Re: linux-next: build warnings after merge of the tip tree

Masami Hiramatsu
2022-03-23T06:28:53Z

On Tue, 22 Mar 2022 22:42:36 -0400
Steven Rostedt <rostedt@goodmis.org> wrote:

> On Wed, 23 Mar 2022 11:23:23 +0900
> Masami Hiramatsu <mhiramat@kernel.org> wrote:
>
> > I see the __fexit__ is needed, but why __ftail__ is needed? I guess because
> > func_B is notrace, in that case the __fexit__ will not be in the func_B.
> > Am I correct?
>
> I believe Peter and I agreed that the "best" solution so far, that has the
> least amount of regressions (doesn't remove anything currently being
> function graph traced, nor removes current tail calls) is:
>
> > At that point giving us something like:
> >
> > 1:
> > pushsection __ftail_loc
> > .long 1b - .
> > popsection
> >
> > jmp.d32 func_B
> > call __fexit__
> > ret
>
>
> Functions with a tail call will not have a __fexit__ and we can not rely on
> the function that is the tail call to do the __fexit__ for the parent
> function. Thus, the compromise is to add a label where the jmp to the
> tail-call function is, and when we want to trace the return of that
> function, we first have to patch the jmp into a call, which will then
> return back to the call to __fexit__.

Got it. So the tail call "jump" will be replaced with a normal call when
we trace it.

That's a good idea :)

>
> -- Steve

--
Masami Hiramatsu <mhiramat@kernel.org>