From: Jens Axboe <axboe@kernel.dk>
Date: Wed, 4 Oct 2017 17:22:24 +0000 (-0600)
Subject: blk-mq: document the need to have STARTED and COMPLETED share a byte
X-Git-Url: http://git.lede-project.org./?a=commitdiff_plain;h=fc13457f74dcf054b0d17efb7b94b46fdf17f412;p=openwrt%2Fstaging%2Fblogic.git

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 a7b46b754a7c..076cbab9c3e0 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;
diff --git a/block/blk.h b/block/blk.h
index fda5a4632aba..6ac43dfd68a7 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,
 };