Subject: -mprofile-kernel vs. notrace in ppc64(le) Linux kernels

From: Torsten Duwe <[email-masked]>
Date: Sat, 26 Sep 2015 16:30:08 +0200

As I mentioned earlier this year, it's a bad idea to call _mcount from
MMU helper functions (e.g. hash_page...), when the profiling/tracing/
live-patching/whatever framewok might in turn cause another such fault.
Jikos suggested to use fine-grained control of these functions with the
"notrace" keyword in the Linux kernel. It is mapped to GCC's (4.8, FWIW)
__attribute__((no_instrument_function)), which, to my surprise, works
for -p and -pg nicely, but does not affect -mprofile-kernel at all!

Should we consider this a bug? IMHO it is. I can see in the GCC sources
that -mprofile-kernel is more like a low-level hack in rs6000.c,
quite far below the RTL code generator, so the no_instrument_function
attribute is probably hard to check for.

What is -mprofile-kernel good for, if it bears such a risk of crashing
the kernel? Is it the right hook for ppc64 live patching? How to protect
those critical functions? Filter -mprofile-kernel for those object files?
Ask the GCC experts to fix this?

Any ideas welcome!

	Torsten