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

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Date:   Tue, 12 Nov 2019 17:48:16 -0500
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Peter Zijlstra <peterz@...radead.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, 12 Nov 2019 23:24:13 +0100
Peter Zijlstra <peterz@...radead.org> 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.

Hmm, I don't think there's a case where ftrace_make_nop() or
ftrace_make_call() ever use the queued function. I added this:

 static int
 ftrace_modify_code_direct(unsigned long ip, const char *old_code,
 			  const char *new_code)
 {
 	int ret = ftrace_verify_code(ip, old_code);
 	if (ret)
 		return ret;
 
 	/* replace the text with the new text */
- 	if (ftrace_poke_late)
+ 	if (ftrace_poke_late) {
+		printk("POKE LATE!\n");
 		text_poke_queue((void *)ip, new_code, MCOUNT_INSN_SIZE, NULL);
- 	else
+ 	} else
 		text_poke_early((void *)ip, new_code, MCOUNT_INSN_SIZE);
 	return 0;
 }

And that printk() never printed, even after running the ftracetests.

> 
> But sure, I can try and dig that out again and write a comment.
> 
> 
> > > +	text_poke_bp((void *)ip, new, MCOUNT_INSN_SIZE, NULL); // BATCH  
> > 
> > What do you mean by "BATCH" ?  
> 
> Ah, that was a question, can/should we use text_poke_queue() there?

Probably not, as that's just a single location or two. It modifies the
static trampolines if only a single user is registered to ftrace.

-- Steve