From 728abda6a6654ee7f4e903dc921c6307065e1644 Mon Sep 17 00:00:00 2001 From: Arnaldo Carvalho de Melo Date: Sun, 5 Jul 2015 14:12:42 -0300 Subject: tools: Adopt {READ,WRITE_ONCE} from the kernel We need it to build rbtree.c after this cset: commit d72da4a4d973 Author: Peter Zijlstra Date: Wed May 27 11:09:36 2015 +0930 rbtree: Make lockless searches non-fatal Cc: Adrian Hunter Cc: Borislav Petkov Cc: David Ahern Cc: Frederic Weisbecker Cc: Jiri Olsa Cc: Namhyung Kim Cc: Peter Zijlstra Cc: Stephane Eranian Link: http://lkml.kernel.org/n/tip-qlnzhezv5ddwst0w9fydju0y@git.kernel.org Signed-off-by: Arnaldo Carvalho de Melo --- tools/include/linux/compiler.h | 58 ++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 58 insertions(+) (limited to 'tools/include/linux') diff --git a/tools/include/linux/compiler.h b/tools/include/linux/compiler.h index f0e72674c52d..9098083869c8 100644 --- a/tools/include/linux/compiler.h +++ b/tools/include/linux/compiler.h @@ -41,4 +41,62 @@ #define ACCESS_ONCE(x) (*(volatile typeof(x) *)&(x)) +#include + +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; + case 8: *(__u64 *)res = *(volatile __u64 *)p; break; + default: + barrier(); + __builtin_memcpy((void *)res, (const void *)p, size); + barrier(); + } +} + +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 + * READ_ONCE, WRITE_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, + * WRITE_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 WRITE_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) \ + ({ union { typeof(x) __val; char __c[1]; } __u; __read_once_size(&(x), __u.__c, sizeof(x)); __u.__val; }) + +#define WRITE_ONCE(x, val) \ + ({ union { typeof(x) __val; char __c[1]; } __u = { .__val = (val) }; __write_once_size(&(x), __u.__c, sizeof(x)); __u.__val; }) + #endif /* _TOOLS_LINUX_COMPILER_H */ -- cgit v1.2.1