[tip: locking/kcsan] kcsan: Pass option tsan-instrument-read-before-write to Clang

[tip-bot2 for Marco Elver]

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

Commit-ID:     52dfbb97a90fbf6a9826f15a71fca37861330a13
Gitweb:        https://git.kernel.org/tip/52dfbb97a90fbf6a9826f15a71fca37861330a13
Author:        Marco Elver <elver@xxxxxxxxxx>
AuthorDate:    Thu, 21 May 2020 16:20:40 +02:00
Committer:     Borislav Petkov <bp@xxxxxxx>
CommitterDate: Fri, 22 May 2020 14:57:39 +02:00

kcsan: Pass option tsan-instrument-read-before-write to Clang

Clang (unlike GCC) removes reads before writes with matching addresses
in the same basic block. This is an optimization for TSAN, since writes
will always cause conflict if the preceding read would have.

However, for KCSAN we cannot rely on this option, because we apply
several special rules to writes, in particular when the
KCSAN_ASSUME_PLAIN_WRITES_ATOMIC option is selected. To avoid missing
potential data races, pass the -tsan-instrument-read-before-write option
to Clang if it is available [1].

[1] https://github.com/llvm/llvm-project/commit/151ed6aa38a3ec6c01973b35f684586b6e1c0f7e

Signed-off-by: Marco Elver <elver@xxxxxxxxxx>
Signed-off-by: Borislav Petkov <bp@xxxxxxx>
Acked-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Acked-by: Will Deacon <will@xxxxxxxxxx>
Link: https://lkml.kernel.org/r/20200521142047.169334-5-elver@xxxxxxxxxx
---
 scripts/Makefile.kcsan | 1 +
 1 file changed, 1 insertion(+)

diff --git a/scripts/Makefile.kcsan b/scripts/Makefile.kcsan
index 75d2942..bd4da1a 100644
--- a/scripts/Makefile.kcsan
+++ b/scripts/Makefile.kcsan
@@ -13,6 +13,7 @@ endif
 # of some options does not break KCSAN nor causes false positive reports.
 CFLAGS_KCSAN := -fsanitize=thread \
 	$(call cc-option,$(call cc-param,tsan-instrument-func-entry-exit=0) -fno-optimize-sibling-calls) \
+	$(call cc-option,$(call cc-param,tsan-instrument-read-before-write=1)) \
 	$(call cc-param,tsan-distinguish-volatile=1)
 
 endif # CONFIG_KCSAN