linux-kernel - Re: [PATCH 1/1] arch/powerpc: Rework local

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Message-ID: <87ef7atjnk.fsf@dja-thinkpad.axtens.net>
Date:   Thu, 14 Mar 2019 10:54:39 +1100
From:   Daniel Axtens <dja@...ens.net>
To:     Alastair D'Silva <alastair@....ibm.com>
Cc:     Alastair D'Silva <alastair@...ilva.org>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        Nicholas Piggin <npiggin@...il.com>,
        Mahesh Salgaonkar <mahesh@...ux.vnet.ibm.com>,
        "Naveen N. Rao" <naveen.n.rao@...ux.vnet.ibm.com>,
        Christophe Leroy <christophe.leroy@....fr>,
        Michal Hocko <mhocko@...e.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Mike Rapoport <rppt@...ux.vnet.ibm.com>,
        linuxppc-dev@...ts.ozlabs.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/1] arch/powerpc: Rework local_paca to avoid LTO warnings

"Alastair D'Silva" <alastair@....ibm.com> writes:

> From: Alastair D'Silva <alastair@...ilva.org>
>
> When building an LTO kernel, the existing code generates warnings:
>     ./arch/powerpc/include/asm/paca.h:37:30: warning: register of
>         ‘local_paca’ used for multiple global register variables
>      register struct paca_struct *local_paca asm("r13");
>                               ^
>     ./arch/powerpc/include/asm/paca.h:37:30: note: conflicts with
>         ‘local_paca’
>
> This patch reworks local_paca into an inline getter & setter function,
> which addresses the warning.
>
> Generated ASM from this patch is broadly similar (addresses have
> changed and the compiler uses different GPRs in some places).

Ditto to Christophe's comment; I'd love to know how to build this so I
can actually see the differences. Perhaps you could bundle up all the
required changes and send it as a patch series with a cover letter
explaining this?

> +static inline struct paca_struct *get_paca_no_preempt_check(void)
> +{
> +	register struct paca_struct *paca asm("r13");
> +	return paca;
> +}

Isn't the convention to have the { on the same line as the function, or
am I horrible mis-remembering things?

Should these functions be __always_inline?

Regards,
Daniel

> +
> +static inline struct paca_struct *get_paca(void)
> +{
> +#if defined(CONFIG_DEBUG_PREEMPT) && defined(CONFIG_SMP)
> +	/*
> +	 * Add standard checks that preemption cannot occur when using get_paca():
> +	 * otherwise the paca_struct it points to may be the wrong one just after.
> +	 */
> +	debug_smp_processor_id();
> +#endif
> +	return get_paca_no_preempt_check();
> +}
> +
> +#define local_paca	get_paca_no_preempt_check()
> +
> +static inline void set_paca(struct paca_struct *new)
> +{
> +	register struct paca_struct *paca asm("r13");
> +	paca = new;
> +}
> +
> +
>  extern void copy_mm_to_paca(struct mm_struct *mm);
>  extern struct paca_struct **paca_ptrs;
>  extern void initialise_paca(struct paca_struct *new_paca, int cpu);
> diff --git a/arch/powerpc/kernel/paca.c b/arch/powerpc/kernel/paca.c
> index 913bfca09c4f..ae5c243f9d5a 100644
> --- a/arch/powerpc/kernel/paca.c
> +++ b/arch/powerpc/kernel/paca.c
> @@ -172,7 +172,7 @@ void __init initialise_paca(struct paca_struct *new_paca, int cpu)
>  void setup_paca(struct paca_struct *new_paca)
>  {
>  	/* Setup r13 */
> -	local_paca = new_paca;
> +	set_paca(new_paca);
>  
>  #ifdef CONFIG_PPC_BOOK3E
>  	/* On Book3E, initialize the TLB miss exception frames */
> -- 
> 2.20.1