Re: [PATCH -v5 05/17] x86/ftrace: Use text_poke()

[Steven Rostedt]

On Tue, 12 Nov 2019 23:24:13 +0100
Peter Zijlstra <peterz@xxxxxxxxxxxxx> 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@xxxxxxxxxxxxx> 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