linux-kernel - Re: question about blk_schedule_flush

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Message-ID: <1306136216.18455.3.camel@twins>
Date:	Mon, 23 May 2011 09:36:56 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Con Kolivas <kernel@...ivas.org>
Cc:	linux-kernel@...r.kernel.org, axboe@...nel.dk, mingo@...e.hu
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?

Yeah, that's quite all-right, both pre_schedule() and idle_balance()
already did that too.

We can safely release the runqueue lock after deactivate_task() and
before put_prev_task().
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