[tip: locking/kcsan] include/linux/compiler.h: Introduce data_race(expr) macro

[tip: locking/kcsan] include/linux/compiler.h: Introduce data_race(expr) macro

[tip-bot2 for Marco Elver]

The following commit has been merged into the locking/kcsan branch of tip:

Commit-ID:     c48981eeb0d56e107691df590007d6699441a689
Gitweb:        https://git.kernel.org/tip/c48981eeb0d56e107691df590007d6699441a689
Author:        Marco Elver <elver@google.com>
AuthorDate:    Thu, 14 Nov 2019 19:02:55 +01:00
Committer:     Paul E. McKenney <paulmck@kernel.org>
CommitterDate: Sat, 16 Nov 2019 07:23:13 -08:00

include/linux/compiler.h: Introduce data_race(expr) macro

This introduces the data_race(expr) macro, which can be used to annotate
expressions for purposes of (1) documenting, and (2) giving tooling such
as KCSAN information about which data races are deemed "safe".

More context:
http://lkml.kernel.org/r/CAHk-=wg5CkOEF8DTez1Qu0XTEFw_oHhxN98bDnFqbY7HL5AB2g@mail.gmail.com

Signed-off-by: Marco Elver <elver@google.com>
Cc: Alan Stern <stern@rowland.harvard.edu>
Cc: Eric Dumazet <edumazet@google.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
---
 include/linux/compiler.h | 20 ++++++++++++++++++++
 1 file changed, 20 insertions(+)

diff --git a/include/linux/compiler.h b/include/linux/compiler.h
index c42fa83..7d3e777 100644
--- a/include/linux/compiler.h
+++ b/include/linux/compiler.h
@@ -310,6 +310,26 @@ unsigned long read_word_at_a_time(const void *addr)
 	__u.__val;					\
 })
 
+#include <linux/kcsan.h>
+
+/*
+ * data_race: macro to document that accesses in an expression may conflict with
+ * other concurrent accesses resulting in data races, but the resulting
+ * behaviour is deemed safe regardless.
+ *
+ * This macro *does not* affect normal code generation, but is a hint to tooling
+ * that data races here should be ignored.
+ */
+#define data_race(expr)                                                        \
+	({                                                                     \
+		typeof(({ expr; })) __val;                                     \
+		kcsan_nestable_atomic_begin();                                 \
+		__val = ({ expr; });                                           \
+		kcsan_nestable_atomic_end();                                   \
+		__val;                                                         \
+	})
+#else
+
 #endif /* __KERNEL__ */
 
 /*