linux-kernel - Re: [PATCH] s390: add unreachable() to dump_fault

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Date:   Fri, 17 May 2019 09:06:26 +0200
From:   Martin Schwidefsky <schwidefsky@...ibm.com>
To:     Masahiro Yamada <yamada.masahiro@...ionext.com>
Cc:     Heiko Carstens <heiko.carstens@...ibm.com>,
        linux-s390@...r.kernel.org, Arnd Bergmann <arnd@...db.de>,
        Claudio Imbrenda <imbrenda@...ux.vnet.ibm.com>,
        linux-kernel@...r.kernel.org,
        Gerald Schaefer <gerald.schaefer@...ibm.com>,
        Souptick Joarder <jrdr.linux@...il.com>,
        Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        Philipp Rudo <prudo@...ux.ibm.com>
Subject: Re: [PATCH] s390: add unreachable() to dump_fault_info() to fix
 -Wmaybe-uninitialized

On Fri, 17 May 2019 15:49:22 +0900
Masahiro Yamada <yamada.masahiro@...ionext.com> wrote:

> When CONFIG_OPTIMIZE_INLINING is enabled for s390, I see this warning:
> 
> arch/s390/mm/fault.c:127:15: warning: 'asce' may be used uninitialized in this function [-Wmaybe-uninitialized]
>   switch (asce & _ASCE_TYPE_MASK) {
> arch/s390/mm/fault.c:177:16: note: 'asce' was declared here
>   unsigned long asce;
>                 ^~~~
> 
> If get_fault_type() is not inlined, the compiler cannot deduce that
> all the possible paths in the 'switch' statement are covered.
> 
> Of course, we could mark get_fault_type() as __always_inline to get
> back the original behavior, but I do not think it sensible to force
> inlining just for the purpose of suppressing the warning. Since this
> is just a matter of warning, I want to keep as much room for compiler
> optimization as possible.
> 
> I added unreachable() to teach the compiler that the 'default' label
> is unreachable.
> 
> I got rid of the 'inline' marker. Even without the 'inline' hint,
> the compiler inlines functions based on its inlining heuristic.
> 
> Fixes: 9012d011660e ("compiler: allow all arches to enable CONFIG_OPTIMIZE_INLINING")
> Signed-off-by: Masahiro Yamada <yamada.masahiro@...ionext.com>

Added to our internal tree and I will add it to s390/linux soon. Thanks.

> ---
> 
>  arch/s390/mm/fault.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/s390/mm/fault.c b/arch/s390/mm/fault.c
> index c220399ae196..91ce03fd0c84 100644
> --- a/arch/s390/mm/fault.c
> +++ b/arch/s390/mm/fault.c
> @@ -85,7 +85,7 @@ static inline int notify_page_fault(struct pt_regs *regs)
>   * Find out which address space caused the exception.
>   * Access register mode is impossible, ignore space == 3.
>   */
> -static inline enum fault_type get_fault_type(struct pt_regs *regs)
> +static enum fault_type get_fault_type(struct pt_regs *regs)
>  {
>  	unsigned long trans_exc_code;
> 
> @@ -211,6 +211,8 @@ static void dump_fault_info(struct pt_regs *regs)
>  		asce = S390_lowcore.kernel_asce;
>  		pr_cont("kernel ");
>  		break;
> +	default:
> +		unreachable();
>  	}
>  	pr_cont("ASCE.\n");
>  	dump_pagetable(asce, regs->int_parm_long & __FAIL_ADDR_MASK);


-- 
blue skies,
   Martin.

"Reality continues to ruin my life." - Calvin.