From: Marco Elver <elver@xxxxxxxxxx>
Previous to the change to distinguish read-write accesses, when
CONFIG_KCSAN_ASSUME_PLAIN_WRITES_ATOMIC=y is set, KCSAN would consider
the non-atomic bitops as atomic. We want to partially revert to this
behaviour, but with one important distinction: report racing
modifications, since lost bits due to non-atomicity are certainly
possible.
Given the operations here only modify a single bit, assuming
non-atomicity of the writer is sufficient may be reasonable for certain
usage (and follows the permissible nature of the "assume plain writes
atomic" rule). In other words:
1. We want non-atomic read-modify-write races to be reported;
this is accomplished by kcsan_check_read(), where any
concurrent write (atomic or not) will generate a report.
2. We do not want to report races with marked readers, but -do-
want to report races with unmarked readers; this is
accomplished by the instrument_write() ("assume atomic
write" with Kconfig option set).
With the above rules, when KCSAN_ASSUME_PLAIN_WRITES_ATOMIC is selected,
it is hoped that KCSAN's reporting behaviour is better aligned with
current expected permissible usage for non-atomic bitops.
Note that, a side-effect of not telling KCSAN that the accesses are
read-writes, is that this information is not displayed in the access
summary in the report. It is, however, visible in inline-expanded stack
traces. For now, it does not make sense to introduce yet another special
case to KCSAN's runtime, only to cater to the case here.
Cc: Dmitry Vyukov <dvyukov@xxxxxxxxxx>
Cc: Paul E. McKenney <paulmck@xxxxxxxxxx>
Cc: Will Deacon <will@xxxxxxxxxx>
Cc: Arnd Bergmann <arnd@xxxxxxxx>
Cc: Daniel Axtens <dja@xxxxxxxxxx>
Cc: Michael Ellerman <mpe@xxxxxxxxxxxxxx>
Cc: <linux-arch@xxxxxxxxxxxxxxx>
Signed-off-by: Marco Elver <elver@xxxxxxxxxx>
Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
---
.../asm-generic/bitops/instrumented-non-atomic.h | 30 +++++++++++++++++++---
1 file changed, 27 insertions(+), 3 deletions(-)
diff --git a/include/asm-generic/bitops/instrumented-non-atomic.h b/include/asm-generic/bitops/instrumented-non-atomic.h
index f86234c..37363d5 100644
--- a/include/asm-generic/bitops/instrumented-non-atomic.h
+++ b/include/asm-generic/bitops/instrumented-non-atomic.h
@@ -58,6 +58,30 @@ static inline void __change_bit(long nr, volatile unsigned long *addr)
arch___change_bit(nr, addr);
}
+static inline void __instrument_read_write_bitop(long nr, volatile unsigned long *addr)
+{
+ if (IS_ENABLED(CONFIG_KCSAN_ASSUME_PLAIN_WRITES_ATOMIC)) {
+ /*
+ * We treat non-atomic read-write bitops a little more special.
+ * Given the operations here only modify a single bit, assuming
+ * non-atomicity of the writer is sufficient may be reasonable
+ * for certain usage (and follows the permissible nature of the
+ * assume-plain-writes-atomic rule):
+ * 1. report read-modify-write races -> check read;
+ * 2. do not report races with marked readers, but do report
+ * races with unmarked readers -> check "atomic" write.
+ */
+ kcsan_check_read(addr + BIT_WORD(nr), sizeof(long));
+ /*
+ * Use generic write instrumentation, in case other sanitizers
+ * or tools are enabled alongside KCSAN.
+ */
+ instrument_write(addr + BIT_WORD(nr), sizeof(long));
+ } else {
+ instrument_read_write(addr + BIT_WORD(nr), sizeof(long));
+ }
+}
+
/**
* __test_and_set_bit - Set a bit and return its old value
* @nr: Bit to set
@@ -68,7 +92,7 @@ static inline void __change_bit(long nr, volatile unsigned long *addr)
*/
static inline bool __test_and_set_bit(long nr, volatile unsigned long *addr)
{
- instrument_read_write(addr + BIT_WORD(nr), sizeof(long));
+ __instrument_read_write_bitop(nr, addr);
return arch___test_and_set_bit(nr, addr);
}
@@ -82,7 +106,7 @@ static inline bool __test_and_set_bit(long nr, volatile unsigned long *addr)
*/
static inline bool __test_and_clear_bit(long nr, volatile unsigned long *addr)
{
- instrument_read_write(addr + BIT_WORD(nr), sizeof(long));
+ __instrument_read_write_bitop(nr, addr);
return arch___test_and_clear_bit(nr, addr);
}
@@ -96,7 +120,7 @@ static inline bool __test_and_clear_bit(long nr, volatile unsigned long *addr)
*/
static inline bool __test_and_change_bit(long nr, volatile unsigned long *addr)
{
- instrument_read_write(addr + BIT_WORD(nr), sizeof(long));
+ __instrument_read_write_bitop(nr, addr);
return arch___test_and_change_bit(nr, addr);
}
--
2.9.5
