summaryrefslogtreecommitdiff
path: root/tools
diff options
context:
space:
mode:
authorArnaldo Carvalho de Melo <acme@redhat.com>2015-07-05 14:12:42 -0300
committerArnaldo Carvalho de Melo <acme@redhat.com>2015-07-05 15:05:00 -0300
commit728abda6a6654ee7f4e903dc921c6307065e1644 (patch)
treea4cba7310c2cfae2039229c354e5dff77216286d /tools
parentb9df84fd7c05cc300d6d14f022b8a00773ebcf8c (diff)
downloadlwn-728abda6a6654ee7f4e903dc921c6307065e1644.tar.gz
lwn-728abda6a6654ee7f4e903dc921c6307065e1644.zip
tools: Adopt {READ,WRITE_ONCE} from the kernel
We need it to build rbtree.c after this cset: commit d72da4a4d973 Author: Peter Zijlstra <peterz@infradead.org> Date: Wed May 27 11:09:36 2015 +0930 rbtree: Make lockless searches non-fatal Cc: Adrian Hunter <adrian.hunter@intel.com> Cc: Borislav Petkov <bp@suse.de> Cc: David Ahern <dsahern@gmail.com> Cc: Frederic Weisbecker <fweisbec@gmail.com> Cc: Jiri Olsa <jolsa@redhat.com> Cc: Namhyung Kim <namhyung@kernel.org> Cc: Peter Zijlstra <peterz@infradead.org> Cc: Stephane Eranian <eranian@google.com> Link: http://lkml.kernel.org/n/tip-qlnzhezv5ddwst0w9fydju0y@git.kernel.org Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Diffstat (limited to 'tools')
-rw-r--r--tools/include/linux/compiler.h58
1 files changed, 58 insertions, 0 deletions
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 <linux/types.h>
+
+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 */