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):

1 files changed, 4 insertions, 4 deletions

diff --git a/include/asm-x86/system_64.h b/include/asm-x86/system_64.h
index 5022aecc333d..fb4bcf99e665 100644
--- a/include/asm-x86/system_64.h
+++ b/include/asm-x86/system_64.h
@@ -85,7 +85,7 @@ static inline void write_cr0(unsigned long val)
 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 long val)
 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 long val)
 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 long val)
 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;
 }