[PATCH 2/2] kcsan: Change data_race() to no longer require marking racing accesses

[Marco Elver]

Thus far, accesses marked with data_race() would still require the
racing access to be marked in some way (be it with READ_ONCE(),
WRITE_ONCE(), or data_race() itself), as otherwise KCSAN would still
report a data race.  This requirement, however, seems to be unintuitive,
and some valid use-cases demand *not* marking other accesses, as it
might hide more serious bugs (e.g. diagnostic reads).

Therefore, this commit changes data_race() to no longer require marking
racing accesses (although it's still recommended if possible).

The alternative would have been introducing another variant of
data_race(), however, since usage of data_race() already needs to be
carefully reasoned about, distinguishing between these cases likely adds
more complexity in the wrong place.

Link: 20200331131002.GA30975@willie-the-truck">https://lkml.kernel.org/r/20200331131002.GA30975@willie-the-truck
Signed-off-by: Marco Elver <elver@xxxxxxxxxx>
Cc: Paul E. McKenney <paulmck@xxxxxxxxxx>
Cc: Will Deacon <will@xxxxxxxxxx>
Cc: Qian Cai <cai@xxxxxx>
---
 include/linux/compiler.h | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/include/linux/compiler.h b/include/linux/compiler.h
index f504edebd5d7..1729bd17e9b7 100644
--- a/include/linux/compiler.h
+++ b/include/linux/compiler.h
@@ -326,9 +326,9 @@ unsigned long read_word_at_a_time(const void *addr)
 #define data_race(expr)                                                        \
 	({                                                                     \
 		typeof(({ expr; })) __val;                                     \
-		kcsan_nestable_atomic_begin();                                 \
+		kcsan_disable_current();                                       \
 		__val = ({ expr; });                                           \
-		kcsan_nestable_atomic_end();                                   \
+		kcsan_enable_current();                                        \
 		__val;                                                         \
 	})
 #else
-- 
2.26.0.rc2.310.g2932bb562d-goog