On Fri, 8 Jul 2016 22:24:55 +0200
Torsten Duwe [off-list ref] wrote:

On Fri, Jul 08, 2016 at 11:57:10AM -0400, Steven Rostedt wrote:

quoted

On Fri, 8 Jul 2016 10:48:24 -0500
Josh Poimboeuf [off-list ref] wrote:  

quoted

Here, with -fprolog-pad, it's already a nop, so no change is needed.
  

Yes, exactly.

quoted

That's what I was thinking. But as I stated in another email (probably
in the air when you wrote this), the call to ftrace_modify_code() may be
completely circumvented by ftrace_make_nop() if the addr is MCOUNT_ADDR.  

Only on the _first_ invocation. Later on, tracing can be switched on and off,
and then the instructions need to be changed just like with fentry (or
profile-kernel ;-)

Understood, but ftrace_modify_code() will only receive addr ==
MCOUNT_ADDR on boot up or when a module is loaded. In both cases, with
-fprolog-pad it will already be a nop, hence no need to call
ftrace_modify_code(), in those cases.

In all other cases, addr will point to a ftrace trampoline.

-- Steve