diff --git a/include/linux/compiler.h b/include/linux/compiler.h index 923d093c9ce..dc78e1246ba 100644 --- a/include/linux/compiler.h +++ b/include/linux/compiler.h @@ -164,6 +164,74 @@ void ftrace_likely_update(struct ftrace_branch_data *f, int val, int expect); (typeof(ptr)) (__ptr + (off)); }) #endif +#include + +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; + 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; + 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__ */