diff options
Diffstat (limited to 'include/linux/compiler.h')
| -rw-r--r-- | include/linux/compiler.h | 74 |
1 files changed, 74 insertions, 0 deletions
diff --git a/include/linux/compiler.h b/include/linux/compiler.h index d5ad7b1118fc..a1c81f80978e 100644 --- a/include/linux/compiler.h +++ b/include/linux/compiler.h @@ -186,6 +186,80 @@ 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 <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) +{ + 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(); + } +} + +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 + default: + barrier(); + __builtin_memcpy((void *)p, (const void *)res, size); + data_access_exceeds_word_size(); + barrier(); + } +} + +/* + * Prevent the compiler from merging or refetching reads or writes. The + * compiler is also forbidden from reordering successive instances of + * READ_ONCE, ASSIGN_ONCE and ACCESS_ONCE (see below), but only when the + * compiler is aware of some particular ordering. One way to make the + * compiler aware of ordering is to put the two invocations of READ_ONCE, + * ASSIGN_ONCE or ACCESS_ONCE() in different C statements. + * + * In contrast to ACCESS_ONCE 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 ASSIGN_ONCE() will fall back to memcpy and print a + * compile-time warning. + * + * Their two major use cases are: (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. + */ + +#define READ_ONCE(x) \ + ({ typeof(x) __val; __read_once_size(&x, &__val, sizeof(__val)); __val; }) + +#define ASSIGN_ONCE(val, x) \ + ({ typeof(x) __val; __val = val; __assign_once_size(&x, &__val, sizeof(__val)); __val; }) + #endif /* __KERNEL__ */ #endif /* __ASSEMBLY__ */ |