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authorMarco Elver <elver@google.com>2019-11-14 21:03:00 +0300
committerPaul E. McKenney <paulmck@kernel.org>2019-11-16 18:23:15 +0300
commitbf07132f96d426bcbf2098227fb680915cf44498 (patch)
treef39448c1c0e68b6f607d21ea3506915273822ba2 /include/linux
parent88ecd153be9519f259b87a9f6f4c8383a8b3bbf1 (diff)
downloadlinux-bf07132f96d426bcbf2098227fb680915cf44498.tar.xz
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@google.com> Acked-by: Paul E. McKenney <paulmck@kernel.org> Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Diffstat (limited to 'include/linux')
-rw-r--r--include/linux/seqlock.h11
1 files 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)