diff options
author | Marco Elver <elver@google.com> | 2019-11-14 19:03:00 +0100 |
---|---|---|
committer | Greg Kroah-Hartman <gregkh@linuxfoundation.org> | 2020-10-01 13:17:25 +0200 |
commit | 06a90303633ffcd18bece6e41bb4a7787b9f34c2 (patch) | |
tree | b92ae2b79c0d9a9ae536b4da4b128dc4e4133b86 /include/linux | |
parent | a9d4bca63493623412e3d0b5e605c4a7d022ffc5 (diff) |
seqlock: Require WRITE_ONCE surrounding raw_seqcount_barrier
[ 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@google.com>
Acked-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
Diffstat (limited to 'include/linux')
-rw-r--r-- | include/linux/seqlock.h | 11 |
1 files changed, 9 insertions, 2 deletions
diff --git a/include/linux/seqlock.h b/include/linux/seqlock.h index bcf4cf26b8c8..a42a29952889 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) |