linux-kernel - Re: [PATCH 05/10] block: remove per-queue plugging

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Message-ID: <20110413070827.11b2a71d@notabene.brown>
Date:	Wed, 13 Apr 2011 07:08:27 +1000
From:	NeilBrown <neilb@...e.de>
To:	Dave Chinner <david@...morbit.com>
Cc:	Jens Axboe <jaxboe@...ionio.com>,
	"hch@...radead.org" <hch@...radead.org>,
	Mike Snitzer <snitzer@...hat.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"dm-devel@...hat.com" <dm-devel@...hat.com>,
	"linux-raid@...r.kernel.org" <linux-raid@...r.kernel.org>
Subject: Re: [PATCH 05/10] block: remove per-queue plugging

On Wed, 13 Apr 2011 00:34:52 +1000 Dave Chinner <david@...morbit.com> wrote:

> On Tue, Apr 12, 2011 at 03:45:52PM +0200, Jens Axboe wrote:
> Not until you
> > schedule() or call blk_finish_plug(), both of which are events that you
> > can control.
> 
> Well, not really - now taking any sleeping lock or waiting on
> anything can trigger a plug flush where previously you had to
> explicitly issue them. I'm not saying what we had is better, just
> that there are implicit flushes with your changes that are
> inherently uncontrollable...

It's not just sleeping locks - if preempt is enabled a schedule can happen at
any time - at any depth.  I've seen a spin_unlock do it.

NeilBrown
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