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Date:   Tue, 26 Mar 2019 15:58:43 +1000
From:   Nicholas Piggin <npiggin@gmail.com>
Subject: Re: [PATCH v2]  arch/powerpc: Rework local_paca to avoid LTO warnings
To:     Alastair D'Silva <alastair@au1.ibm.com>, alastair@d-silva.org
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        Benjamin Herrenschmidt <benh@kernel.crashing.org>,
        Christophe Leroy <christophe.leroy@c-s.fr>,
        linux-kernel@vger.kernel.org, linuxppc-dev@lists.ozlabs.org,
        Mahesh Salgaonkar <mahesh@linux.vnet.ibm.com>,
        Michal Hocko <mhocko@suse.com>,
        Michael Ellerman <mpe@ellerman.id.au>,
        "Naveen N. Rao" <naveen.n.rao@linux.vnet.ibm.com>,
        Paul Mackerras <paulus@samba.org>,
        Mike Rapoport <rppt@linux.vnet.ibm.com>
References: <20190313034208.13134-1-alastair@au1.ibm.com>
        <20190314023125.10076-1-alastair@au1.ibm.com>
In-Reply-To: <20190314023125.10076-1-alastair@au1.ibm.com>
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Alastair D'Silva's on March 14, 2019 12:31 pm:
> From: Alastair D'Silva <alastair@d-silva.org>
>=20
> When building an LTO kernel, the existing code generates warnings:
>     ./arch/powerpc/include/asm/paca.h:37:30: warning: register of
>         =E2=80=98local_paca=E2=80=99 used for multiple global register va=
riables
>      register struct paca_struct *local_paca asm("r13");
>                                   ^
>     ./arch/powerpc/include/asm/paca.h:37:30: note: conflicts with
>         =E2=80=98local_paca=E2=80=99

Isn't this a bogus warning? It doesn't look like there's a way to=20
define it any other way.

>=20
> This patch reworks local_paca into an inline getter & setter function,
> which addresses the warning.
>=20
> Changelog:
> V2
>   - Address whitespace issues
>   - keep new implementation close to where the old implementation was
>=20
> Signed-off-by: Alastair D'Silva <alastair@d-silva.org>
> ---
>  arch/powerpc/include/asm/paca.h | 37 +++++++++++++++++++++++++--------
>  arch/powerpc/kernel/paca.c      |  2 +-
>  2 files changed, 29 insertions(+), 10 deletions(-)
>=20
> diff --git a/arch/powerpc/include/asm/paca.h b/arch/powerpc/include/asm/p=
aca.h
> index e843bc5d1a0f..2fa0b43357c9 100644
> --- a/arch/powerpc/include/asm/paca.h
> +++ b/arch/powerpc/include/asm/paca.h
> @@ -34,19 +34,38 @@
>  #include <asm/cpuidle.h>
>  #include <asm/atomic.h>
> =20
> -register struct paca_struct *local_paca asm("r13");
> -
>  #if defined(CONFIG_DEBUG_PREEMPT) && defined(CONFIG_SMP)
>  extern unsigned int debug_smp_processor_id(void); /* from linux/smp.h */
> -/*
> - * Add standard checks that preemption cannot occur when using get_paca(=
):
> - * otherwise the paca_struct it points to may be the wrong one just afte=
r.
> - */
> -#define get_paca()	((void) debug_smp_processor_id(), local_paca)
> -#else
> -#define get_paca()	local_paca
>  #endif
> =20
> +static inline struct paca_struct *get_paca_no_preempt_check(void)
> +{
> +	register struct paca_struct *paca asm("r13");
> +
> +	return paca;
> +}

Problem is it now changes the global register variable to a local=20
register variable. The compiler would presumably be within its rights
to "cache" that return value or use another register for it, which
is not really what we want.

Thanks,
Nick

=