linux-kernel - Re: [PATCH -v5 05/17] x86/ftrace: Use text

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Message-ID: <20191113085354.GE4131@hirez.programming.kicks-ass.net>
Date:   Wed, 13 Nov 2019 09:53:54 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     x86@...nel.org, linux-kernel@...r.kernel.org, mhiramat@...nel.org,
        bristot@...hat.com, jbaron@...mai.com,
        torvalds@...ux-foundation.org, tglx@...utronix.de,
        mingo@...nel.org, namit@...are.com, hpa@...or.com, luto@...nel.org,
        ard.biesheuvel@...aro.org, jpoimboe@...hat.com, jeyu@...nel.org,
        alexei.starovoitov@...il.com
Subject: Re: [PATCH -v5 05/17] x86/ftrace: Use text_poke()

On Tue, Nov 12, 2019 at 11:24:13PM +0100, Peter Zijlstra wrote:
> On Tue, Nov 12, 2019 at 01:25:36PM -0500, Steven Rostedt wrote:
> > On Mon, 11 Nov 2019 14:12:57 +0100
> > Peter Zijlstra <peterz@...radead.org> wrote:
> 
> > >  int ftrace_arch_code_modify_post_process(void)
> > >      __releases(&text_mutex)
> > >  {
> > > -	set_all_modules_text_ro();
> > > -	set_kernel_text_ro();
> > > +	text_poke_finish();
> > 
> > Why is the text_poke_finish() needed here? Can we add a comment about
> > why?
> 
> I think this is because of the text_poke_queue() in
> ftrace_modify_code_direct(). I seem to have forgotten the code-flow
> between the core and arch parts of ftrace again.
> 
> But sure, I can try and dig that out again and write a comment.

These are the two callgraphs:

  ftrace_module_enable()
    ftrace_arch_code_modify_prepare()
      do_for_each_ftrace_rec()
	__ftrace_replace_code()
	  ftrace_make_{call,nop}()
	    ftrace_modify_code_direct()
	      text_poke_queue()

    ftrace_arch_code_modify_post_process()
      text_poke_finish();


  ftrace_run_update_code()
    ftrace_arch_code_modify_prepare()
    arch_ftrace_update_code()
      ftrace_modify_all_code()
	ftrace_replace_code()
	  for_ftrace_rec_iter()
	    text_poke_queue()
	  text_poke_finish()
    ftrace_arch_code_modify_post_process()
      text_poke_finish()


So while it is superfluous for ftrace_run_update_code() it is required
for ftrace_module_enable(), and, as I said, pairs with the
text_poke_queue() in ftrace_modify_code_direct().

I'll stick in a comment.