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block: flush: fix IO hang in case of flood fua req

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This patch fixes one issue reported by Kent, which can
be triggered in bcachefs over sata disk. Actually it
is a generic issue in block flush vs. blk-tag.

Cc: Christoph Hellwig <hch@infradead.org>
Reported-by: Kent Overstreet <kent.overstreet@gmail.com>
Signed-off-by: Ming Lei <tom.leiming@gmail.com>
Signed-off-by: Jens Axboe <axboe@fb.com>
This commit is contained in:
Ming Lei 2016-10-26 16:57:15 +08:00 committed by Jens Axboe
parent 423221d174
commit 94d7dea448
1 changed files with 28 additions and 0 deletions
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28
block/blk-flush.c
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@ -342,6 +342,34 @@ static void flush_data_end_io(struct request *rq, int error)
struct request_queue *q = rq->q;
struct blk_flush_queue *fq = blk_get_flush_queue(q, NULL);
/*
* Updating q->in_flight[] here for making this tag usable
* early. Because in blk_queue_start_tag(),
* q->in_flight[BLK_RW_ASYNC] is used to limit async I/O and
* reserve tags for sync I/O.
*
* More importantly this way can avoid the following I/O
* deadlock:
*
* - suppose there are 40 fua requests comming to flush queue
* and queue depth is 31
* - 30 rqs are scheduled then blk_queue_start_tag() can't alloc
* tag for async I/O any more
* - all the 30 rqs are completed before FLUSH_PENDING_TIMEOUT
* and flush_data_end_io() is called
* - the other rqs still can't go ahead if not updating
* q->in_flight[BLK_RW_ASYNC] here, meantime these rqs
* are held in flush data queue and make no progress of
* handling post flush rq
* - only after the post flush rq is handled, all these rqs
* can be completed
*/
elv_completed_request(q, rq);
/* for avoiding double accounting */
rq->cmd_flags &= ~REQ_STARTED;
/*
* After populating an empty queue, kick it to avoid stall. Read
* the comment in flush_end_io().