Merge branch 'rwonce/rework' of git://git.kernel.org/pub/scm/linux/kernel/git/will/linux

Pull READ/WRITE_ONCE rework from Will Deacon:
 "This the READ_ONCE rework I've been working on for a while, which
  bumps the minimum GCC version and improves code-gen on arm64 when
  stack protector is enabled"

[ Side note: I'm _really_ tempted to raise the minimum gcc version to
  4.9, so that we can just say that we require _Generic() support.

  That would allow us to more cleanly handle a lot of the cases where we
  depend on very complex macros with 'sizeof' or __builtin_choose_expr()
  with __builtin_types_compatible_p() etc.

  This branch has a workaround for sparse not handling _Generic(),
  either, but that was already fixed in the sparse development branch,
  so it's really just gcc-4.9 that we'd require.   - Linus ]

* 'rwonce/rework' of git://git.kernel.org/pub/scm/linux/kernel/git/will/linux:
  compiler_types.h: Use unoptimized __unqual_scalar_typeof for sparse
  compiler_types.h: Optimize __unqual_scalar_typeof compilation time
  compiler.h: Enforce that READ_ONCE_NOCHECK() access size is sizeof(long)
  compiler-types.h: Include naked type in __pick_integer_type() match
  READ_ONCE: Fix comment describing 2x32-bit atomicity
  gcov: Remove old GCC 3.4 support
  arm64: barrier: Use '__unqual_scalar_typeof' for acquire/release macros
  locking/barriers: Use '__unqual_scalar_typeof' for load-acquire macros
  READ_ONCE: Drop pointer qualifiers when reading from scalar types
  READ_ONCE: Enforce atomicity for {READ,WRITE}_ONCE() memory accesses
  READ_ONCE: Simplify implementations of {READ,WRITE}_ONCE()
  arm64: csum: Disable KASAN for do_csum()
  fault_inject: Don't rely on "return value" from WRITE_ONCE()
  net: tls: Avoid assigning 'const' pointer to non-const pointer
  netfilter: Avoid assigning 'const' pointer to non-const pointer
  compiler/gcc: Raise minimum GCC version for kernel builds to 4.8

1 files changed, 69 insertions, 79 deletions

diff --git a/include/linux/compiler.h b/include/linux/compiler.h
index 6325d64e3c3b..33d3a2e5abab 100644
--- a/include/linux/compiler.h
+++ b/include/linux/compiler.h
@@ -230,60 +230,6 @@ 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
@@ -293,11 +239,7 @@ 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,
@@ -309,23 +251,69 @@ 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)						\
+/*
+ * Use __READ_ONCE() instead of READ_ONCE() if you do not require any
+ * atomicity or dependency ordering guarantees. Note that this may result
+ * in tears!
+ */
+#define __READ_ONCE(x)	(*(const volatile __unqual_scalar_typeof(x) *)&(x))
+
+#define __READ_ONCE_SCALAR(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;							\
+	__unqual_scalar_typeof(x) __x = __READ_ONCE(x);			\
+	smp_read_barrier_depends();					\
+	(typeof(x))__x;							\
 })
-#define READ_ONCE(x) __READ_ONCE(x, 1)
 
+#define READ_ONCE(x)							\
+({									\
+	compiletime_assert_rwonce_type(x);				\
+	__READ_ONCE_SCALAR(x);						\
+})
+
+#define __WRITE_ONCE(x, val)				\
+do {							\
+	*(volatile typeof(x) *)&(x) = (val);		\
+} while (0)
+
+#define WRITE_ONCE(x, val)				\
+do {							\
+	compiletime_assert_rwonce_type(x);		\
+	__WRITE_ONCE(x, val);				\
+} while (0)
+
+#ifdef CONFIG_KASAN
 /*
- * Use READ_ONCE_NOCHECK() instead of READ_ONCE() if you need
- * to hide memory access from 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 READ_ONCE_NOCHECK(x) __READ_ONCE(x, 0)
+# 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 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)						\
+({									\
+	unsigned long __x;						\
+	compiletime_assert(sizeof(x) == sizeof(__x),			\
+		"Unsupported access size for READ_ONCE_NOCHECK().");	\
+	__x = __read_once_word_nocheck(&(x));				\
+	smp_read_barrier_depends();					\
+	(typeof(x))__x;							\
+})
 
 static __no_kasan_or_inline
 unsigned long read_word_at_a_time(const void *addr)
@@ -334,14 +322,6 @@ 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__ */
 
 /*
@@ -406,6 +386,16 @@ static inline void *offset_to_ptr(const int *off)
 	compiletime_assert(__native_word(t),				\
 		"Need native word sized stores/loads for atomicity.")
 
+/*
+ * Yes, this permits 64-bit accesses on 32-bit architectures. These will
+ * actually be atomic in some cases (namely Armv7 + LPAE), but for others we
+ * rely on the access being split into 2x32-bit accesses for a 32-bit quantity
+ * (e.g. a virtual address) and a strong prevailing wind.
+ */
+#define compiletime_assert_rwonce_type(t)					\
+	compiletime_assert(__native_word(t) || sizeof(t) == sizeof(long long),	\
+		"Unsupported access size for {READ,WRITE}_ONCE().")
+
 /* &a[0] degrades to a pointer: a different type from an array */
 #define __must_be_array(a)	BUILD_BUG_ON_ZERO(__same_type((a), &(a)[0]))