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blk-mq: Clarify comments in blk_mq_dispatch_rq_list() 

The blk_mq_dispatch_rq_list() implementation got modified several
times but the comments in that function were not updated every
time. Since it is nontrivial what is going on, update the comments
in blk_mq_dispatch_rq_list().

Signed-off-by: Bart Van Assche <bart.vanassche@sandisk.com>
Cc: Omar Sandoval <osandov@fb.com>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Hannes Reinecke <hare@suse.de>
Signed-off-by: Jens Axboe <axboe@fb.com>
This commit is contained in:
Bart Van Assche 2017-04-07 11:16:51 -07:00 committed by Jens Axboe
parent 705cda97ee
commit 710c785f80
1 changed files with 18 additions and 10 deletions
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28
block/blk-mq.c
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@ -1063,8 +1063,8 @@ bool blk_mq_dispatch_rq_list(struct request_queue *q, struct list_head *list)
*/
if (!list_empty(list)) {
/*
* If we got a driver tag for the next request already,
* free it again.
* If an I/O scheduler has been configured and we got a driver
* tag for the next request already, free it again.
*/
rq = list_first_entry(list, struct request, queuelist);
blk_mq_put_driver_tag(rq);
@ -1074,16 +1074,24 @@ bool blk_mq_dispatch_rq_list(struct request_queue *q, struct list_head *list)
spin_unlock(&hctx->lock);
/*
* the queue is expected stopped with BLK_MQ_RQ_QUEUE_BUSY, but
* it's possible the queue is stopped and restarted again
* before this. Queue restart will dispatch requests. And since
* requests in rq_list aren't added into hctx->dispatch yet,
* the requests in rq_list might get lost.
* If SCHED_RESTART was set by the caller of this function and
* it is no longer set that means that it was cleared by another
* thread and hence that a queue rerun is needed.
*
* blk_mq_run_hw_queue() already checks the STOPPED bit
* If TAG_WAITING is set that means that an I/O scheduler has
* been configured and another thread is waiting for a driver
* tag. To guarantee fairness, do not rerun this hardware queue
* but let the other thread grab the driver tag.
*
* If RESTART or TAG_WAITING is set, then let completion restart
* the queue instead of potentially looping here.
* If no I/O scheduler has been configured it is possible that
* the hardware queue got stopped and restarted before requests
* were pushed back onto the dispatch list. Rerun the queue to
* avoid starvation. Notes:
* - blk_mq_run_hw_queue() checks whether or not a queue has
* been stopped before rerunning a queue.
* - Some but not all block drivers stop a queue before
* returning BLK_MQ_RQ_QUEUE_BUSY. Two exceptions are scsi-mq
* and dm-rq.
*/
if (!blk_mq_sched_needs_restart(hctx) &&
!test_bit(BLK_MQ_S_TAG_WAITING, &hctx->state))