[Bug 215686] (Possibly theoretical) race condition in kern.devstat.all sysctl

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=215686

            Bug ID: 215686
           Summary: (Possibly theoretical) race condition in
                    kern.devstat.all sysctl
           Product: Base System
           Version: CURRENT
          Hardware: Any
                OS: Any
            Status: New
          Severity: Affects Many People
          Priority: ---
         Component: kern
          Assignee: freebsd-bugs at FreeBSD.org
          Reporter: shea at shealevy.com

The code path which copies out devstat nodes to the sysctl req does not use the
necessary barriers to make the 'sequence0' and 'sequence1' fields meaningful.
For any given device, it's possible that matching sequence numbers would be
exported to userspace while the rest of the structure is otherwise
inconsistent.

>From my reading of devstat_start_transaction and devstat_end_transaction, we
want to ensure that none of the field reads are reordered to be before the read
of sequence0 and none are reordered to be after the read of sequence1, thus
ensuring that if the sequence numbers match, the data in the structure is a
consistent snapshot of some point in time.

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