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 <h...@infradead.org>
Reported-by: Kent Overstreet <kent.overstr...@gmail.com>
Signed-off-by: Ming Lei <tom.leim...@gmail.com>
---
block/blk-flush.c | 28 ++++++++++++++++++++++++++++
1 file changed, 28 insertions(+)
diff --git a/block/blk-flush.c b/block/blk-flush.c
index 6a14b68b9135..3c882cbc7541 100644
--- a/block/blk-flush.c
+++ b/block/blk-flush.c
@@ -343,6 +343,34 @@ static void flush_data_end_io(struct request *rq, int
error)
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().
*/
--
2.7.4
