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

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Date:	Mon, 28 Sep 2015 19:36:05 +0200
From:	Roman Peniaev <r.peniaev@...il.com>
To:	Jens Axboe <axboe@...nel.dk>
Cc:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/1] blk-core: fix queue stuck on attempt to submit
 request from unplug

On Mon, Sep 28, 2015 at 7:27 PM, Jens Axboe <axboe@...nel.dk> wrote:
> 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
>> (readahead.c:read_pages):
>>
>> 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
>>          q->make_request_fn
>>          [blk_queue_bio]
>>              if (current->plug)
>>                  list_add_tail(&req->queuelist, &plug->list);
>> 3. blk_finish_plug
>>          blk_flush_plug_list
>>              queue_unplugged
>>                  __blk_run_queue
>>                      XXX_request_fn [some request handler of block device]
>>                          generic_make_request
>>                              q->make_request_fn
>>                              [blk_queue_bio]
>>                                  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.

Practically, this stuck was observed while doing some experiments with md,
which was switched from bio mode to request mode.

But what drawbacks has this loop-till-empty? Because it seems that those
corner cases with plugging can be easily fixed. Or I am missing something?

Also e.g. flush_plug_callbacks() does exactly the same:

while (!list_empty(&plug->cb_list)) {
      list_splice_init(&plug->cb_list, &callbacks);

      while (!list_empty(&callbacks)) {
       [...]
      }
}


--
Roman



>
>
> --
> Jens Axboe
>
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