[Bug 201517] Kernel hang on boot with KDTRACE_HOOKS enabled after r284998

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=201517

            Bug ID: 201517
           Summary: Kernel hang on boot with KDTRACE_HOOKS enabled after
                    r284998
           Product: Base System
           Version: 10.1-STABLE
          Hardware: Any
                OS: Any
            Status: New
          Severity: Affects Some People
          Priority: ---
         Component: kern
          Assignee: freebsd-bugs at FreeBSD.org
          Reporter: matthew at reztek.cz

Updating world on a 10-STABLE machine to the point of 10.2-BETA in order to
catch the last OpenSSL update resulted in a kernel that hangs on boot. The hang
occurs approximately when USB buses should be probed. The floppy drive
controller was probed and the drive is still spinning (stuck with light on),
firewire ports are detected, and sometimes but not always the USB ports are
detected, but never gets as far as detecting other disks. Kernel must be booted
without any drm2 drivers loaded in order to see where it hangs. If KMS init has
started the display outputs are still off (no signal, monitor in powersave)
when the kernel hangs.

Bisecting (manually) the commits between the versions and test booting a dozen
or so kernels revealed the lockstat improvements MFHed from r284297 in r284998
are the culprit. The version before that commit is fine, but that exact
revision and any after hang on boot as described. Current revision with that
changeset reversed works ok. Current revision with all changesets works ok if
KDTRACE_HOOKS is disabled, so the offending change must be in one of the
sections that is #ifdef KDTRACE_HOOKS.

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