Commit 9ba52e58 authored May 04, 2015 by Shaohua Li Committed by Jens Axboe May 04, 2015

blk-mq: don't lose requests if a stopped queue restarts



Normally if driver is busy to dispatch a request the logic is like below:
block layer:					driver:
	__blk_mq_run_hw_queue
a.						blk_mq_stop_hw_queue
b.	rq add to ctx->dispatch

later:
1.						blk_mq_start_hw_queue
2.	__blk_mq_run_hw_queue

But it's possible step 1-2 runs between a and b. And since rq isn't in
ctx->dispatch yet, step 2 will not run rq. The rq might get lost if
there are no subsequent requests kick in.

Signed-off-by: Shaohua Li <shli@fb.com>
Signed-off-by: Jens Axboe <axboe@fb.com>

parent b2387ddc

block/blk-mq.c

+10 −0

Original line number	Diff line number	Diff line
		@@ -858,6 +858,16 @@ static void __blk_mq_run_hw_queue(struct blk_mq_hw_ctx *hctx)
		spin_lock(&hctx->lock);
		list_splice(&rq_list, &hctx->dispatch);
		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.
		*
		* blk_mq_run_hw_queue() already checks the STOPPED bit
		**/
		blk_mq_run_hw_queue(hctx, true);
		}
		}