Re: [PATCH -v5 00/17] Rewrite x86/ftrace to use text_poke (and more)

Masami Hiramatsu
2019-11-26T00:11:12Z

On Mon, 25 Nov 2019 12:32:45 -0500
Steven Rostedt <rostedt@goodmis.org> wrote:

> On Mon, 25 Nov 2019 12:55:34 +0900
> Masami Hiramatsu <mhiramat@kernel.org> wrote:
>
> > void text_poke_bp_batch(struct text_poke_loc *tp, unsigned int nr_entries)
> > ...
> > on_each_cpu(do_sync_core, NULL, 1);
> > /*
> > * sync_core() implies an smp_mb() and orders this store against
> > * the writing of the new instruction.
> > */
> > bp_patching.vec = NULL;
> > bp_patching.nr_entries = 0;
> > }
> > -----
> >
> > I think the "on_each_cpu(do_sync_core, NULL, 1);" can sync the pipeline
> > but doesn't ensure all ongoing int3 handling is done. Thus, we may need a
>
> How does it not ensure all ongoing int3 handling is done? int3 is done
> with interrupts disabled, and the on_each_cpu() requires all CPUs to
> have had their interrupts enabled, thus int3 handling should be
> completed. Perhaps we need another sync core?
>
> on_each_cpu(do_sync_core, NULL, 1);
> bp_patching.nr_entries = 0;
> on_each_cpu(do_sync_core, NULL, 1);
> bp_patching.vec = NULL;

OK, let me check.

The 1st sync_core will ensure the poking "int3" is removed. Thus any
int3-hit address (ip) should NOT match the bp_patching.vec[*].addr after
that. At this point, "if (likely(!bp_patching.nr_entries))" check does not
work.

And the 2nd sync_core will ensure all poke_int3_handler() will see the
bp_patching.nr_entries = 0.
After this point, "if (likely(!bp_patching.nr_entries))" works and
poke_int3_handler() will exit soon. (before touching bp_patching.vec)

So this looks good to me.

Thank you!

--
Masami Hiramatsu <mhiramat@kernel.org>