linux-kernel - Re: [PATCH] ARM: SWP emulation: Restore original *data when failed

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Message-ID: <20151015082417.GX32532@n2100.arm.linux.org.uk>
Date:	Thu, 15 Oct 2015 09:24:17 +0100
From:	Russell King - ARM Linux <linux@....linux.org.uk>
To:	Shengjiu Wang <shengjiu.wang@...escale.com>
Cc:	nico@...aro.org, ard.biesheuvel@...aro.org,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] ARM: SWP emulation: Restore original *data when failed

On Wed, Oct 14, 2015 at 10:51:17AM +0800, Shengjiu Wang wrote:
> __user_swpX_asm maybe failed in first STREX operation, emulate_swpX
> will try again, but the *data has been changed in first time. which
> cause the result is wrong. So need to recover the *data when failed.
> 
> Signed-off-by: Shengjiu Wang <shengjiu.wang@...escale.com>
> ---
>  arch/arm/kernel/swp_emulate.c | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/arch/arm/kernel/swp_emulate.c b/arch/arm/kernel/swp_emulate.c
> index 5b26e7e..c61fbf92 100644
> --- a/arch/arm/kernel/swp_emulate.c
> +++ b/arch/arm/kernel/swp_emulate.c
> @@ -41,6 +41,7 @@
>  	"1:	strex"B"	%0, %2, [%3]\n"			\
>  	"	cmp		%0, #0\n"			\
>  	"	movne		%0, %4\n"			\
> +	"	movne		%1, %2\n"			\
>  	"2:\n"							\
>  	"	.section	 .text.fixup,\"ax\"\n"		\
>  	"	.align		2\n"				\

I think I'd prefer this to be:

	__asm__ __volatile__(					\
	"0:	ldrex"B"	%2, [%3]\n"			\
	"1:	strex"B"	%0, %1, [%3]\n" 		\
	"	cmp		%0, #0\n"			\
	"	moveq		%1, %2\n"			\
	"	movne		%0, %4\n"			\

so that we're not loading into %1 (an in-out non-temporary) but rather
loading it into a temporary - and only overwriting the saved register
value if the swap succeeds.

Thanks.

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