[PATCH 4/6] blk-mq: Avoid that I/O hangs in bt_get()

[Bart Van Assche <bvanassche@acm.org>]

Avoid that if there are fewer hardware queues than CPU threads that
bt_get() can hang. The symptoms of the hang were as follows:
* All tags allocated for a particular hardware queue.
* (nr_tags) pending commands for that hardware queue.
* No pending commands for the software queues associated with that
  hardware queue.

The call stack that corresponds to the hang is as follows:

io_schedule+0x9c/0x130
bt_get+0xef/0x180
blk_mq_get_tag+0x9f/0xd0
__blk_mq_alloc_request+0x16/0x1f0
blk_mq_map_request+0x123/0x130
blk_mq_make_request+0x69/0x280
generic_make_request+0xc0/0x110
submit_bio+0x64/0x130
do_blockdev_direct_IO+0x1dc8/0x2da0
__blockdev_direct_IO+0x47/0x50
blkdev_direct_IO+0x49/0x50
generic_file_read_iter+0x546/0x610
blkdev_read_iter+0x32/0x40
aio_run_iocb+0x1f8/0x400
do_io_submit+0x121/0x490
SyS_io_submit+0xb/0x10
system_call_fastpath+0x12/0x17

Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Robert Elliott <elliott@hp.com>
Cc: Ming Lei <ming.lei@canonical.com>
Cc: Alexander Gordeev <agordeev@redhat.com>
Cc: <stable@vger.kernel.org> # v3.13+
---
 block/blk-mq-tag.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/block/blk-mq-tag.c b/block/blk-mq-tag.c
index 70d9f9e..795996e 100644
--- a/block/blk-mq-tag.c
+++ b/block/blk-mq-tag.c
@@ -256,6 +256,12 @@ static int bt_get(struct blk_mq_alloc_data *data,
 		if (tag != -1)
 			break;
 
+		blk_mq_run_hw_queue(hctx, false);
+
+		tag = __bt_get(hctx, bt, last_tag);
+		if (tag != -1)
+			break;
+
 		blk_mq_put_ctx(data->ctx);
 
 		io_schedule();
-- 
2.1.2