linux-kernel - Re: [PATCH 7/8] mq-deadline: add blk-mq adaptation of the deadline IO scheduler

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Date:   Wed, 1 Feb 2017 22:20:02 -0700
From:   Jens Axboe <axboe@...com>
To:     Paolo Valente <paolo.valente@...aro.org>
CC:     Jens Axboe <axboe@...nel.dk>, <linux-block@...r.kernel.org>,
        Linux-Kernal <linux-kernel@...r.kernel.org>, <osandov@...com>
Subject: Re: [PATCH 7/8] mq-deadline: add blk-mq adaptation of the deadline IO
 scheduler

On 02/01/2017 04:56 AM, Paolo Valente wrote:
>> +/*
>> + * add rq to rbtree and fifo
>> + */
>> +static void dd_insert_request(struct blk_mq_hw_ctx *hctx, struct request *rq,
>> +			      bool at_head)
>> +{
>> +	struct request_queue *q = hctx->queue;
>> +	struct deadline_data *dd = q->elevator->elevator_data;
>> +	const int data_dir = rq_data_dir(rq);
>> +
>> +	if (blk_mq_sched_try_insert_merge(q, rq))
>> +		return;
>> +
> 
> A related doubt: shouldn't blk_mq_sched_try_insert_merge be invoked
> with the scheduler lock held too, as blk_mq_sched_try_merge, to
> protect (at least) q->last_merge?
>
> In bfq this function is invoked with the lock held.

It doesn't matter which lock you use, as long as:

1) You use the same one consistently
2) It has the same scope as the queue lock (the one you call the
   scheduler lock)

mq-deadline sets up a per-queue structure, deadline_data, and it has a
lock embedded in that structure. This is what mq-deadline uses to
serialize access to its data structures, as well as those in the queue
(like last_merge).

-- 
Jens Axboe