[tip: locking/kcsan] kcsan: Rework data_race() so that it can be used by READ_ONCE()

[tip-bot2 for Will Deacon]

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

Commit-ID:     88f1be32068d4323aa31236452352d6019a03ccc
Gitweb:        https://git.kernel.org/tip/88f1be32068d4323aa31236452352d6019a03ccc
Author:        Will Deacon <will@xxxxxxxxxx>
AuthorDate:    Mon, 11 May 2020 21:41:48 +01:00
Committer:     Thomas Gleixner <tglx@xxxxxxxxxxxxx>
CommitterDate: Tue, 12 May 2020 11:04:17 +02:00

kcsan: Rework data_race() so that it can be used by READ_ONCE()

Rework the data_race() macro so that it:

  - Accepts expressions which evaluate to a 'const' type
  - Attempts to discard volatile qualifiers from scalar types, avoiding
    pointless stack spills
  - Uses __kcsan_{disable,enable}_current(), allowing its use from code
    that is built independently from the kernel, such as the vDSO

This will allow for its use by {READ,WRITE}_ONCE() in a subsequent patch.
At the same time, fix-up some weird whitespace issues.

Signed-off-by: Will Deacon <will@xxxxxxxxxx>
Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Acked-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Cc: Marco Elver <elver@xxxxxxxxxx>
Link: https://lkml.kernel.org/r/20200511204150.27858-17-will@xxxxxxxxxx

---
 include/linux/compiler.h | 17 +++++++++--------
 1 file changed, 9 insertions(+), 8 deletions(-)

diff --git a/include/linux/compiler.h b/include/linux/compiler.h
index 548294e..cb2e3b3 100644
--- a/include/linux/compiler.h
+++ b/include/linux/compiler.h
@@ -314,14 +314,15 @@ unsigned long read_word_at_a_time(const void *addr)
  * This macro *does not* affect normal code generation, but is a hint
  * to tooling that data races here are to be ignored.
  */
-#define data_race(expr)                                                        \
-	({                                                                     \
-		typeof(({ expr; })) __val;                                     \
-		kcsan_disable_current();                                       \
-		__val = ({ expr; });                                           \
-		kcsan_enable_current();                                        \
-		__val;                                                         \
-	})
+#define data_race(expr)							\
+({									\
+	__kcsan_disable_current();					\
+	({								\
+		__unqual_scalar_typeof(({ expr; })) __v = ({ expr; });	\
+		__kcsan_enable_current();				\
+		__v;							\
+	});								\
+})
 
 #endif /* __KERNEL__ */