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

Steven Rostedt
2019-11-12T22:48:22Z

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