diff options
Diffstat (limited to 'include/linux/compiler.h')
| -rw-r--r-- | include/linux/compiler.h | 20 |
1 files changed, 18 insertions, 2 deletions
diff --git a/include/linux/compiler.h b/include/linux/compiler.h index 867722591be2..05be2352fef8 100644 --- a/include/linux/compiler.h +++ b/include/linux/compiler.h @@ -250,7 +250,23 @@ static __always_inline void __write_once_size(volatile void *p, void *res, int s ({ 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); __write_once_size(&(x), &__val, sizeof(__val)); __val; }) + ({ union { typeof(x) __val; char __c[1]; } __u = { .__val = (val) }; __write_once_size(&(x), __u.__c, sizeof(x)); __u.__val; }) + +/** + * READ_ONCE_CTRL - Read a value heading a control dependency + * @x: The value to be read, heading the control dependency + * + * Control dependencies are tricky. See Documentation/memory-barriers.txt + * for important information on how to use them. Note that in many cases, + * use of smp_load_acquire() will be much simpler. Control dependencies + * should be avoided except on the hottest of hotpaths. + */ +#define READ_ONCE_CTRL(x) \ +({ \ + typeof(x) __val = READ_ONCE(x); \ + smp_read_barrier_depends(); /* Enforce control dependency. */ \ + __val; \ +}) #endif /* __KERNEL__ */ @@ -450,7 +466,7 @@ static __always_inline void __write_once_size(volatile void *p, void *res, int s * with an explicit memory barrier or atomic instruction that provides the * required ordering. * - * If possible use READ_ONCE/ASSIGN_ONCE instead. + * If possible use READ_ONCE()/WRITE_ONCE() instead. */ #define __ACCESS_ONCE(x) ({ \ __maybe_unused typeof(x) __var = (__force typeof(x)) 0; \ |