linux-kernel - Re: [PATCH -v5 00/17] Rewrite x86/ftrace to use text

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Date:   Tue, 26 Nov 2019 09:11:04 +0900
From:   Masami Hiramatsu <mhiramat@...nel.org>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     Peter Zijlstra <peterz@...radead.org>, x86@...nel.org,
        linux-kernel@...r.kernel.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 00/17] Rewrite x86/ftrace to use text_poke (and
 more)

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

> On Mon, 25 Nov 2019 12:55:34 +0900
> Masami Hiramatsu <mhiramat@...nel.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@...nel.org>