[PATCHv2 02/10] kernel: Provide READ_ONCE and ASSIGN_ONCE

[Christian Borntraeger <borntraeger@de.ibm.com>]

ACCESS_ONCE does not work reliably on non-scalar types. For
example gcc 4.6 and 4.7 might remove the volatile tag for such
accesses during the SRA (scalar replacement of aggregates) step
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=58145)

Let's provide READ_ONCE/ASSIGN_ONCE that will do all accesses via
scalar types.

Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
---
 include/linux/compiler.h | 34 ++++++++++++++++++++++++++++++++++
 1 file changed, 34 insertions(+)

diff --git a/include/linux/compiler.h b/include/linux/compiler.h
index d5ad7b1..0ff01f2 100644
--- a/include/linux/compiler.h
+++ b/include/linux/compiler.h
@@ -186,6 +186,40 @@ void ftrace_likely_update(struct ftrace_branch_data *f, int val, int expect);
 # define __UNIQUE_ID(prefix) __PASTE(__PASTE(__UNIQUE_ID_, prefix), __LINE__)
 #endif
 
+#include <linux/types.h>
+
+static __always_inline void __assign_once_size(volatile void *p, void *res, int size)
+{
+	switch (size) {
+	case 1: *(volatile u8 *)p = *(u8 *)res; break;
+	case 2: *(volatile u16 *)p = *(u16 *)res; break;
+	case 4: *(volatile u32 *)p = *(u32 *)res; break;
+#ifdef CONFIG_64BIT
+	case 8: *(volatile u64 *)p = *(u64 *)res; break;
+#endif
+       }
+}
+
+#define ASSIGN_ONCE(val, p) \
+      ({ typeof(p) __val; __val = val; __assign_once_size(&p, &__val, sizeof(__val)); __val; })
+
+
+static __always_inline void __read_once_size(volatile void *p, void *res, int size)
+{
+	switch (size) {
+	case 1: *(u8 *)res = *(volatile u8 *)p; break;
+	case 2: *(u16 *)res = *(volatile u16 *)p; break;
+	case 4: *(u32 *)res = *(volatile u32 *)p; break;
+#ifdef CONFIG_64BIT
+	case 8: *(u64 *)res = *(volatile u64 *)p; break;
+#endif
+       }
+}
+
+#define READ_ONCE(p) \
+      ({ typeof(p) __val; __read_once_size(&p, &__val, sizeof(__val)); __val; })
+
+
 #endif /* __KERNEL__ */
 
 #endif /* __ASSEMBLY__ */
-- 
1.9.3