Subject: [PATCH] ARM: SWP emulation: Restore original *data when failed

From: [email-masked] (Will Deacon)
Date: Thu, 15 Oct 2015 14:02:47 +0100

On Thu, Oct 15, 2015 at 10:17:47AM +0100, Vladimir Murzin wrote:
> We might need the same change for arm64 counterpart (see
> arch/arm64/kernel/armv8_deprecated.c).

Something like below?

Will

>>From 63c3e83073cfac2e011adf0ed6f335275cc977a7 Mon Sep 17 00:00:00 2001
From: Will Deacon <[email-masked]>
Date: Thu, 15 Oct 2015 13:55:53 +0100
Subject: [PATCH] arm64: compat: fix stxr failure case in SWP emulation

If the STXR instruction fails in the SWP emulation code, we leave *data
overwritten with the loaded value, therefore corrupting the data written
by a subsequent, successful attempt.

This patch re-jigs the code so that we only write back to *data once we
know that the update has happened.

Reported-by: Shengjiu Wang <[email-masked]>
Reported-by: Vladimir Murzin <[email-masked]>
Signed-off-by: Will Deacon <[email-masked]>
---
 arch/arm64/kernel/armv8_deprecated.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

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"			\
-- 
2.1.4