linux-kernel - Re: [PATCH v2 cmpxchg 12/13] sh: Emulate one-byte cmpxchg

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Message-ID: <1376850f47279e3a3f4f40e3de2784ae3ac30414.camel@physik.fu-berlin.de>
Date: Thu, 02 May 2024 06:52:53 +0200
From: John Paul Adrian Glaubitz <glaubitz@...sik.fu-berlin.de>
To: "Paul E. McKenney" <paulmck@...nel.org>, linux-arch@...r.kernel.org, 
	linux-kernel@...r.kernel.org
Cc: elver@...gle.com, akpm@...ux-foundation.org, tglx@...utronix.de, 
 peterz@...radead.org, dianders@...omium.org, pmladek@...e.com,
 arnd@...db.de,  torvalds@...ux-foundation.org, kernel-team@...a.com, Andi
 Shyti <andi.shyti@...ux.intel.com>, Palmer Dabbelt <palmer@...osinc.com>,
 Masami Hiramatsu <mhiramat@...nel.org>, linux-sh@...r.kernel.org
Subject: Re: [PATCH v2 cmpxchg 12/13] sh: Emulate one-byte cmpxchg

Hi Paul,

On Wed, 2024-05-01 at 16:01 -0700, Paul E. McKenney wrote:
> Use the new cmpxchg_emu_u8() to emulate one-byte cmpxchg() on sh.
> 
> [ paulmck: Drop two-byte support per Arnd Bergmann feedback. ]
> [ paulmck: Apply feedback from Naresh Kamboju. ]
> [ Apply Geert Uytterhoeven feedback. ]
> 
> Signed-off-by: Paul E. McKenney <paulmck@...nel.org>
> Cc: Andi Shyti <andi.shyti@...ux.intel.com>
> Cc: Palmer Dabbelt <palmer@...osinc.com>
> Cc: Masami Hiramatsu <mhiramat@...nel.org>
> Cc: Arnd Bergmann <arnd@...db.de>
> Cc: <linux-sh@...r.kernel.org>
> ---
>  arch/sh/Kconfig               | 1 +
>  arch/sh/include/asm/cmpxchg.h | 3 +++
>  2 files changed, 4 insertions(+)
> 
> diff --git a/arch/sh/Kconfig b/arch/sh/Kconfig
> index 2ad3e29f0ebec..f47e9ccf4efd2 100644
> --- a/arch/sh/Kconfig
> +++ b/arch/sh/Kconfig
> @@ -16,6 +16,7 @@ config SUPERH
>  	select ARCH_HIBERNATION_POSSIBLE if MMU
>  	select ARCH_MIGHT_HAVE_PC_PARPORT
>  	select ARCH_WANT_IPC_PARSE_VERSION
> +	select ARCH_NEED_CMPXCHG_1_EMU
>  	select CPU_NO_EFFICIENT_FFS
>  	select DMA_DECLARE_COHERENT
>  	select GENERIC_ATOMIC64
> diff --git a/arch/sh/include/asm/cmpxchg.h b/arch/sh/include/asm/cmpxchg.h
> index 5d617b3ef78f7..1e5dc5ccf7bf5 100644
> --- a/arch/sh/include/asm/cmpxchg.h
> +++ b/arch/sh/include/asm/cmpxchg.h
> @@ -9,6 +9,7 @@
>  
>  #include <linux/compiler.h>
>  #include <linux/types.h>
> +#include <linux/cmpxchg-emu.h>
>  
>  #if defined(CONFIG_GUSA_RB)
>  #include <asm/cmpxchg-grb.h>
> @@ -56,6 +57,8 @@ static inline unsigned long __cmpxchg(volatile void * ptr, unsigned long old,
>  		unsigned long new, int size)
>  {
>  	switch (size) {
> +	case 1:
> +		return cmpxchg_emu_u8(ptr, old, new);
>  	case 4:
>  		return __cmpxchg_u32(ptr, old, new);
>  	}

Thanks for the patch. However, I don't quite understand its purpose.

There is already a case for 8-byte cmpxchg in the switch statement below:

        case 1:                                         \
                __xchg__res = xchg_u8(__xchg_ptr, x);   \
                break;

Does cmpxchg_emu_u8() have any advantages over the native xchg_u8()?

Thanks,
Adrian

-- 
 .''`.  John Paul Adrian Glaubitz
: :' :  Debian Developer
`. `'   Physicist
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