linux-kernel - [PATCH AUTOSEL 5.4 084/330] seqlock: Require WRITE_ONCE surrounding raw_seqcount

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Date:   Thu, 17 Sep 2020 21:57:04 -0400
From:   Sasha Levin <sashal@...nel.org>
To:     linux-kernel@...r.kernel.org, stable@...r.kernel.org
Cc:     Marco Elver <elver@...gle.com>,
        "Paul E . McKenney" <paulmck@...nel.org>,
        Sasha Levin <sashal@...nel.org>
Subject: [PATCH AUTOSEL 5.4 084/330] seqlock: Require WRITE_ONCE surrounding raw_seqcount_barrier

From: Marco Elver <elver@...gle.com>

[ Upstream commit bf07132f96d426bcbf2098227fb680915cf44498 ]

This patch proposes to require marked atomic accesses surrounding
raw_write_seqcount_barrier. We reason that otherwise there is no way to
guarantee propagation nor atomicity of writes before/after the barrier
[1]. For example, consider the compiler tears stores either before or
after the barrier; in this case, readers may observe a partial value,
and because readers are unaware that writes are going on (writes are not
in a seq-writer critical section), will complete the seq-reader critical
section while having observed some partial state.
[1] https://lwn.net/Articles/793253/

This came up when designing and implementing KCSAN, because KCSAN would
flag these accesses as data-races. After careful analysis, our reasoning
as above led us to conclude that the best thing to do is to propose an
amendment to the raw_seqcount_barrier usage.

Signed-off-by: Marco Elver <elver@...gle.com>
Acked-by: Paul E. McKenney <paulmck@...nel.org>
Signed-off-by: Paul E. McKenney <paulmck@...nel.org>
Signed-off-by: Sasha Levin <sashal@...nel.org>
---
 include/linux/seqlock.h | 11 +++++++++--
 1 file changed, 9 insertions(+), 2 deletions(-)

diff --git a/include/linux/seqlock.h b/include/linux/seqlock.h
index bcf4cf26b8c89..a42a29952889c 100644
--- a/include/linux/seqlock.h
+++ b/include/linux/seqlock.h
@@ -243,6 +243,13 @@ static inline void raw_write_seqcount_end(seqcount_t *s)
  * usual consistency guarantee. It is one wmb cheaper, because we can
  * collapse the two back-to-back wmb()s.
  *
+ * Note that, writes surrounding the barrier should be declared atomic (e.g.
+ * via WRITE_ONCE): a) to ensure the writes become visible to other threads
+ * atomically, avoiding compiler optimizations; b) to document which writes are
+ * meant to propagate to the reader critical section. This is necessary because
+ * neither writes before and after the barrier are enclosed in a seq-writer
+ * critical section that would ensure readers are aware of ongoing writes.
+ *
  *      seqcount_t seq;
  *      bool X = true, Y = false;
  *
@@ -262,11 +269,11 @@ static inline void raw_write_seqcount_end(seqcount_t *s)
  *
  *      void write(void)
  *      {
- *              Y = true;
+ *              WRITE_ONCE(Y, true);
  *
  *              raw_write_seqcount_barrier(seq);
  *
- *              X = false;
+ *              WRITE_ONCE(X, false);
  *      }
  */
 static inline void raw_write_seqcount_barrier(seqcount_t *s)
-- 
2.25.1