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From:   Gabriel Krisman Bertazi <krisman@collabora.com>
To:     Bart Van Assche <bvanassche@acm.org>
Cc:     =?utf-8?Q?Andr=C3=A9?= Almeida <andrealmeid@collabora.com>,
        linux-block@vger.kernel.org, linux-kernel@vger.kernel.org,
        axboe@kernel.dk, kernel@collabora.com
Subject: Re: [PATCH v2 1/1] blk-mq: fill header with kernel-doc
Organization: Collabora
References: <20191008001416.12656-1-andrealmeid@collabora.com>
        <854l0j19go.fsf@collabora.com>
        <6aa48cd2-5f23-a4be-f777-d65bf755a976@acm.org>
Date:   Tue, 08 Oct 2019 14:46:31 -0400
In-Reply-To: <6aa48cd2-5f23-a4be-f777-d65bf755a976@acm.org> (Bart Van Assche's
        message of "Tue, 8 Oct 2019 10:30:58 -0700")
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Bart Van Assche <bvanassche@acm.org> writes:

> On 10/8/19 9:35 AM, Gabriel Krisman Bertazi wrote:
>> André Almeida <andrealmeid@collabora.com> writes:
>>
>>> diff --git a/include/linux/blk-mq.h b/include/linux/blk-mq.h
>>> index e0fce93ac127..8b745f229789 100644
>>> --- a/include/linux/blk-mq.h
>>> +++ b/include/linux/blk-mq.h
>>> @@ -10,74 +10,153 @@ struct blk_mq_tags;
>>>   struct blk_flush_queue;
>>>     /**
>>> - * struct blk_mq_hw_ctx - State for a hardware queue facing the hardware block device
>>> + * struct blk_mq_hw_ctx - State for a hardware queue facing the hardware
>>> + * block device
>>>    */
>>>   struct blk_mq_hw_ctx {
>>>   	struct {
>>> +		/** @lock: Lock for accessing dispatch queue */
>>>   		spinlock_t		lock;
>>> +		/**
>>> +		 * @dispatch: Queue of dispatched requests, waiting for
>>> +		 * workers to send them to the hardware.
>>> +		 */
>>
>> It's been a few years since I looked at the block layer, but isn't
>> this used to hold requests that were taken from the blk_mq_ctx,  but
>> couldn't be dispatched because the queue was full?
>
> I don't think so. I think that you are looking for the requeue_list
> member of struct request_queue.
>

Hmm, sorry, but I'm confused.  I'm sure I'm missing something simple,
since I haven't touched this in a while, so maybe you can quickly point
me in the right direction?

I see blk_mq_requeue_request() being used by device drivers to retry
requests that failed, but if I read the code correctly, the flushed
queue seems to be moved to hctx->dispatch when the device
driver returned BLK_STS_RESOURCE or BLK_STS_DEV_RESOURCE to
blk_mq_dispatch_rq_list(). I thought BLK_STS_RESOURCE was returned by
the driver on .queue_rq() to signal there was no more resources on the
hardware to service further requests.

-- 
Gabriel Krisman Bertazi