Re: [PATCH 1/1] blk-core: fix queue stuck on attempt to submit request from unplug

From: Jens Axboe
Date: Mon Sep 28 2015 - 13:27:26 EST

On 09/27/2015 02:44 PM, Roman Pen wrote:
In case of several stacked block devices, which both were inited by
blk_init_queue call, you can catch the queue stuck, if first device
in stack makes bio submit being in a flush of a plug list.

Let's consider this regular scenario taking readahead into account

1. Start plug
2. Read pages in loop
3. Finish plug

This example generates backtrace as follows:

1. blk_start_plug
2. generic_make_request
if (current->plug)
list_add_tail(&req->queuelist, &plug->list);
3. blk_finish_plug
XXX_request_fn [some request handler of block device]
if (current->plug)
list_add_tail(&req->queuelist, &plug->list);

So the problem is, that on step 3. XXX_request_fn makes
another request, which again will be put to plug list,
because plug is till active, thus new request will be
stuck forever in the queue.

How to fix?
Do flush plug list till it becomes empty.

What devices submit IO from request_fn? Seems like something that would be better handled out of a make_request_fn hook for that device, which would also avoid this corner case.

Jens Axboe

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