linux-kernel - Re: [PATCH 2/6] block: allow blk_execute_rq_nowait() to be called from IRQ context

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Message-ID: <4EA7C094.8000204@kernel.dk>
Date:	Wed, 26 Oct 2011 10:11:00 +0200
From:	Jens Axboe <axboe@...nel.dk>
To:	Tejun Heo <tj@...nel.org>
CC:	vgoyal@...hat.com, jgarzik@...ox.com, davem@...emloft.net,
	hch@...radead.org, ctalbott@...gle.com, rni@...gle.com,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/6] block: allow blk_execute_rq_nowait() to be called
 from IRQ context

On 2011-10-26 03:02, Tejun Heo wrote:
> Currently blk_execute_rq_nowait() directly calls __blk_run_queue() and
> thus must be called from sleepable context.  This patch updates the
> function such that it can be called from non-sleepable context and
> schedules async execution in such cases.  This will be used to
> unexport elv_add_request().
> 
> While at it, add FIXME comment for REQ_TYPE_PM_RESUME special case.
> 
> -v2: hch pointed out that blk_execute_rq_nowait() can be hot path for
>      some drivers.  Retained direct execution from sleepable context.

Ugh, this looks nasty:

> +	bool may_sleep = !preempt_count() && !irqs_disabled();

please don't ever do that. Pass the context in instead.

> +	/*
> +	 * Some drivers beat this path pretty hard.  As an optimization, if
> +	 * we're being called from sleepable context, run @q directly.
> +	 */
> +	if (may_sleep) {
> +		__blk_run_queue(q);
> +		/*
> +		 * The queue is stopped so it won't be run.
> +		 * FIXME: Please kill me along with REQ_TYPE_PM_RESUME.
> +		 */
> +		if (rq->cmd_type == REQ_TYPE_PM_RESUME)
> +			q->request_fn(q);

This is very nasty as well.

-- 
Jens Axboe

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