kcsan: Introduce KCSAN_ACCESS_ASSERT access type

The KCSAN_ACCESS_ASSERT access type may be used to introduce dummy reads
and writes to assert certain properties of concurrent code, where bugs
could not be detected as normal data races.

For example, a variable that is only meant to be written by a single
CPU, but may be read (without locking) by other CPUs must still be
marked properly to avoid data races. However, concurrent writes,
regardless if WRITE_ONCE() or not, would be a bug. Using
kcsan_check_access(&x, sizeof(x), KCSAN_ACCESS_ASSERT) would allow
catching such bugs.

To support KCSAN_ACCESS_ASSERT the following notable changes were made:

  * If an access is of type KCSAN_ASSERT_ACCESS, disable various filters
    that only apply to data races, so that all races that KCSAN observes are
    reported.
  * Bug reports that involve an ASSERT access type will be reported as
    "KCSAN: assert: race in ..." instead of "data-race"; this will help
    more easily distinguish them.
  * Update a few comments to just mention 'races' where we do not always
    mean pure data races.

Signed-off-by: Marco Elver <elver@google.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>

1 files changed, 14 insertions, 10 deletions

diff --git a/lib/Kconfig.kcsan b/lib/Kconfig.kcsan
index 9785bbf9a1d1..f0b791143c6a 100644
--- a/lib/Kconfig.kcsan
+++ b/lib/Kconfig.kcsan
@@ -4,13 +4,17 @@ config HAVE_ARCH_KCSAN
 	bool
 
 menuconfig KCSAN
-	bool "KCSAN: dynamic data race detector"
+	bool "KCSAN: dynamic race detector"
 	depends on HAVE_ARCH_KCSAN && DEBUG_KERNEL && !KASAN
 	select STACKTRACE
 	help
-	  The Kernel Concurrency Sanitizer (KCSAN) is a dynamic data race
-	  detector, which relies on compile-time instrumentation, and uses a
-	  watchpoint-based sampling approach to detect data races.
+	  The Kernel Concurrency Sanitizer (KCSAN) is a dynamic race detector,
+	  which relies on compile-time instrumentation, and uses a
+	  watchpoint-based sampling approach to detect races.
+
+	  KCSAN's primary purpose is to detect data races. KCSAN can also be
+	  used to check properties, with the help of provided assertions, of
+	  concurrent code where bugs do not manifest as data races.
 
 	  See <file:Documentation/dev-tools/kcsan.rst> for more details.
 
@@ -85,14 +89,14 @@ config KCSAN_SKIP_WATCH_RANDOMIZE
 	  KCSAN_WATCH_SKIP.
 
 config KCSAN_REPORT_ONCE_IN_MS
-	int "Duration in milliseconds, in which any given data race is only reported once"
+	int "Duration in milliseconds, in which any given race is only reported once"
 	default 3000
 	help
-	  Any given data race is only reported once in the defined time window.
-	  Different data races may still generate reports within a duration
-	  that is smaller than the duration defined here. This allows rate
-	  limiting reporting to avoid flooding the console with reports.
-	  Setting this to 0 disables rate limiting.
+	  Any given race is only reported once in the defined time window.
+	  Different races may still generate reports within a duration that is
+	  smaller than the duration defined here. This allows rate limiting
+	  reporting to avoid flooding the console with reports.  Setting this
+	  to 0 disables rate limiting.
 
 # The main purpose of the below options is to control reported data races (e.g.
 # in fuzzer configs), and are not expected to be switched frequently by other