READ_ONCE: Simplify implementations of {READ,WRITE}_ONCE()

The implementations of {READ,WRITE}_ONCE() suffer from a significant
amount of indirection and complexity due to a historic GCC bug:

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=58145

which was originally worked around by 230fa253 ("kernel: Provide
READ_ONCE and ASSIGN_ONCE").

Since GCC 4.8 is fairly vintage at this point and we emit a warning if
we detect it during the build, return {READ,WRITE}_ONCE() to their former
glory with an implementation that is easier to understand and, crucially,
more amenable to optimisation. A side effect of this simplification is
that WRITE_ONCE() no longer returns a value, but nobody seems to be
relying on that and the new behaviour is aligned with smp_store_release().
Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Michael Ellerman <mpe@ellerman.id.au>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Christian Borntraeger <borntraeger@de.ibm.com>
Signed-off-by: Will Deacon <will@kernel.org>

include/linux/compiler.h

@@ -177,60 +177,6 @@ void ftrace_likely_update(struct ftrace_likely_data *f, int val,
 # define __UNIQUE_ID(prefix) __PASTE(__PASTE(__UNIQUE_ID_, prefix), __LINE__)
 #endif
 
-#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();						\
-	}								\
-})
-
-static __always_inline
-void __read_once_size(const volatile void *p, void *res, int size)
-{
-	__READ_ONCE_SIZE;
-}
-
-#ifdef CONFIG_KASAN
-/*
- * We can't declare function 'inline' because __no_sanitize_address confilcts
- * with inlining. Attempt to inline it may cause a build failure.
- * 	https://gcc.gnu.org/bugzilla/show_bug.cgi?id=67368
- * '__maybe_unused' allows us to avoid defined-but-not-used warnings.
- */
-# define __no_kasan_or_inline __no_sanitize_address notrace __maybe_unused
-#else
-# define __no_kasan_or_inline __always_inline
-#endif
-
-static __no_kasan_or_inline
-void __read_once_size_nocheck(const volatile void *p, void *res, int size)
-{
-	__READ_ONCE_SIZE;
-}
-
-static __always_inline void __write_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;
-	case 8: *(volatile __u64 *)p = *(__u64 *)res; break;
-	default:
-		barrier();
-		__builtin_memcpy((void *)p, (const void *)res, size);
-		barrier();
-	}
-}
-
 /*
  * Prevent the compiler from merging or refetching reads or writes. The
  * compiler is also forbidden from reordering successive instances of
@@ -240,11 +186,7 @@ static __always_inline void __write_once_size(volatile void *p, void *res, int s
  * statements.
  *
  * These two macros will also work on aggregate data types like structs or
- * unions. If the size of the accessed data type exceeds the word size of
- * the machine (e.g., 32 bits or 64 bits) READ_ONCE() and WRITE_ONCE() will
- * fall back to memcpy(). There's at least two memcpy()s: one for the
- * __builtin_memcpy() and then one for the macro doing the copy of variable
- * - '__u' allocated on the stack.
+ * unions.
  *
  * Their two major use cases are: (1) Mediating communication between
  * process-level code and irq/NMI handlers, all running on the same CPU,
@@ -256,23 +198,49 @@ static __always_inline void __write_once_size(volatile void *p, void *res, int s
 #include <asm/barrier.h>
 #include <linux/kasan-checks.h>
 
-#define __READ_ONCE(x, check)						\
+#define __READ_ONCE(x)	(*(volatile typeof(x) *)&(x))
+
+#define READ_ONCE(x)							\
 ({									\
-	union { typeof(x) __val; char __c[1]; } __u;			\
-	if (check)							\
-		__read_once_size(&(x), __u.__c, sizeof(x));		\
-	else								\
-		__read_once_size_nocheck(&(x), __u.__c, sizeof(x));	\
-	smp_read_barrier_depends(); /* Enforce dependency ordering from x */ \
-	__u.__val;							\
+	typeof(x) __x = __READ_ONCE(x);					\
+	smp_read_barrier_depends();					\
+	__x;								\
 })
-#define READ_ONCE(x) __READ_ONCE(x, 1)
+
+#define WRITE_ONCE(x, val)				\
+do {							\
+	*(volatile typeof(x) *)&(x) = (val);		\
+} while (0)
+
+#ifdef CONFIG_KASAN
+/*
+ * We can't declare function 'inline' because __no_sanitize_address conflicts
+ * with inlining. Attempt to inline it may cause a build failure.
+ *     https://gcc.gnu.org/bugzilla/show_bug.cgi?id=67368
+ * '__maybe_unused' allows us to avoid defined-but-not-used warnings.
+ */
+# define __no_kasan_or_inline __no_sanitize_address notrace __maybe_unused
+#else
+# define __no_kasan_or_inline __always_inline
+#endif
+
+static __no_kasan_or_inline
+unsigned long __read_once_word_nocheck(const void *addr)
+{
+	return __READ_ONCE(*(unsigned long *)addr);
+}
 
 /*
- * Use READ_ONCE_NOCHECK() instead of READ_ONCE() if you need
- * to hide memory access from KASAN.
+ * Use READ_ONCE_NOCHECK() instead of READ_ONCE() if you need to load a
+ * word from memory atomically but without telling KASAN. This is usually
+ * used by unwinding code when walking the stack of a running process.
  */
-#define READ_ONCE_NOCHECK(x) __READ_ONCE(x, 0)
+#define READ_ONCE_NOCHECK(x)						\
+({									\
+	unsigned long __x = __read_once_word_nocheck(&(x));		\
+	smp_read_barrier_depends();					\
+	__x;								\
+})
 
 static __no_kasan_or_inline
 unsigned long read_word_at_a_time(const void *addr)
@@ -281,14 +249,6 @@ unsigned long read_word_at_a_time(const void *addr)
 	return *(unsigned long *)addr;
 }
 
-#define WRITE_ONCE(x, val) \
-({							\
-	union { typeof(x) __val; char __c[1]; } __u =	\
-		{ .__val = (__force typeof(x)) (val) }; \
-	__write_once_size(&(x), __u.__c, sizeof(x));	\
-	__u.__val;					\
-})
-
 #endif /* __KERNEL__ */
 
 /*