[patch 20/29] x86: mark read_crX() asm code as volatile

[Greg Kroah-Hartman]

2.6.23-stable review patch.  If anyone has any objections, please let us
know.

------------------
From: Kirill Korotaev <dev@xxxxxxxxxx>

patch c1217a75ea102d4e69321f210fab60bc47b9a48e in mainline.

x86: mark read_crX() asm code as volatile

Some gcc versions (I checked at least 4.1.1 from RHEL5 & 4.1.2 from gentoo)
can generate incorrect code with read_crX()/write_crX() functions mix up,
due to cached results of read_crX().

The small app for x8664 below compiled with -O2 demonstrates this
(i686 does the same thing):

Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Signed-off-by: Ingo Molnar <mingo@xxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxx>

---
 include/asm-i386/system.h   |    2 +-
 include/asm-x86_64/system.h |    8 ++++----
 2 files changed, 5 insertions(+), 5 deletions(-)

--- a/include/asm-i386/system.h
+++ b/include/asm-i386/system.h
@@ -141,7 +141,7 @@ static inline unsigned long native_read_
 {
 	unsigned long val;
 	/* This could fault if %cr4 does not exist */
-	asm("1: movl %%cr4, %0		\n"
+	asm volatile("1: movl %%cr4, %0		\n"
 		"2:				\n"
 		".section __ex_table,\"a\"	\n"
 		".long 1b,2b			\n"
--- a/include/asm-x86_64/system.h
+++ b/include/asm-x86_64/system.h
@@ -85,7 +85,7 @@ static inline void write_cr0(unsigned lo
 static inline unsigned long read_cr2(void)
 {
 	unsigned long cr2;
-	asm("movq %%cr2,%0" : "=r" (cr2));
+	asm volatile("movq %%cr2,%0" : "=r" (cr2));
 	return cr2;
 }
 
@@ -97,7 +97,7 @@ static inline void write_cr2(unsigned lo
 static inline unsigned long read_cr3(void)
 { 
 	unsigned long cr3;
-	asm("movq %%cr3,%0" : "=r" (cr3));
+	asm volatile("movq %%cr3,%0" : "=r" (cr3));
 	return cr3;
 }
 
@@ -109,7 +109,7 @@ static inline void write_cr3(unsigned lo
 static inline unsigned long read_cr4(void)
 { 
 	unsigned long cr4;
-	asm("movq %%cr4,%0" : "=r" (cr4));
+	asm volatile("movq %%cr4,%0" : "=r" (cr4));
 	return cr4;
 }
 
@@ -121,7 +121,7 @@ static inline void write_cr4(unsigned lo
 static inline unsigned long read_cr8(void)
 {
 	unsigned long cr8;
-	asm("movq %%cr8,%0" : "=r" (cr8));
+	asm volatile("movq %%cr8,%0" : "=r" (cr8));
 	return cr8;
 }
 

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