linux-kernel - Re: [PATCH] blk-mq: Avoid that I/O hangs in bt

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Date:	Mon, 08 Dec 2014 15:55:14 +0100
From:	Bart Van Assche <bvanassche@....org>
To:	Jens Axboe <axboe@...nel.dk>
CC:	Christoph Hellwig <hch@....de>, Robert Elliott <Elliott@...com>,
	linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] blk-mq: Avoid that I/O hangs in bt_get()

On 11/06/14 14:41, Bart Van Assche wrote:
> With kernel 3.18-rc3 and with can_queue=62 I can trigger a hang in
> bt_get() easily.

(once more replying to my own e-mail)

Hello Jens,

Finally I found the time to look further into this. The patch below 
seems to be sufficient to prevent this hang. However, I'm not a block 
layer expert so it's not clear to me whether the patch below makes sense ?

Thanks,

Bart.

[PATCH] blk-mq: Fix bt_get() hang

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.
---
  block/blk-mq-tag.c | 2 ++
  1 file changed, 2 insertions(+)

diff --git a/block/blk-mq-tag.c b/block/blk-mq-tag.c
index 67ab88b..e88af88 100644
--- a/block/blk-mq-tag.c
+++ b/block/blk-mq-tag.c
@@ -256,6 +256,8 @@ static int bt_get(struct blk_mq_alloc_data *data,
  			break;
  		}

+		blk_mq_run_hw_queue(hctx, false);
+
  		blk_mq_put_ctx(data->ctx);

  		io_schedule();
--
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