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@linux.ibm.com, peterz@infradead.org,
tglx@linutronix.de, will@kernel.org, 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 5/8] seqlock: Require WRITE_ONCE surrounding raw_seqcount_barrier
Date: Wed, 16 Oct 2019 10:39:56 +0200 [thread overview]
Message-ID: <20191016083959.186860-6-elver@google.com> (raw)
In-Reply-To: <20191016083959.186860-1-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>
---
include/linux/seqlock.h | 7 +++++++
1 file changed, 7 insertions(+)
diff --git a/include/linux/seqlock.h b/include/linux/seqlock.h
index 1e425831a7ed..5d50aad53b47 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;
*
--
2.23.0.700.g56cf767bdb-goog
next prev parent reply other threads:[~2019-10-16 8:41 UTC|newest]
Thread overview: 25+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-10-16 8:39 [PATCH 0/8] Add Kernel Concurrency Sanitizer (KCSAN) Marco Elver
2019-10-16 8:39 ` [PATCH 1/8] kcsan: Add Kernel Concurrency Sanitizer infrastructure Marco Elver
2019-10-16 9:42 ` Boqun Feng
2019-10-16 10:06 ` Marco Elver
2019-10-17 0:25 ` Boqun Feng
2019-10-16 11:49 ` Andrey Konovalov
2019-10-16 13:52 ` Marco Elver
2019-10-16 15:16 ` Mark Rutland
2019-10-16 15:53 ` Marco Elver
2019-10-16 18:43 ` Peter Zijlstra
2019-10-16 19:34 ` Marco Elver
2019-10-17 7:47 ` Peter Zijlstra
2019-10-17 7:49 ` Marco Elver
2019-10-16 8:39 ` [PATCH 2/8] objtool, kcsan: Add KCSAN runtime functions to whitelist Marco Elver
2019-10-16 8:39 ` [PATCH 3/8] build, kcsan: Add KCSAN build exceptions Marco Elver
2019-10-16 8:39 ` [PATCH 4/8] seqlock, kcsan: Add annotations for KCSAN Marco Elver
2019-10-16 8:39 ` Marco Elver [this message]
2019-10-16 8:39 ` [PATCH 6/8] asm-generic, kcsan: Add KCSAN instrumentation for bitops Marco Elver
2019-10-16 8:39 ` [PATCH 7/8] locking/atomics, kcsan: Add KCSAN instrumentation Marco Elver
2019-10-16 11:18 ` Mark Rutland
2019-10-16 11:47 ` Marco Elver
2019-10-16 8:39 ` [PATCH 8/8] x86, kcsan: Enable KCSAN for x86 Marco Elver
2019-10-16 16:14 ` Dave Hansen
2019-10-16 17:04 ` Marco Elver
2019-10-17 14:15 ` [PATCH 0/8] Add Kernel Concurrency Sanitizer (KCSAN) Marco Elver
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