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 a7b46b754a7c8107e34ee7cd3e7733220af7fb1b..076cbab9c3e0f5c47c7e8807c730c9ad050a036e 100644 (file)
--- 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;

diff --git a/block/blk.h b/block/blk.h
index fda5a4632aba3cc3cfcc1942a358329c93f9882c..6ac43dfd68a7d489b295b5416fa1723b6e1cd7e8 100644 (file)
--- 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,
 };