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>
diff --git a/block/blk-mq.c b/block/blk-mq.c
index a7b46b7..076cbab 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -2923,6 +2923,12 @@ EXPORT_SYMBOL_GPL(blk_mq_poll);
static int __init blk_mq_init(void)
{
+ /*
+ * See comment in block/blk.h rq_atomic_flags enum
+ */
+ BUILD_BUG_ON((REQ_ATOM_STARTED / BITS_PER_BYTE) !=
+ (REQ_ATOM_COMPLETE / BITS_PER_BYTE));
+
cpuhp_setup_state_multi(CPUHP_BLK_MQ_DEAD, "block/mq:dead", NULL,
blk_mq_hctx_notify_dead);
return 0;
