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

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Message-ID: <Y5DiaT8+eQDLfJcW@dev-arch.thelio-3990X>
Date:   Wed, 7 Dec 2022 11:58:49 -0700
From:   Nathan Chancellor <nathan@...nel.org>
To:     Daniel Jordan <daniel.m.jordan@...cle.com>
Cc:     Masahiro Yamada <masahiroy@...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 Tue, Dec 06, 2022 at 03:15:26PM -0500, Daniel Jordan wrote:
> 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.

Ah, thank you for pointing out __ref, that seems to be exactly what we
want here. I will send a v2 marking padata_work_init() as __ref shortly.

Cheers,
Nathan