include/linux/compiler.h | 33 +++++++++++++++++----------------
 1 file changed, 17 insertions(+), 16 deletions(-)

diff --git a/include/linux/compiler.h b/include/linux/compiler.h
index 5e88e7e33abe..8b4282194f16 100644
--- a/include/linux/compiler.h
+++ b/include/linux/compiler.h
@@ -179,18 +179,18 @@ void ftrace_likely_update(struct ftrace_likely_data *f, int val,
 
 #include <uapi/linux/types.h>
 
-#define __READ_ONCE_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;		\
-	case 8: *(__u64 *)res = *(volatile __u64 *)p; break;		\
-	default:							\
-		barrier();						\
-		__builtin_memcpy((void *)res, (const void *)p, size);	\
-		barrier();						\
-	}								\
+/* "unsigned long" or "unsigned long long" - make it fit in a register if possible */
+#define __READ_ONCE_TYPE(size) \
+	__typeof__(__builtin_choose_expr(size > sizeof(0UL), 0ULL, 0UL))
+
+#define __READ_ONCE_SIZE							\
+({										\
+	switch (size) {								\
+	case 1: *(unsigned long *)res = *(volatile __u8 *)p; break;		\
+	case 2: *(unsigned long *)res = *(volatile __u16 *)p; break;		\
+	case 4: *(unsigned long *)res = *(volatile __u32 *)p; break;		\
+	case 8: *(unsigned long long *)res = *(volatile __u64 *)p; break;	\
+	}									\
 })
 
 static __always_inline
@@ -258,13 +258,14 @@ static __always_inline void __write_once_size(volatile void *p, void *res, int s
 
 #define __READ_ONCE(x, check)						\
 ({									\
-	union { typeof(x) __val; char __c[1]; } __u;			\
+	__READ_ONCE_TYPE(sizeof(x)) __u;				\
+	compiletime_assert(sizeof(x) <= sizeof(__u), "READ_ONCE type");	\
 	if (check)							\
-		__read_once_size(&(x), __u.__c, sizeof(x));		\
+		__read_once_size(&(x), &__u, sizeof(x));		\
 	else								\
-		__read_once_size_nocheck(&(x), __u.__c, sizeof(x));	\
+		__read_once_size_nocheck(&(x), &__u, sizeof(x));	\
 	smp_read_barrier_depends(); /* Enforce dependency ordering from x */ \
-	__u.__val;							\
+	(__typeof__(x))__u;						\
 })
 #define READ_ONCE(x) __READ_ONCE(x, 1)