linux-kernel - [PATCH v4 07/10] seqlock: Require WRITE_ONCE surrounding raw_seqcount

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Date:   Thu, 14 Nov 2019 19:03:00 +0100
From:   Marco Elver <elver@...gle.com>
To:     elver@...gle.com
Cc:     akiyks@...il.com, stern@...land.harvard.edu, glider@...gle.com,
        parri.andrea@...il.com, andreyknvl@...gle.com, luto@...nel.org,
        ard.biesheuvel@...aro.org, arnd@...db.de, boqun.feng@...il.com,
        bp@...en8.de, dja@...ens.net, dlustig@...dia.com,
        dave.hansen@...ux.intel.com, dhowells@...hat.com,
        dvyukov@...gle.com, hpa@...or.com, mingo@...hat.com,
        j.alglave@....ac.uk, joel@...lfernandes.org, corbet@....net,
        jpoimboe@...hat.com, luc.maranget@...ia.fr, mark.rutland@....com,
        npiggin@...il.com, paulmck@...nel.org, peterz@...radead.org,
        tglx@...utronix.de, will@...nel.org, edumazet@...gle.com,
        kasan-dev@...glegroups.com, linux-arch@...r.kernel.org,
        linux-doc@...r.kernel.org, linux-efi@...r.kernel.org,
        linux-kbuild@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, x86@...nel.org
Subject: [PATCH v4 07/10] seqlock: Require WRITE_ONCE surrounding raw_seqcount_barrier

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>
---
v3:
* Add missing comment that was in preceding seqlock patch.
---
 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 61232bc223fd..f52c91be8939 100644
--- a/include/linux/seqlock.h
+++ b/include/linux/seqlock.h
@@ -265,6 +265,13 @@ 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;
  *
@@ -284,11 +291,11 @@ 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)
-- 
2.24.0.rc1.363.gb1bccd3e3d-goog