tools: virtio/linux/compiler.h: Add data_race() define.

Port over the definition of data_race() so we can build tools/virtio.

cc -g -O2 -Werror -Wno-maybe-uninitialized -Wall -I.
-I../include/ -I ../../usr/include/ -Wno-pointer-sign
-fno-strict-overflow -fno-strict-aliasing -fno-common
-MMD -U_FORTIFY_SOURCE -include ../../include/linux/kconfig.h
-mfunction-return=thunk -fcf-protection=none
-mindirect-branch-register -pthread
-c -o virtio_ring.o ../../drivers/virtio/virtio_ring.c
../../drivers/virtio/virtio_ring.c: in function'vring_interrupt':
../../drivers/virtio/virtio_ring.c:2711:17: error：Implicit declaration function'data_race' [-Wimplicit-function-declaration]
 2711 |                 data_race(vq->event_triggered = true);
      |                 ^~~~~~~~~

Signed-off-by: Yufeng Wang <[email protected]>
Message-Id: <[email protected]>
Signed-off-by: Michael S. Tsirkin <[email protected]>

Author: Yufeng Wang

tools/virtio/linux/compiler.h

@@ -10,4 +10,29 @@
 #define READ_ONCE(var) (*((volatile typeof(var) *)(&(var))))
 
 #define __aligned(x) __attribute((__aligned__(x)))
+
+/**
+ * data_race - mark an expression as containing intentional data races
+ *
+ * This data_race() macro is useful for situations in which data races
+ * should be forgiven.  One example is diagnostic code that accesses
+ * shared variables but is not a part of the core synchronization design.
+ * For example, if accesses to a given variable are protected by a lock,
+ * except for diagnostic code, then the accesses under the lock should
+ * be plain C-language accesses and those in the diagnostic code should
+ * use data_race().  This way, KCSAN will complain if buggy lockless
+ * accesses to that variable are introduced, even if the buggy accesses
+ * are protected by READ_ONCE() or WRITE_ONCE().
+ *
+ * This macro *does not* affect normal code generation, but is a hint
+ * to tooling that data races here are to be ignored.  If the access must
+ * be atomic *and* KCSAN should ignore the access, use both data_race()
+ * and READ_ONCE(), for example, data_race(READ_ONCE(x)).
+ */
+#define data_race(expr)							\
+({									\
+	__auto_type __v = (expr);					\
+	__v;								\
+})
+
 #endif