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

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

include/linux/seqlock.h

+9 −2
Original line number Diff line number Diff line
@@ -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)