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From: Marco Elver <elver@google.com>
To: elver@google.com
Cc: akiyks@gmail.com, stern@rowland.harvard.edu, glider@google.com,
	parri.andrea@gmail.com, andreyknvl@google.com, luto@kernel.org,
	ard.biesheuvel@linaro.org, arnd@arndb.de, boqun.feng@gmail.com,
	bp@alien8.de, dja@axtens.net, dlustig@nvidia.com,
	dave.hansen@linux.intel.com, dhowells@redhat.com,
	dvyukov@google.com, hpa@zytor.com, mingo@redhat.com,
	j.alglave@ucl.ac.uk, joel@joelfernandes.org, corbet@lwn.net,
	jpoimboe@redhat.com, luc.maranget@inria.fr, mark.rutland@arm.com,
	npiggin@gmail.com, paulmck@kernel.org, peterz@infradead.org,
	tglx@linutronix.de, will@kernel.org, edumazet@google.com,
	kasan-dev@googlegroups.com, linux-arch@vger.kernel.org,
	linux-doc@vger.kernel.org, linux-efi@vger.kernel.org,
	linux-kbuild@vger.kernel.org, linux-kernel@vger.kernel.org,
	linux-mm@kvack.org, x86@kernel.org
Subject: [PATCH v4 07/10] seqlock: Require WRITE_ONCE surrounding raw_seqcount_barrier
Date: Thu, 14 Nov 2019 19:03:00 +0100
Message-ID: <20191114180303.66955-8-elver@google.com> (raw)
In-Reply-To: <20191114180303.66955-1-elver@google.com>

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@google.com>
Acked-by: Paul E. McKenney <paulmck@kernel.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


  parent reply index

Thread overview: 40+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-11-14 18:02 [PATCH v4 00/10] Add Kernel Concurrency Sanitizer (KCSAN) Marco Elver
2019-11-14 18:02 ` [PATCH v4 01/10] kcsan: Add Kernel Concurrency Sanitizer infrastructure Marco Elver
2019-11-19 19:27   ` Qian Cai
2019-11-19 19:54     ` Marco Elver
2019-11-19 21:42       ` Qian Cai
2019-11-19 21:53         ` Marco Elver
2020-01-03  5:13   ` Qian Cai
2020-01-06 12:46     ` Marco Elver
2020-01-14 11:08       ` Qian Cai
2020-01-14 12:51         ` Marco Elver
2020-01-14 19:22         ` Paul E. McKenney
2020-01-14 20:30           ` Qian Cai
2020-01-14 21:34             ` Paul E. McKenney
2020-01-14 21:48               ` Qian Cai
2020-01-14 22:09                 ` Paul E. McKenney
2019-11-14 18:02 ` [PATCH v4 02/10] include/linux/compiler.h: Introduce data_race(expr) macro Marco Elver
2019-11-14 18:02 ` [PATCH v4 03/10] kcsan: Add Documentation entry in dev-tools Marco Elver
2019-11-14 18:02 ` [PATCH v4 04/10] objtool, kcsan: Add KCSAN runtime functions to whitelist Marco Elver
2019-11-14 18:02 ` [PATCH v4 05/10] build, kcsan: Add KCSAN build exceptions Marco Elver
2019-11-14 18:02 ` [PATCH v4 06/10] seqlock, kcsan: Add annotations for KCSAN Marco Elver
2019-11-14 18:03 ` Marco Elver [this message]
2019-11-14 18:03 ` [PATCH v4 08/10] asm-generic, kcsan: Add KCSAN instrumentation for bitops Marco Elver
2019-11-15 11:55   ` Marco Elver
2019-11-14 18:03 ` [PATCH v4 09/10] locking/atomics, kcsan: Add KCSAN instrumentation Marco Elver
2019-11-14 18:03 ` [PATCH v4 10/10] x86, kcsan: Enable KCSAN for x86 Marco Elver
2019-11-14 19:50 ` [PATCH v4 00/10] Add Kernel Concurrency Sanitizer (KCSAN) Paul E. McKenney
2019-11-14 21:33   ` Marco Elver
2019-11-14 22:15     ` Paul E. McKenney
2019-11-15 12:02       ` Marco Elver
2019-11-15 16:41         ` Paul E. McKenney
2019-11-15 17:14           ` Marco Elver
2019-11-15 20:43             ` Paul E. McKenney
2019-11-16  8:20               ` Marco Elver
2019-11-16 15:34                 ` Paul E. McKenney
2019-11-16 18:09                   ` Marco Elver
2019-11-16 18:28                     ` Paul E. McKenney
2019-11-19 19:50 ` Qian Cai
2019-11-19 20:12 ` Qian Cai
2019-11-19 21:50   ` Marco Elver
2019-11-20 15:54     ` Marco Elver

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