From fc13457f74dcf054b0d17efb7b94b46fdf17f412 Mon Sep 17 00:00:00 2001 From: Jens Axboe Date: Wed, 4 Oct 2017 11:22:24 -0600 Subject: 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 Signed-off-by: Jens Axboe --- block/blk-mq.c | 6 ++++++ block/blk.h | 7 +++++++ 2 files changed, 13 insertions(+) (limited to 'block') 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, }; -- cgit v1.2.3-55-g7522