Commit bf07132f authored by Marco Elver's avatar Marco Elver Committed by Paul E. McKenney

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: default avatarMarco Elver <elver@google.com>
Acked-by: default avatarPaul E. McKenney <paulmck@kernel.org>
Signed-off-by: default avatarPaul E. McKenney <paulmck@kernel.org>
parent 88ecd153
......@@ -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)
......
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