blk-mq: document the need to have STARTED and COMPLETED share a byte

For memory ordering guarantees on stores, we need to ensure that
these two bits share the same byte of storage in the unsigned
long. Add a comment as to why, and a BUILD_BUG_ON() to ensure that
we don't violate this requirement.

Suggested-by: Boqun Feng <boqun.feng@gmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

1 files changed, 7 insertions, 0 deletions

diff --git a/block/blk.h b/block/blk.h
index fda5a4632aba3..6ac43dfd68a7d 100644
--- a/block/blk.h
+++ b/block/blk.h
@@ -122,8 +122,15 @@ void blk_account_io_done(struct request *req);
  * Internal atomic flags for request handling
  */
 enum rq_atomic_flags {
+	/*
+	 * Keep these two bits first - not because we depend on the
+	 * value of them, but we do depend on them being in the same
+	 * byte of storage to ensure ordering on writes. Keeping them
+	 * first will achieve that nicely.
+	 */
 	REQ_ATOM_COMPLETE = 0,
 	REQ_ATOM_STARTED,
+
 	REQ_ATOM_POLL_SLEPT,
 };