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

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Message-ID: <20151027154424.GG1689@arm.com>
Date:	Tue, 27 Oct 2015 15:44:24 +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 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.

Will

--->8

diff --git a/arch/arm64/kernel/armv8_deprecated.c b/arch/arm64/kernel/armv8_deprecated.c
index 6039d1eb5912..937f5e58a4d3 100644
--- a/arch/arm64/kernel/armv8_deprecated.c
+++ b/arch/arm64/kernel/armv8_deprecated.c
@@ -288,17 +288,19 @@ static void register_insn_emulation_sysctl(struct ctl_table *table)
 	"1:	stxr"B"		%w0, %w1, [%3]\n"		\
 	"	cbz		%w0, 2f\n"			\
 	"	mov		%w0, %w4\n"			\
+	"	b		3f\n"				\
 	"2:\n"							\
 	"	mov		%w1, %w2\n"			\
+	"3:\n"							\
 	"	.pushsection	 .fixup,\"ax\"\n"		\
 	"	.align		2\n"				\
-	"3:	mov		%w0, %w5\n"			\
-	"	b		2b\n"				\
+	"4:	mov		%w0, %w5\n"			\
+	"	b		3b\n"				\
 	"	.popsection"					\
 	"	.pushsection	 __ex_table,\"a\"\n"		\
 	"	.align		3\n"				\
-	"	.quad		0b, 3b\n"			\
-	"	.quad		1b, 3b\n"			\
+	"	.quad		0b, 4b\n"			\
+	"	.quad		1b, 4b\n"			\
 	"	.popsection\n"					\
 	ALTERNATIVE("nop", SET_PSTATE_PAN(1), ARM64_HAS_PAN,	\
 		CONFIG_ARM64_PAN)				\
--
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