From: Nathan Chancellor <nathan@kernel.org>
To: Masahiro Yamada <masahiroy@kernel.org>
Cc: Nick Desaulniers <ndesaulniers@google.com>,
Tom Rix <trix@redhat.com>, Nicolas Schier <nicolas@fjasle.eu>,
Sami Tolvanen <samitolvanen@google.com>,
Vincent Donnefort <vdonnefort@google.com>,
linux-kbuild@vger.kernel.org, linux-kernel@vger.kernel.org,
llvm@lists.linux.dev, patches@lists.linux.dev,
Daniel Jordan <daniel.m.jordan@oracle.com>,
Steffen Klassert <steffen.klassert@secunet.com>,
linux-crypto@vger.kernel.org
Subject: Re: [PATCH v2 1/2] padata: Mark padata_work_init() as __ref
Date: Mon, 12 Dec 2022 10:05:02 -0700 [thread overview]
Message-ID: <Y5dfPgNF8E2EpNCM@dev-arch.thelio-3990X> (raw)
In-Reply-To: <CAK7LNARoxqSzjpM0twcssMkf9X_PppzqtUo_opq=CX+zixma8g@mail.gmail.com>
On Mon, Dec 12, 2022 at 10:07:24PM +0900, Masahiro Yamada wrote:
> On Thu, Dec 8, 2022 at 4:17 AM Nathan Chancellor <nathan@kernel.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)
> >
> > LLVM has optimized padata_work_init() to include the address of
> > padata_mt_helper() directly, which causes modpost to complain since
> > padata_work_init() is not __init, whereas padata_mt_helper() is. In
> > reality, padata_work_init() is only called with padata_mt_helper() as
> > the work_fn argument in code that is __init, so this warning will not
> > result in any problems. Silence it with __ref, which makes it clear to
> > modpost that padata_work_init() can only use padata_mt_helper() in
> > __init code.
> >
> > Suggested-by: Daniel Jordan <daniel.m.jordan@oracle.com>
> > Signed-off-by: Nathan Chancellor <nathan@kernel.org>
> > ---
> > Cc: Steffen Klassert <steffen.klassert@secunet.com>
> > Cc: Daniel Jordan <daniel.m.jordan@oracle.com>
> > Cc: linux-crypto@vger.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..4c3137fe8449 100644
> > --- a/kernel/padata.c
> > +++ b/kernel/padata.c
> > @@ -83,8 +83,8 @@ static struct padata_work *padata_work_alloc(void)
> > return pw;
> > }
> >
> > -static void padata_work_init(struct padata_work *pw, work_func_t work_fn,
> > - void *data, int flags)
> > +static __ref void padata_work_init(struct padata_work *pw, work_func_t work_fn,
> > + void *data, int flags)
> > {
> > if (flags & PADATA_WORK_ONSTACK)
> > INIT_WORK_ONSTACK(&pw->pw_work, work_fn);
> >
> > base-commit: 76dcd734eca23168cb008912c0f69ff408905235
> > --
> > 2.38.1
> >
>
> It took me a while to understand why LTO can embed
> padata_mt_helper's address into padata_work_init().
Sorry about that, I can try to expand on this in both the commit message
and in-code comment if I end up adding it.
> There are 3 call-sites to padata_work_init().
>
> (1) __init padata_work_alloc_mt()
> --> padata_work_init(..., padata_mt_helper, ...)
>
> (2) padata_do_parallel()
> --> padata_work_init(..., padata_parallel_worker, ...)
>
> (3) __init padata_do_multithreaded()
> --> padata_work_init(..., padata_mt_helper, ...)
>
>
> The function call (2) is squashed away.
>
>
> With only (1) and (3) remaining, the 2nd parameter to
> padata_work_init() is always padata_mt_helper,
> therefore LLVM embeds padata_mt_hlper's address
> directly into padata_work_init().
>
> I am not sure if the compiler should do this level of optimization
> because kernel/padata.c does not seem to be a special case.
> Perhaps, we might be hit with more cases that need __ref annotation,
> which is only required by LTO.
That's possible. I did only see this once instance in all my builds but
allmodconfig + ThinLTO might not be too interesting of a case,
since the sanitizers will be enabled, which makes optimization more
difficult. I could try to enable ThinLTO with some distribution
configurations to see if there are any more instances that crop up.
> One note is that, we could discard padata_work_init()
> because (1) and (3) are both annotated as __init.
> So, another way of fixing is
> static __always_inline void padata_work_init(...)
> because the compiler would determine padata_work_init()
> would be small enough if the caller and callee belonged to
> the same section.
>
> I do not have a strong opinion.
> Honestly, I do not know what the best approach would be to fix this.
Agreed to both points, it is really up to the padata maintainers.
> If we go with the __ref annotation, I can pick this, but
> at least can you add some comments?
>
>
> include/linux/init.h says:
> "optimally document why the __ref is needed and why it's OK"
>
>
> I think this is the case that needs some comments
> because LTO optimization looks too tricky to me.
Sure thing, I will send a v3 either Tuesday or Wednesday with an updated
commit message and code comment if we end up going this route.
Thank you for the review!
Cheers,
Nathan
next prev parent reply other threads:[~2022-12-12 17:05 UTC|newest]
Thread overview: 10+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-12-07 19:16 [PATCH v2 0/2] Fix lack of section mismatch warnings with LTO Nathan Chancellor
2022-12-07 19:16 ` [PATCH v2 1/2] padata: Mark padata_work_init() as __ref Nathan Chancellor
2022-12-07 19:56 ` Daniel Jordan
2022-12-12 13:07 ` Masahiro Yamada
2022-12-12 17:05 ` Nathan Chancellor [this message]
2022-12-12 19:21 ` Daniel Jordan
2022-12-12 20:06 ` Nathan Chancellor
2022-12-13 16:48 ` Nathan Chancellor
2022-12-07 19:16 ` [PATCH v2 2/2] modpost: Include '.text.*' in TEXT_SECTIONS Nathan Chancellor
2022-12-12 13:13 ` Masahiro Yamada
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