tools headers: Restore READ_ONCE() C++ compatibility

Our userspace <linux/compiler.h> defines READ_ONCE() in a way that clang
doesn't like, as we have an anonymous union in which neither field is
initialized.

WRITE_ONCE() is fine since it initializes the __val field. For
READ_ONCE() we can keep clang and GCC happy with a dummy initialization
of the __c field, so let's do that.

At the same time, let's split READ_ONCE() and WRITE_ONCE() over several
lines for legibility, as we do in the in-kernel <linux/compiler.h>.

Reported-by: Li Zhijian <lizhijian@cn.fujitsu.com>
Reported-by: Sandipan Das <sandipan@linux.vnet.ibm.com>
Tested-by: Sandipan Das <sandipan@linux.vnet.ibm.com>
Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Fixes: 6aa7de059173a986 ("locking/atomics: COCCINELLE/treewide: Convert trivial ACCESS_ONCE() patterns to READ_ONCE()/WRITE_ONCE()")
Link: http://lkml.kernel.org/r/20180404163445.16492-1-mark.rutland@arm.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

diff --git a/tools/include/linux/compiler.h b/tools/include/linux/compiler.h
index 04e32f9..1827c2f 100644 (file)
--- a/tools/include/linux/compiler.h
+++ b/tools/include/linux/compiler.h
@@ -151,11 +151,21 @@ static __always_inline void __write_once_size(volatile void *p, void *res, int s
  * 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; })
+#define READ_ONCE(x)                                   \
+({                                                     \
+       union { typeof(x) __val; char __c[1]; } __u =   \
+               { .__c = { 0 } };                       \
+       __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;                                      \
+})
 
 
 #ifndef __fallthrough