linux-kernel - Re: [PATCH 12/12] blkcg: kill the mind-bending blkg->dev

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Message-ID: <20120119154919.GA10908@redhat.com>
Date:	Thu, 19 Jan 2012 10:49:19 -0500
From:	Vivek Goyal <vgoyal@...hat.com>
To:	Tejun Heo <tj@...nel.org>
Cc:	axboe@...nel.dk, ctalbott@...gle.com, rni@...gle.com,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 12/12] blkcg: kill the mind-bending blkg->dev

On Wed, Jan 18, 2012 at 05:11:30PM -0800, Tejun Heo wrote:
> blkg->dev is dev_t recording the device number of the block device for
> the associated request_queue.  It is used to identify the associated
> block device when printing out configuration or stats.
> 
> This is redundant to begin with.  A blkg is an association between a
> cgroup and a request_queue and it of course is possible to reach
> request_queue from blkg and synchronization conventions are in place
> for safe q dereferencing, so this shouldn't be necessary from the
> beginning.  Furthermore, it's initialized by sscanf()ing the device
> name of backing_dev_info.  The mind boggles.
> 
> Anyways, if blkg is visible under rcu lock, we *know* that the
> associated request_queue hasn't gone away yet and its bdi is
> registered and alive - blkg can't be created for request_queue which
> hasn't been fully initialized and it can't go away before blkg is
> removed.
> 
> Let stat and conf read functions get device name from
> blkg->q->backing_dev_info.dev and pass it down to printing functions
> and remove blkg->dev.

I think one of the reasons for not using blkg->q->backing_dev_info.dev was
that there were still drivers where multiple gendisks were sharing the
request queue.

Thanks
Vivek
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