@@ -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)
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> --- v3: * Add missing comment that was in preceding seqlock patch. --- include/linux/seqlock.h | 11 +++++++++-- 1 file changed, 9 insertions(+), 2 deletions(-)