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blk-mq: document the need to have STARTED and COMPLETED share a byte

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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 <[email protected]>
Signed-off-by: Jens Axboe <[email protected]>
This commit is contained in:
Jens Axboe 2017-10-04 11:22:24 -06:00
commit fc13457f74
2 changed files with 13 additions and 0 deletions
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7
block/blk.h
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@ -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,
};