linux-kernel - Re: [PATCH] ARM: avoid cmpxchg warning in kernel/acct.c

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Date:	Tue, 2 Jun 2015 11:07:50 +0200
From:	Frans Klaver <fransklaver@...il.com>
To:	Arnd Bergmann <arnd@...db.de>
Cc:	linux-arm-kernel@...ts.infradead.org,
	Russell King - ARM Linux <linux@....linux.org.uk>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Al Viro <viro@...iv.linux.org.uk>
Subject: Re: [PATCH] ARM: avoid cmpxchg warning in kernel/acct.c

On Tue, May 12, 2015 at 11:29 PM, Arnd Bergmann <arnd@...db.de> wrote:
> A recent change in kernel/acct.c added a new warning for many
> configurations on ARM:
>
> kernel/acct.c: In function 'acct_pin_kill':
> arch/arm/include/asm/cmpxchg.h:122:3: warning: value computed is not used [-Wunused-value]
>   ((__typeof__(*(ptr)))__cmpxchg_local_generic((ptr), (unsigned long)(o),\
>
> The code is in fact correct, it's just a cmpxchg() call that intentionally
> ignores the result, and no other code does that. The warning does not
> show up on x86 because of the way that its cmpxchg() macro is written.
>
> This changes the ARM implementation to use a similar construct with a
> compound expression instead of a typecast, which causes the compiler to
> not complain about an unused result.
>
> Signed-off-by: Arnd Bergmann <arnd@...db.de>
> Fixes: 3b994d98a815 ("get rid of the second argument of acct_kill()")
> ---
> This helps build some of the ARM defconfigs without warnings once more.
>
> diff --git a/arch/arm/include/asm/cmpxchg.h b/arch/arm/include/asm/cmpxchg.h
> index abb2c3769b01..04be894f7c4e 100644
> --- a/arch/arm/include/asm/cmpxchg.h
> +++ b/arch/arm/include/asm/cmpxchg.h
> @@ -119,8 +119,8 @@ static inline unsigned long __xchg(unsigned long x, volatile void *ptr, int size
>   * them available.
>   */
>  #define cmpxchg_local(ptr, o, n)                                              \
> -       ((__typeof__(*(ptr)))__cmpxchg_local_generic((ptr), (unsigned long)(o),\
> -                       (unsigned long)(n), sizeof(*(ptr))))
> +       ({(__typeof__(*(ptr)))__cmpxchg_local_generic((ptr), (unsigned long)(o),\
> +                       (unsigned long)(n), sizeof(*(ptr)));})
>  #define cmpxchg64_local(ptr, o, n) __cmpxchg64_local_generic((ptr), (o), (n))
>
>  #ifndef CONFIG_SMP
> @@ -202,10 +202,10 @@ static inline unsigned long __cmpxchg_mb(volatile void *ptr, unsigned long old,
>  }
>
>  #define cmpxchg(ptr,o,n)                                               \
> -       ((__typeof__(*(ptr)))__cmpxchg_mb((ptr),                        \
> +       ({(__typeof__(*(ptr)))__cmpxchg_mb((ptr),                       \
>                                           (unsigned long)(o),           \
>                                           (unsigned long)(n),           \
> -                                         sizeof(*(ptr))))
> +                                         sizeof(*(ptr)));})
>
>  static inline unsigned long __cmpxchg_local(volatile void *ptr,
>                                             unsigned long old,
>
>

Works for me.

Thanks,
Frans
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