linux-kernel - Re: question about blk_schedule_flush

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Message-ID: <1306136629.4876.13.camel@marge.simson.net>
Date:	Mon, 23 May 2011 09:43:49 +0200
From:	Mike Galbraith <efault@....de>
To:	Con Kolivas <kernel@...ivas.org>
Cc:	linux-kernel@...r.kernel.org, axboe@...nel.dk, mingo@...e.hu,
	peterz@...radead.org
Subject: Re: question about blk_schedule_flush_plug

On Mon, 2011-05-23 at 17:05 +1000, Con Kolivas wrote:
> I was looking at the scheduler changes going into 2.6.39 again and wondered 
> about the use of blk_schedule_flush_plug smack in the middle of schedule()
> 
> It looks like this:
> 			if (blk_needs_flush_plug(prev)) {
> 				raw_spin_unlock(&rq->lock);
> 				blk_schedule_flush_plug(prev);
> 				raw_spin_lock(&rq->lock);
> 			}
> 
> Now call me suspicious but to my eyes it looks really dubious unlocking the 
> runqueue like that right in the heart of schedule().
> 
> Comments?

Releasing/retaking rq->lock is nothing new:

static void idle_balance(int this_cpu, struct rq *this_rq)
{
...
        /*
         * Drop the rq->lock, but keep IRQ/preempt disabled.
         */
        raw_spin_unlock(&this_rq->lock);

See also need_resched, and double_lock_balance() instances.

	-Mike

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