Date:   Tue, 30 Nov 2021 12:44:29 +0100
In-Reply-To: <20211130114433.2580590-1-elver@google.com>
Message-Id: <20211130114433.2580590-22-elver@google.com>
Mime-Version: 1.0
References: <20211130114433.2580590-1-elver@google.com>
Subject: [PATCH v3 21/25] sched, kcsan: Enable memory barrier instrumentation
From:   Marco Elver <elver@google.com>
To:     elver@google.com, "Paul E. McKenney" <paulmck@kernel.org>
Cc:     Alexander Potapenko <glider@google.com>,
        Boqun Feng <boqun.feng@gmail.com>,
        Borislav Petkov <bp@alien8.de>,
        Dmitry Vyukov <dvyukov@google.com>,
        Ingo Molnar <mingo@kernel.org>,
        Mark Rutland <mark.rutland@arm.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Waiman Long <longman@redhat.com>,
        Will Deacon <will@kernel.org>, kasan-dev@googlegroups.com,
        linux-arch@vger.kernel.org, linux-doc@vger.kernel.org,
        linux-kbuild@vger.kernel.org, linux-kernel@vger.kernel.org,
        linux-mm@kvack.org, llvm@lists.linux.dev, x86@kernel.org
Content-Type: text/plain; charset="UTF-8"
Precedence: bulk

There's no fundamental reason to disable KCSAN for scheduler code,
except for excessive noise and performance concerns (instrumenting
scheduler code is usually a good way to stress test KCSAN itself).

However, several core sched functions imply memory barriers that are
invisible to KCSAN without instrumentation, but are required to avoid
false positives. Therefore, unconditionally enable instrumentation of
memory barriers in scheduler code. Also update the comment to reflect
this and be a bit more brief.

Signed-off-by: Marco Elver <elver@google.com>
---
 kernel/sched/Makefile | 7 +++----
 1 file changed, 3 insertions(+), 4 deletions(-)

diff --git a/kernel/sched/Makefile b/kernel/sched/Makefile
index c7421f2d05e1..c83b37af155b 100644
--- a/kernel/sched/Makefile
+++ b/kernel/sched/Makefile
@@ -11,11 +11,10 @@ ccflags-y += $(call cc-disable-warning, unused-but-set-variable)
 # that is not a function of syscall inputs. E.g. involuntary context switches.
 KCOV_INSTRUMENT := n
 
-# There are numerous data races here, however, most of them are due to plain accesses.
-# This would make it even harder for syzbot to find reproducers, because these
-# bugs trigger without specific input. Disable by default, but should re-enable
-# eventually.
+# Disable KCSAN to avoid excessive noise and performance degradation. To avoid
+# false positives ensure barriers implied by sched functions are instrumented.
 KCSAN_SANITIZE := n
+KCSAN_INSTRUMENT_BARRIERS := y
 
 ifneq ($(CONFIG_SCHED_OMIT_FRAME_POINTER),y)
 # According to Alan Modra <alan@linuxcare.com.au>, the -fno-omit-frame-pointer is
-- 
2.34.0.rc2.393.gf8c9666880-goog