Function tracing can trace in NMIs and such. If the TSC is determined
to be unstable, the tracing clock will switch to the global clock on
boot up, unless "trace_clock" is specified on the kernel command line.

The global clock disables interrupts to access sched_clock_cpu(), and in
doing so can be done within lockdep internals (because of function
tracing and NMIs). This can trigger false lockdep splats.

The trace_clock_global() is special, best not to trace the irq logic
within it.

Link: http://lkml.kernel.org/r/20180404145015.77bde42d@gandalf.local.homeSigned-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>