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Commit fc13457f authored Oct 04, 2017 by Jens Axboe

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>

parent a7af0af3

block/blk-mq.c

+6 −0

Original line number	Diff line number	Diff line
		@@ -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;

block/blk.h

+7 −0

Original line number	Diff line number	Diff line
		@@ -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,
		};