locking: Remove atomicy checks from {READ,WRITE}_ONCE

[karo-tx-linux.git] / include / linux / compiler.h

diff --git a/include/linux/compiler.h b/include/linux/compiler.h
index 17f624cdf53c5609066389bd3da3375fcdfe2827..0e41ca0e59275deb33b7c7220dffd1ff39cbb2f7 100644 (file)
--- a/include/linux/compiler.h
+++ b/include/linux/compiler.h
@@ -192,29 +192,16 @@ void ftrace_likely_update(struct ftrace_branch_data *f, int val, int expect);
 
 #include <uapi/linux/types.h>
 
-static __always_inline void data_access_exceeds_word_size(void)
-#ifdef __compiletime_warning
-__compiletime_warning("data access exceeds word size and won't be atomic")
-#endif
-;
-
-static __always_inline void data_access_exceeds_word_size(void)
-{
-}
-
-static __always_inline void __read_once_size(volatile void *p, void *res, int size)
+static __always_inline void __read_once_size(const 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
        default:
                barrier();
                __builtin_memcpy((void *)res, (const void *)p, size);
-               data_access_exceeds_word_size();
                barrier();
        }
 }
@@ -225,13 +212,10 @@ static __always_inline void __write_once_size(volatile void *p, void *res, int s
        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
        default:
                barrier();
                __builtin_memcpy((void *)p, (const void *)res, size);
-               data_access_exceeds_word_size();
                barrier();
        }
 }
@@ -259,10 +243,10 @@ static __always_inline void __write_once_size(volatile void *p, void *res, int s
  */
 
 #define READ_ONCE(x) \
-       ({ typeof(x) __val; __read_once_size(&x, &__val, sizeof(__val)); __val; })
+       ({ union { typeof(x) __val; char __c[1]; } __u; __read_once_size(&(x), __u.__c, sizeof(x)); __u.__val; })
 
 #define WRITE_ONCE(x, val) \
-       ({ typeof(x) __val; __val = val; __write_once_size(&x, &__val, sizeof(__val)); __val; })
+       ({ typeof(x) __val = (val); __write_once_size(&(x), &__val, sizeof(__val)); __val; })
 
 #endif /* __KERNEL__ */
 
@@ -451,12 +435,23 @@ static __always_inline void __write_once_size(volatile void *p, void *res, int s
  * to make the compiler aware of ordering is to put the two invocations of
  * ACCESS_ONCE() in different C statements.
  *
- * This macro does absolutely -nothing- to prevent the CPU from reordering,
- * merging, or refetching absolutely anything at any time.  Its main intended
- * use is to mediate communication between process-level code and irq/NMI
- * handlers, all running on the same CPU.
+ * ACCESS_ONCE will only work on scalar types. For union types, ACCESS_ONCE
+ * on a union member will work as long as the size of the member matches the
+ * size of the union and the size is smaller than word size.
+ *
+ * The major use cases of ACCESS_ONCE used to be (1) Mediating communication
+ * between process-level code and irq/NMI handlers, all running on the same CPU,
+ * and (2) Ensuring that the compiler does not  fold, spindle, or otherwise
+ * mutilate accesses that either do not require ordering or that interact
+ * with an explicit memory barrier or atomic instruction that provides the
+ * required ordering.
+ *
+ * If possible use READ_ONCE/ASSIGN_ONCE instead.
  */
-#define ACCESS_ONCE(x) (*(volatile typeof(x) *)&(x))
+#define __ACCESS_ONCE(x) ({ \
+        __maybe_unused typeof(x) __var = (__force typeof(x)) 0; \
+       (volatile typeof(x) *)&(x); })
+#define ACCESS_ONCE(x) (*__ACCESS_ONCE(x))
 
 /* Ignore/forbid kprobes attach on very low level functions marked by this attribute: */
 #ifdef CONFIG_KPROBES