more "sysctl net.inet.tcp.hostcache.list" command gives the same
result - it hangs forever with no output. Other "sysctl SOMETHING" commands
works fine. 

# ps ax| grep sysctl
operator     787    0.0  0.0    4236  1004  -  D    Tue16         2:42.96
/sbin/sysctl net.inet.tcp.hostcache.list
operator    1908    0.0  0.0    4236  1004  -  D    Mon02         2:45.74
/sbin/sysctl net.inet.tcp.hostcache.list
operator    2358    0.0  0.0    4236  1004  -  D    Wed10         1:33.93
/sbin/sysctl net.inet.tcp.hostcache.list
operator    3270    0.0  0.0    4236  1004  -  D    Sun16         2:47.22
/sbin/sysctl net.inet.tcp.hostcache.list


After rebot 'sysctl net.inet.tcp.hostcache.list' works again as expected for
another 3 or 4 months.

The issue is reproducing on about 60 hevy loaded http(s) servers with different
hw specs.

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