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

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Message-ID: <20151028161604.GI18966@arm.com>
Date:	Wed, 28 Oct 2015 16:16:05 +0000
From:	Will Deacon <will.deacon@....com>
To:	Catalin Marinas <catalin.marinas@....com>
Cc:	Vladimir Murzin <vladimir.murzin@....com>,
	Shengjiu Wang <shengjiu.wang@...escale.com>,
	Russell King - ARM Linux <linux@....linux.org.uk>,
	"ard.biesheuvel@...aro.org" <ard.biesheuvel@...aro.org>,
	Punit Agrawal <Punit.Agrawal@....com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"nico@...aro.org" <nico@...aro.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH] ARM: SWP emulation: Restore original *data when failed

On Tue, Oct 27, 2015 at 03:44:24PM +0000, Will Deacon wrote:
> On Fri, Oct 16, 2015 at 11:37:29AM +0100, Catalin Marinas wrote:
> > On Fri, Oct 16, 2015 at 08:52:24AM +0100, Vladimir Murzin wrote:
> > > > On 15/10/15 14:02, Will Deacon wrote:
> > > >> diff --git a/arch/arm64/kernel/armv8_deprecated.c b/arch/arm64/kernel/armv8_deprecated.c
> > > >> index bcee7abac68e..6039d1eb5912 100644
> > > >> --- a/arch/arm64/kernel/armv8_deprecated.c
> > > >> +++ b/arch/arm64/kernel/armv8_deprecated.c
> > > >> @@ -284,12 +284,12 @@ static void register_insn_emulation_sysctl(struct ctl_table *table)
> > > >>  	__asm__ __volatile__(					\
> > > >>  	ALTERNATIVE("nop", SET_PSTATE_PAN(0), ARM64_HAS_PAN,	\
> > > >>  		    CONFIG_ARM64_PAN)				\
> > > >> -	"	mov		%w2, %w1\n"			\
> > > >> -	"0:	ldxr"B"		%w1, [%3]\n"			\
> > > >> -	"1:	stxr"B"		%w0, %w2, [%3]\n"		\
> > > >> +	"0:	ldxr"B"		%w2, [%3]\n"			\
> > > >> +	"1:	stxr"B"		%w0, %w1, [%3]\n"		\
> > > >>  	"	cbz		%w0, 2f\n"			\
> > > >>  	"	mov		%w0, %w4\n"			\
> > > >>  	"2:\n"							\
> > > >> +	"	mov		%w1, %w2\n"			\
> > > >>  	"	.pushsection	 .fixup,\"ax\"\n"		\
> > > >>  	"	.align		2\n"				\
> > > >>  	"3:	mov		%w0, %w5\n"			\
> > > 
> > > On the second thought looks like we still update *data in case stxr
> > > fails (or I need more coffee).
> > 
> > I'm on the second cup and I see the same problem. Even if stxr fails, we
> > fall back through "mov %w1, %w2", so *data is always updated with the
> > loaded value. Maybe something like below on top of Will's patch:
> 
> Yeah, sorry, my original patch was an untested mess. I think we can avoid
> the "cc" clobber by adding a branch to the slow-path, as below.
> 
> Still needs testing, mind.

Right, it passes the simple test case I wrote (which fails under mainline).

Will
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