linux-kernel - Re: [PATCH 1/2] padata: Do not mark padata_mt_helper() as _

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Date:   Tue, 6 Dec 2022 15:15:26 -0500
From:   Daniel Jordan <daniel.m.jordan@...cle.com>
To:     Masahiro Yamada <masahiroy@...nel.org>
Cc:     Nathan Chancellor <nathan@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Tom Rix <trix@...hat.com>, Nicolas Schier <nicolas@...sle.eu>,
        Sami Tolvanen <samitolvanen@...gle.com>,
        Vincent Donnefort <vdonnefort@...gle.com>,
        linux-kbuild@...r.kernel.org, linux-kernel@...r.kernel.org,
        llvm@...ts.linux.dev, patches@...ts.linux.dev,
        Steffen Klassert <steffen.klassert@...unet.com>,
        linux-crypto@...r.kernel.org
Subject: Re: [PATCH 1/2] padata: Do not mark padata_mt_helper() as __init

On Thu, Dec 01, 2022 at 07:35:59AM +0900, Masahiro Yamada wrote:
> On Wed, Nov 30, 2022 at 4:02 AM Nathan Chancellor <nathan@...nel.org> wrote:
> >
> > When building arm64 allmodconfig + ThinLTO with clang and a proposed
> > modpost update to account for -ffuncton-sections, the following warning
> > appears:
> >
> >   WARNING: modpost: vmlinux.o: section mismatch in reference: padata_work_init (section: .text.padata_work_init) -> padata_mt_helper (section: .init.text)
> >   WARNING: modpost: vmlinux.o: section mismatch in reference: padata_work_init (section: .text.padata_work_init) -> padata_mt_helper (section: .init.text)
> >
> > In both cases, an __init function calls padata_work_init(), which is not
> > marked __init, with padata_mt_helper(), another __init function, as a
> > work function argument.
> >
> > padata_work_init() is called from non-init paths, otherwise it could be
> > marked __init to resolve the warning. Instead, remove __init from
> > padata_mt_helper() to resolve the warning.
> >
> > Signed-off-by: Nathan Chancellor <nathan@...nel.org>
> > ---
> > Cc: Steffen Klassert <steffen.klassert@...unet.com>
> > Cc: Daniel Jordan <daniel.m.jordan@...cle.com>
> > Cc: linux-crypto@...r.kernel.org
> > ---
> >  kernel/padata.c | 4 ++--
> >  1 file changed, 2 insertions(+), 2 deletions(-)
> >
> > diff --git a/kernel/padata.c b/kernel/padata.c
> > index e5819bb8bd1d..c2271d7e446d 100644
> > --- a/kernel/padata.c
> > +++ b/kernel/padata.c
> > @@ -45,7 +45,7 @@ struct padata_mt_job_state {
> >  };
> >
> >  static void padata_free_pd(struct parallel_data *pd);
> > -static void __init padata_mt_helper(struct work_struct *work);
> > +static void padata_mt_helper(struct work_struct *work);
> >
> >  static int padata_index_to_cpu(struct parallel_data *pd, int cpu_index)
> >  {
> > @@ -425,7 +425,7 @@ static int padata_setup_cpumasks(struct padata_instance *pinst)
> >         return err;
> >  }
> >
> > -static void __init padata_mt_helper(struct work_struct *w)
> > +static void padata_mt_helper(struct work_struct *w)
> >  {
> >         struct padata_work *pw = container_of(w, struct padata_work, pw_work);
> >         struct padata_mt_job_state *ps = pw->pw_data;
> > --
> > 2.38.1
> >
> 
> This patch seems wrong.
> 
> padata_work_init() does not reference to padata_mt_helper()
> 
> 
> padata_work_alloc_mt() and padata_do_multithreaded() do.

I see LLVM optimizing padata_work_init by embedding padata_mt_helper's
address in its text, which runs afoul of modpost.

I agree with Masahiro, the warning is a false positive since only __init
functions ever cause the embedded address to be used.

We have __ref for situations like this.  That way, padata_mt_helper can
stay properly __init.