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: NBoqun Feng <boqun.feng@gmail.com> Signed-off-by: NJens Axboe <axboe@kernel.dk>
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